PHYSICS : ELECTRIC CIRCUITS CHEMISTRY : ACIDS, BASES AND SALTS BIOLOGY : LIFE PROCESSES

Transcription

1 ClassX CBSE-i Science : ELECTRIC CIRCUITS CHEMISTRY : ACIDS, BASES AND SALTS BIOLOGY : LIFE PROCESSES UNIT 1 Shiksha Kendra, 2, Community Centre, Preet Vihar, Delhi India

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3 CBSE-i Science : ELECTRIC CIRCUITS CHEMISTRY : ACIDS, BASES AND SALTS BIOLOGY : LIFE PROCESSES UNIT 1 ClassX Shiksha Kendra, 2, Community Centre, Preet Vihar, Delhi India

4 The CBSE-International is grateful for permission to reproduce and/or translate copyright material used in this publication. The acknowledgements have been included wherever appropriate and sources from where the material has been taken duly mentioned. In case anything has been missed out, the Board will be pleased to rectify the error at the earliest possible opportunity. All Rights of these documents are reserved. No part of this publication may be reproduced, printed or transmitted in any form without the prior permission of the CBSE-i. This material is meant for the use of schools who are a part of the CBSE-International only.

5 Preface The Curriculum initiated by Central Board of Secondary Education -International (CBSE-i) is a progressive step in making the educational content and methodology more sensitive and responsive to the global needs. It signifies the emergence of a fresh thought process in imparting a curriculum which would restore the independence of the learner to pursue the learning process in harmony with the existing personal, social and cultural ethos. The Central Board of Secondary Education has been providing support to the academic needs of the learners worldwide. It has about schools affiliated to it and over 158 schools situated in more than 23 countries. The Board has always been conscious of the varying needs of the learners in countries abroad and has been working towards contextualizing certain elements of the learning process to the physical, geographical, social and cultural environment in which they are engaged. The International Curriculum being designed by CBSE-i, has been visualized and developed with these requirements in view. The nucleus of the entire process of constructing the curricular structure is the learner. The objective of the curriculum is to nurture the independence of the learner, given the fact that every learner is unique. The learner has to understand, appreciate, protect and build on values, beliefs and traditional wisdom, make the necessary modifications, improvisations and additions wherever and whenever necessary. The recent scientific and technological advances have thrown open the gateways of knowledge at an astonishing pace. The speed and methods of assimilating knowledge have put forth many challenges to the educators, forcing them to rethink their approaches for knowledge processing by their learners. In this context, it has become imperative for them to incorporate those skills which will enable the young learners to become 'life long learners'. The ability to stay current, to upgrade skills with emerging technologies, to understand the nuances involved in change management and the relevant life skills have to be a part of the learning domains of the global learners. The CBSE-i curriculum has taken cognizance of these requirements. The CBSE-i aims to carry forward the basic strength of the Indian system of education while promoting critical and creative thinking skills, effective communication skills, interpersonal and collaborative skills along with information and media skills. There is an inbuilt flexibility in the curriculum, as it provides a foundation and an extension curriculum, in all subject areas to cater to the different pace of learners. The CBSE has introduced the CBSE-i curriculum in schools affiliated to CBSE at the international level in 2010 and is now introducing it to other affiliated schools who meet the requirements for introducing this curriculum. The focus of CBSE-i is to ensure that the learner is stress-free and committed to active learning. The learner would be evaluated on a continuous and comprehensive basis consequent to the mutual interactions between the teacher and the learner. There are some nonevaluative components in the curriculum which would be commented upon by the teachers and the school. The objective of this part or the core of the curriculum is to scaffold the learning experiences and to relate tacit knowledge with formal knowledge. This would involve trans-disciplinary linkages that would form the core of the learning process. Perspectives, SEWA (Social Empowerment through Work and Action), Life Skills and Research would be the constituents of this 'Core'. The Core skills are the most significant aspects of a learner's holistic growth and learning curve. The International Curriculum has been designed keeping in view the foundations of the National Curricular Framework (NCF 2005) NCERT and the experience gathered by the Board over the last seven decades in imparting effective learning to millions of learners, many of whom are now global citizens. The Board does not interpret this development as an alternative to other curricula existing at the international level, but as an exercise in providing the much needed Indian leadership for global education at the school level. The International Curriculum would evolve on its own, building on learning experiences inside the classroom over a period of time. The Board while addressing the issues of empowerment with the help of the schools' administering this system strongly recommends that practicing teachers become skillful learners on their own and also transfer their learning experiences to their peers through the interactive platforms provided by the Board. I profusely thank Shri G. Balasubramanian, former Director (Academics), CBSE, Ms. Abha Adams and her team and Dr. Sadhana Parashar, Head (Innovations and Research) CBSE along with other Education Officers involved in the development and implementation of this material. The CBSE-i website has already started enabling all stakeholders to participate in this initiative through the discussion forums provided on the portal. Any further suggestions are welcome. Vineet Joshi Chairman

6 Acknowledgements Advisory Shri Vineet Joshi, Chairman, CBSE Shri Shashi Bhushan, Director(Academic), CBSE Ideators English : Ms. Sarita Manuja Ms. Renu Anand Ms. Gayatri Khanna Ms. P. Rajeshwary Ms. Neha Sharma Ms. Sarabjit Kaur Ms. Ruchika Sachdev Geography: Ms. Deepa Kapoor Ms. Bharti Dave Ms. Bhagirathi Ms. Archana Sagar Ms. Manjari Rattan Mathematics : Dr. K.P. Chinda Mr. J.C. Nijhawan Ms. Rashmi Kathuria Ms. Reemu Verma Political Science: Ms. Sharmila Bakshi Ms. Srelekha Mukherjee Conceptual Framework Shri G. Balasubramanian, Former Director (Acad), CBSE Ms. Abha Adams, Consultant, Step-by-Step School, Noida Dr. Sadhana Parashar, Head (I & R),CBSE Ms. Aditi Misra Ms. Anuradha Sen Ms. Jaishree Srivastava Dr. Rajesh Hassija Ms. Amita Mishra Ms. Archana Sagar Dr. Kamla Menon Ms. Rupa Chakravarty Ms. Anita Sharma Ms. Geeta Varshney Dr. Meena Dhami Ms. Sarita Manuja Ms. Anita Makkar Ms. Guneet Ohri Ms. Neelima Sharma Ms. Seema Rawat Dr. Anju Srivastava Dr. Indu Khetrapal Dr. N. K. Sehgal Dr. Uma Chaudhry Material Production Groups: Classes IX-X Science : Ms. Charu Maini Ms. S. Anjum Ms. Meenambika Menon Ms. Novita Chopra Ms. Neeta Rastogi Ms. Pooja Sareen Economics: Ms. Mridula Pant Mr. Pankaj Bhanwani Ms. Ambica Gulati Material Production Groups: Classes VI-VIII History : Ms. Jayshree Srivastava Ms. M. Bose Ms. A. Venkatachalam Ms. Smita Bhattacharya English : Science : Mathematics : Geography: Ms. Rachna Pandit Dr. Meena Dhami Ms. Seema Rawat Ms. Suparna Sharma Ms. Neha Sharma Mr. Saroj Kumar Ms. N. Vidya Ms. Leela Grewal Ms. Sonia Jain Ms. Rashmi Ramsinghaney Ms. Mamta Goyal History : Ms. Dipinder Kaur Ms. Seema kapoor Ms. Chhavi Raheja Ms. Leeza Dutta Ms. Sarita Ahuja Ms. Priyanka Sen Political Science: Ms. Kalpana Pant Dr. Kavita Khanna Ms. Kanu Chopra Ms. Keya Gupta Ms. Shilpi Anand Coordinators: Dr. Sadhana Parashar, Ms. Sugandh Sharma, Dr. Srijata Das, Dr. Rashmi Sethi, Head (I and R) E O (Com) E O (Maths) E O (Science) Shri R. P. Sharma, Consultant Ms. Ritu Narang, RO (Innovation) Ms. Sindhu Saxena, R O (Tech) Shri Al Hilal Ahmed, AEO Ms. Seema Lakra, S O Material Production Group: Classes I-V Dr. Indu Khetarpal Ms. Rupa Chakravarty Ms. Anita Makkar Ms. Nandita Mathur Ms. Vandana Kumar Ms. Anuradha Mathur Ms. Kalpana Mattoo Ms. Seema Chowdhary Ms. Anju Chauhan Ms. Savinder Kaur Rooprai Ms. Monika Thakur Ms. Ruba Chakarvarty Ms. Deepti Verma Ms. Seema Choudhary Mr. Bijo Thomas Ms. Mahua Bhattacharya Ms. Ritu Batra Ms. Kalyani Voleti Ms. Preeti Hans, Proof Reader

7 Contents 1. SYLLABUS COVERAGE - Physics 3 Core and Extension 2. SCOPE DOCUMENT- Physics 3 ( Learning Objectives ( Cross Curricular Links ( Suggested Activities 3. LESSON TEMPLATE-physics 5 4. Student -Teacher Support Material-Physics 8 5. Rubrics of Assessment- Physics 65 chemistry 6. SYLLABUS COVERAGE - Chemistry 69 Core and Extension 7. SCOPE DOCUMENT- Chemistry 69 ( Learning Objectives ( Cross Curricular Links ( Suggested Activities 8. LESSON TEMPLATE- Chemistry Student -Teacher Support Material- Chemistry Rubrics of Assessment-Chemistry 151

8 BIOLOGY 10. SYLLABUS COVERAGE - Biology 155 Core and Extension 11. SCOPE DOCUMENT- Biology 156 ( Learning Objectives ( Cross Curricular Links ( Suggested Activities 12. LESSON TEMPLATE- Biology Student -Teacher Support Material- Biology Rubrics of Assessment-Biology 258

9 Learning outcomes - Core Physics Scope Document THERMAL UNIT-1 ELECTRIC CIRCUITS At the end of this unit, students should be able to vexplain, how solids, liquids and gases expand. vunderstand the importance and significance of thermal expansion in different practical situations. vdefine specific heat capacity and specific latent heat. vsolve numericals based on specific heat capacity and specific latent heat. vunderstand, how heat gets transferred through solids, liquids and gases. vdifferentiate between conduction, convection and radiation. vlearn about different methods of heat transfer in different situations in nature and man made devices. 1

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11 Physics Unit 1 - ELECTRIC CIRCUITS Syllabus Coverage S Y L L A B U S Core 2Electric circuit - Drawing electric circuit using symbols 2Electric Current, Electric Potential and the relation between them 2Resistance and the factors on which it depends 2Ohm's Law and its verification 2Series and parallel Combination of resistors 2Heating effect of current and its Practical Applications Scope Document Learning outcomes At the end of this unit, students should be able to 2Explain an electric circuit and Draw a circuit using simple symbols for the components of an electric circuit. 2Define electric potential, potential difference and current and Understand the relation between them. 2Define and Explain resistance and derive an expression for resistance. 2Explain and Verify Ohm's law 3

12 2Understand and describe the two different types of combination of resistors - series and parallel circuits. 2Explain the heating effect of current. 2Understand and explain the practical application of heating effect of current. 2Solve numerical problems on resistance, combination of resistors, ohm's law and heating effect of current and power consumption Cross curricular links 2Mathematics - Solve numerical problems 4

13 Lesson Template Steps to be followed Pre content Warming Up Activity Teacher's Activity Teacher may start the class by showing a video on electricity to build interest of the learners. ideos/physics/electricity.html video. Student's Activity Students will watch and discuss the matter of the video.. Content Development Student Teacher Material 1. Introduction 1.1 Electric Potential 1.2 Electric Potential Difference 2. Electric Circuit Teacher may explain the concept of electric potential and potential difference by using analogies of water flow or gravitational potential energy and check the understanding of the concept using worksheets. Teacher may define and explain the making and representation of electric circuit using activities and examples. Activity 2.1 and /worksheets/ohm_law.html Students will understand the concept and try to attempt Worksheet 1.1 and 1.2 Students will be able to schematically represent an e l e c t r i c c i r c u i t u s i n g appropriate symbols. Worksheet 2 5

14 3 Electric Current 3.1 Conventional Current Direction 4. Journey of an electron 4.1 Resistance and the factors affecting it 4.2 Mathematical Expression for resistance 5. Ohm's Law 6. Series Combination/ circuit 6.1 Series Circuit Calculation Teacher may define and explain the concept of flow of electric c u r r e n t, t h e d i r e c t i o n o f conventional current flow. Expression and unit for electric current. Teacher may explain the term resistance, the effect of various factors on resistance and its unit. Teacher may also help the learners d e r i v e a n e x p r e s s i o n f o r resistance. Activity 4 The learners can be made to attempt an interactive activity showing the effect of length and area of cross section of a wire on its resistance. rk/physics/child/index.html interactive website Teacher may explain Ohm's law and help the learners verify it with the help of Activity 5. Links.aspx for lab activities and videos Teacher may explain series connection and help the learners to derive an expression for the calculation of resultant resistance of resistors connected in series with the help of activity 6 6 Students will understand the c o n c e p t a n d a t t e m p t worksheet 3. Students will understand the term will be able to solve numerical problems. They will try to solve Worksheet 4 Student will understand the c o n c e p t t h r o u g h experimental verification and try to solve worksheets 5. Students may perform the activity and understand s e r i e s c o n n e c t i o n, i t s c h a r a c t e r i s t i c s a n d calculation

15 7. Parallel Combination/ circuit 7.1 Parallel circuit calculation 8. Series and Parallel Circuit 8.1 Series parallel Calculation 9. Heating Effect of Current 9.1 Application of heating effect of current Teacher may explain parallel connection and help the learners to derive an expression for the calculation of resultant resistance of resistors connected in parallel with the help of activity 7 T e a c h e r m a y e x p l a i n t h e combination of series and parallel connection with the help of activity 8 Teacher may show the power point presentation to sum up the concepts dealt till here. Teacher may explain the heating effect of current, Joules law of heating and practical application of heating effect with the help of examples and activity 9 Students may perform the activity and understand parallel connection, its c h a r a c t e r i s t i c s a n d calculation Students may perform the activity and understand the combination circuit and calculation. Students may perform the a c t i v i t y a n d d e r i v e expression for heat energy. 10. Electric Power 10.1 Consumption of electrical energy Teacher may help the learners to derive an expression for power, its unit and to calculate the consumption of electrical energy with the help of solved examples. Students may understand the concept of electric power and consumption of electrical energy. Post Content Revision Worksheet Project Teacher may sum up the concepts dealt during the teaching learning and assess the understanding of the concepts through revision assignments. For worksheets Teacher may ask the learners to do the given projects individually or in groups 7 Students may attempt Revision Worksheets 1, 2 and 3. S t u d e n t s w i l l d o t h e suggested projects.

16 Warm up Activity Learning Objective: The student may see the video and get interested in Electricity and terms and phenomena related to it. The teacher may show the video in the link provided here to introduce the concept of electricity and generate an interest in the learner to study the concept. video. Student-Teacher Material 1. Introduction We already know that objects like a glass rod rubbed with silk, get electrically charged. Such electrically charged objects show some very interesting properties that are not shown by an uncharged objects. However, the charges on such objects are static charges, that is, they do not move. When charges are in motion, they are said to constitute an 'electric current'. A steady continuous current, however, flows only when the charges move in a regular, systematic, orderly and cyclic way. conventional current electrons 8

17 It was (later on) recognised that the (basic) charges that move in a current carrying metallic wire are the negatively charged electrons. These electrons are present within the atoms of all the elements. In a metal, some of these often get detached from their parent atoms. These 'free electrons' then move and roam about within the volume of the metal. The free electrons when moving about in a metal (themselves), however, move about in a completely random and chaotic manner. The number of electrons moving in any one direction equals their number moving in the (exactly) opposite direction. We therefore, need some 'external agent' that can force these electrons to move in an orderly systematic way, within the metal.one of the simplest of such external agents, is the familiar and well known, 'electric cell'. The cell has a potential difference associated with it. It is this characteristic of the cell that makes it 'an agent' that can cause an electric current to flow through a wire. Let us, therefore understand the meaning of the terms ' electric potential' and 'potential difference'. 1.1 Electric Potential The concept of electric potential is closely linked to that of the electric field. Any charge, placed within an electric field, experiences a force. Hence bringing that charge to that point against the force, requires work. The electric potential at any point is defined as the energy required to bring a unit [test] charge from an infinite distance slowly to that point. It is usually measured in volts. One volt is the potential at a point if one joule of work is expended to bring a charge of one coulomb from infinity to that point. Electric potential is a scalar quantity, that is, it has only magnitude but not a direction. We can view it as analogous to height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall 'from a point at a higher potential to one at a lower potential. It was reasoned that the movement of a positive test charge within an electric field is accompanied by changes in potential energy. We know that Potential energy in a gravitational field is the stored energy of position of an object and it 9

18 is related to the location of the object within a field. When we introduce the concept of electric potential we can relate this concept to the potential energy of a positive test charge at various locations within an electric field. While electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent. Electric potential is the potential energy per charge. The concept of electric potential is closely linked to that of the electric field. Any charge, placed within an electric field, experiences a force. Hence bringing that charge to that point against the force, requires work. The electric potential at any point is defined as the energy required to bring a unit [test] charge from an infinite distance slowly to that point. It is usually measured in volts. One volt is the potential at a point if one joule of work is expended to bring a charge of one coulomb from infinity to that point. Electric potential is a scalar quantity, that is, it has only magnitude but not a direction. We can view it as analogous to height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall 'from a point at a higher potential to one at a lower potential. It was reasoned that the movement of a positive test charge within an electric field is accompanied by changes in potential energy. We know that Potential energy in a gravitational field is the stored energy of position of an object and it is related to the location of the object within a field. When we introduce the concept of electric potential we can relate this concept to the potential energy of a positive test charge at various locations within an electric field. While electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent. Electric potential is the potential energy per charge. 10

19 Worksheet 1.1 Check Your Understanding 1. The term electric potential is defined as the amount of. a. electric potential energy b. force acting upon a charge c. potential energy per charge d. force per charge 2. Complete the following statement: When work is done on a positive test charge by an external force to move it from one location to another, potential energy (increases, decreases) and electric potential (increases, decreases). 3. The following diagrams show an electric field (represented by arrows) and two points - labeled A and B - located within the electric field. A positive test charge is shown at point A. For each diagram, indicate whether work must be done upon the charge to move it from point A to point B. Finally, indicate the point (A or B) with the greater electric potential energy and the greater electric potential. A B B A Work done on charge? Yes or No Electric PE is greater at: A B Electric potential is greater at: A B Work done on charge? Yes or No Electric PE is greater at: A B Electric potential is greater at: A B 11

20 B A A Work done on charge? Yes or No Electric PE is greater at: A B Electric potential is greater at: A B B Work done on charge? Yes or No Electric PE is greater at: A B Electric potential is greater at: A B 1.2 Electric Potential Difference A volt is the unit of electric potential. 1 Volt = 1 Joule/Coulomb B E A Electric potential is a location-dependent quantity that expresses the amount of potential energy per unit of charge at a specified location. When a Coulomb of charge (or any given amount of charge) possesses a relatively large quantity of potential energy at a given location, then that location is said to be a location of high electric potential. When we apply our concepts of potential energy and electric potential to circuits, we will begin to refer to the difference in electric potential between two points. It is this electric potential difference that controls the movement of charge in electric circuits. Consider the task of moving a positive test charge within a uniform electric field from location A to location B as shown in the diagram at the right. In moving the charge against the electric field from location A to location B, work will have to be done on the charge by an external force. The work done on the charge changes 12

21 its potential energy to a higher value; and the amount of work that is done is equal to the change in the potential energy. As a result of this change in potential energy, there is also a difference in electric potential between locations A and B. This difference in electric potential is represented by the symbol? V and is formally referred to as the electric potential difference. We say that ÄV = V B - V A = Work Charge =? PE Charge The standard metric unit on electric potential difference is the volt, abbreviated V and named in honor of Alessandra Volta. One Volt is equivalent to one Joule per Coulomb. If the electric potential difference between two locations is 1 volt, then one Coulomb of charge will gain 1 joule of potential energy when moved between those two locations because electric potential difference is expressed in units of volts, it is sometimes referred to as the voltage. High Potential A B C Volts D Low Potential D Cell Role of the Cell : Supplies the energy Pumps the charge from to + terminal Maintains a ÄV across With a clear understanding of electric potential difference, the role of an electrochemical cell or collection of cells (i.e., a battery) in a simple circuit can be correctly understood. The cells simply supply the energy to do work upon the charge to move it from the negative terminal to the positive terminal. By providing energy to the charge, the cell is capable of maintaining an electric potential difference across the two ends of the external circuit. Once the charge has reached the high potential terminal, it will naturally flow through the wires to the low potential terminal in a battery-powered electric circuit, the cells serve the role of a charge pump and supply energy to the charge to lift it back from the low potential position through the cell to the high potential position. As a positive test charge moves through the external circuit, it can pass through a variety of types of circuit elements. Each circuit element serves as an energytransforming device. Light bulbs, motors, and heating elements (such as in 13

22 toasters and hair dryers) are examples of energy-transforming devices. In each of these devices, the electrical potential energy of the charge is transformed into other useful (and non-useful) forms. For instance, in a light bulb, the electric potential energy of the charge is transformed into light energy (a useful form) and thermal energy (a non-useful form). The loss in electric potential while passing through a circuit element is often referred to as a voltage drop. By the time that the positive test charge has returned to the negative terminal, it is at 0 volts and is ready to be re-energized and pumped back up to the high voltage, positive terminal. Check Your Understanding Worksheet Moving an electron within an electric field would change the the electron. a. mass of b. amount of charge on c. potential energy of d. weight of 2. If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to. a. the rate at which water flows through the circuit b. the speed at which water flows through the circuit c. the distance that water flows through the circuit d. the water pressure between the top and bottom of the circuit 3. If the electrical circuit in your Walkman were analogous to a water circuit at a water park, then the battery would be comparable to. a. the obstacles that stand in the path of the moving water b. the pump that moves water from the ground to the elevated positions c. the pipes through which water flows d. the distance that water flows through the circuit 14

23 4. Which of the following is true about the electrical circuit in your flashlight? a. Charge moves around the circuit very fast - nearly as fast as the speed of light. b. The battery generates the charge (electrons) that moves through the wires c. The charge becomes used up as it passes through the light bulb. d. The battery supplies energy that raises charge from low to high voltage. 5. If a battery provides a high voltage, it can. a. do a lot of work over the course of its lifetime b. do a lot of work on each charge it encounters c. push a lot of charge through a circuit d. last a long time The diagram below at the right shows a light bulb connected by wires to the + and - terminals of a car battery. Use the diagram in answering the next four questions. 6. Compared to point D, point A is electric potential. a. 12 V higher in B C b. 12 V lower in c. exactly the same 7. The electric potential energy of a charge is zero at point. A + 12 Volts D 8. Energy is required to force a positive test charge to move. a. through the wire from point A to point B b. through the light bulb from point B to point C c. through the wire from point C to point D d. through the battery from point D to point A 9. The energy required to move +2 C of charge between points D and A is J. a b. 2.0 c. 6.0 d. 12 e

24 10. The following circuit consists of a dry cell and a light bulb. Use >, <, and = symbols to compare the electric potential at A to B and at C to D. Indicate whether the devices add energy to or remove energy from the charge. Add or Remove Enegrgy? C D Add or Remove Enegrgy? A + + B Battery V A VB (>, < or =) V C VD (>, < or =) V B VD (>, < or =) Add or Remove Enegrgy? 11. Use your understanding of the mathematical relationship between work, potential energy, charge and electric potential difference to complete the following statements: a. A 9-volt battery will increase the potential energy of 1 coulomb of charge by joules. b. A 9-volt battery will increase the potential energy of 2 coulombs of charge by joules. c. A 9-volt battery will increase the potential energy of 0.5 coulombs of charge by joules. d. A -volt battery will increase the potential energy of 3 coulombs of charge by 18 joules. e. A -volt battery will increase the potential energy of 2 coulombs of charge by 3 joules. f. A 1.5-volt battery will increase the potential energy of coulombs of charge by 0.75 joules. g. A 12-volt battery will increase the potential energy of coulombs of charge by 6 joules E High Potential Low Potential 16

25 2. Electric Circuit Electric potential is the amount of electric potential energy per unit of charge that would be possessed by a charged object if placed within an electric field at a given location. Electric potential difference is simply the difference in electric potential between two different locations within an electric field. Let us now understand the meaning of an electric circuit. In an electric circuit, charges must continually flow through a complete loop, returning to their original position and cycling through again. If there were a means of moving positive charge from the negative plate back up onto the positive plate, then the movement of positive charge downward through the charge pipe (i.e., the wire) would occur continuously. In such a case, a circuit or loop would be established. Activity 2.1 Learning Objectives: Students will be able to Illustrate the necessity of a closed conducting path for the flow of charges. Observe the effect of connecting and disconnecting a wire. The teacher may provide the students with 1. a battery pack(two to three cells) 2. a torch bulb 3. a few pieces of connecting wires 4. a switch The students may be asked to connect the components and observe that when all connections are made to the battery pack, the light bulb lights. In fact, the lighting of the bulb occurs immediately after the final connection is made. There is no perceivable time delay between when the last connection is made and when the light bulb is perceived to light up. The fact that the light bulb lights and remains lit is evidence that charge is flowing through the light bulb filament and that an electric circuit has been established. A 17

26 circuit is simply a closed loop through which charges can continuously move. To demonstrate that charges are not only moving through the light bulb filament but also through the wires connecting the battery pack and the light bulb, a variation on the above activity is made. A compass is placed beneath the wire at any location such that its needle is placed in alignment with the wire. Once the final connection is made to the battery pack, the light bulb lights and the compass needle deflects. The needle serves as a detector of moving charges within the wire. When it deflects, charges are moving through the wire. And if the wire is disconnected at the battery pack, the light bulb is no longer lit and the compass needle returns to its original orientation. When the light bulb lights, charge is moving through the electrochemical cells of the battery, the wires and the light bulb filaments; the compass needle detects the movement of this charge. It can be said that there is a current - a flow of charge within the circuit. The electric circuit demonstrated by the combination of battery, light bulb and wires consists of two distinct parts: the internal circuit and the external circuit. The part of the circuit containing electrochemical cells of the battery is the internal circuit. The part of the circuit where charge is moving outside the battery pack through the wires and the light bulb is the external circuit. ACTIVITY 2.2 CIRCUIT DIAGRAMS After reading this section students will be able to do the following: 2Explain what circuit diagrams are used for. 2Identify what the symbols in the circuit diagrams stand for. Circuit diagrams are a pictorial way of showing circuits. Electricians and engineers draw circuit diagrams to help them design the actual circuits. Here is an example circuit diagram. V A 18

27 The important thing to note on this diagram is what different symbols stand for. We see that there are straight lines that connect each of the symbols together. These lines represent a wire. A represents an Ammeter. V represents a Voltmeter. represents the resistor. represents a switch. represents a battery. represents a light bulb. Review 1. Circuit diagrams are used to show how all the components connect together to make a circuit. Example 1: Three Dry cells are placed in a battery pack to power a circuit containing three light bulbs connected as shown. We can then draw its circuit diagram by using the schematic symbols shown alongside. Drawing of Circuit Schematic Diagram of Circuit 19

28 Example 2: Three D-cells are placed in a battery pack to power a circuit containing three light bulbs. Drawing of Circuit Schematic Diagram of Circuit Using the verbal description, one can acquire a mental picture of the circuit being described. But this time, the connections of light bulbs is done in a manner such that there is a point on the circuit where the wires branch off from each other. The branching location is referred to as a node. Each light bulb is placed in its own separate branch. These branch wires eventually connect to each other to form a second node. A single wire is used to connect this second node to the negative terminal of the battery. Check Your Understanding Worksheet 2 1. Use circuit symbols to construct schematic diagrams for the following circuits: a. A single cell, light bulb and switch are placed together in a circuit such that the switch can be opened and closed to turn the light bulb on. b. A three-pack of D-cells is placed in a circuit to power a flashlight bulb. c. d. 20

29 2. Use the concept of conventional current to draw an unbroken line on the schematic diagram at the right that indicates the direction of the conventional current. Place an arrowhead on your unbroken line. 3. If an electric circuit could be compared to a water circuit at a water park, then the... Choices:... battery would be analogous to the.... positive terminal of the battery would be analogous to the.... current would be analogous to the.... charge would be analogous to the.... electric potential difference would be analogous to the. A. water pressure B. volume of water flowing down slide per minute C. water D. bottom of the slide E. water pump F. top of the slide 4. Utilize your understanding of the requirements of an electric circuit to state whether charge would flow through the following arrangements of cells, bulbs, wires and switches. If there is no charge flow, then explain why not. a. b. + + Charge Flow: Yes or No? Explanation: Charge Flow: Yes or No? Explanation: 21

30 c. d Charge Flow: Yes or No? Charge Flow: Yes or No? Explanation: Explanation: 5. The diagram shows a light bulb connected to a 12-V car battery. The + and - terminals are shown. B C A + 12 Volts D a. As a + charge moves through the battery from D to A, it (gains, loses) potential energy and (gains, loses) electric potential. The point of highest energy within a battery is the (+, -) terminal. b. As a + charge moves through the external circuit from A to D, it (gains, loses) potential energy and (gains, loses) electric potential. The point of highest energy within the external circuit is closest to the (+, -) terminal. c. Use >, <, and = signs to compare the electric potential (V) at the four points of the circuit. V V V V A B C D 6. In the movie Tango and Cash, Kurt Russell and Sylvester Stallone escape from a prison by jumping off the top of a tall wall through the air and onto a high-voltage power line. Before the jump, Stallone objects to the idea, telling Russell "We're going 22

31 to fry." Russell responds with "You didn't take high school Physics, did you? As long as you're only touching one wire and your feet aren't touching the ground, you don't get electrocuted." Is this a correct statement? 3. Electric Current If the two requirements of an electric circuit are met, then charge will flow through the external circuit. It is said that there is a current - a flow of charge. Using the word current in this context is to simply use it to say that something is happening in the wires - charge is moving. Yet current is a physical quantity that can be measured and expressed numerically. As a physical quantity, current is the rate at which charge flows past a point on a circuit. The current in a circuit can be determined if the quantity of charge Q passing through a cross section of a wire in a time t can be measured. The current is simply the ratio of the quantity of charge and time. Current is a rate quantity and is expressed mathematically as Current = I = Note that the equation above uses the symbol I to represent the quantity current. The standard metric unit for current is the ampere. It is often shortened to Amp and is abbreviated by the unit symbol A. A current of 1 ampere means that there is 1 coulomb of charge passing through a cross section of a wire every 1 second. 1 ampere = 1 coulomb / 1 second I Q t + I I I Electric current in the external circuit is directed from the positive to the negative terminal. 23

32 To test your understanding, determine the current for the following two situations. A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 2mm 20 C 20 C 40 s A 1 mm long cross section of wire is isolated and 2 C of charge is determined to pass through it in 0.5 1mm 2 C 2 C 0.5 s I = ampere I = ampere 3.1 Conventional Current Direction The particles that carry charge through wires in a circuit are mobile electrons. The electric field direction within a circuit is by definition the direction that positive test charges are pushed. Thus, these negatively charged electrons move in the direction opposite the electric field. But while electrons are the charge carriers in metal wires, the charge carriers in other circuits can be positive charges, negative charges or both. In fact, the charge carriers in semiconductors, street lamps and fluorescent lamps are simultaneously both positive and negative charges traveling in opposite directions. Ben Franklin, who conducted extensive scientific studies in both static and current electricity, envisioned positive charges as the carriers of charge. As such, an early convention for the direction of an electric current was established to be in the direction that positive charges would move. The convention has stuck and is still used today. The direction of an electric current is by convention the direction in which a positive charge would move. Thus, the current in the external circuit is directed away from the positive terminal and toward the negative terminal of the battery. Electrons would actually move through the wires in the opposite direction. 24

33 Worksheet 3 Check Your Understanding 1. A current is said to exist whenever. a. a wire is charged b. a battery is present c. electric charges are unbalanced d. electric charges move in a loop 2. Current has a direction. By convention, current is in the direction that. a. + charges move b. - electrons move c. + electrons move d zero charge neutrons move 3. The diagram below depicts a conducting wire. Two cross-sectional areas are located 50 cm apart. Every 2.0 seconds, 10 C of charge flow through each of these areas. The current in this wire is A. 50cm 10 C 10 C 2s a b c d. 1.0 e. 5.0 f. 20g. 10 h. 40 i. none of these 25

34 4. Look at the diagram below and complete the following statements: a. When a current of one ampere How through the bulb located between A and B, there would be a flow of charge at the rate of coulomb per second through this bulb. b. When a charge of 8 C flows past any point along the circuit in 2 seconds, the current is A. c. If 5 C of charge flow past point A (diagram at right) in 10 seconds, then the current is A. d. If the current at point D is 2.0 A, then C of charge flow past point D in 10 seconds. e. If 12 C of charge flow past point A in 3 seconds, then 8 C of charge will flow past point E in seconds. f. True or False: The current at point E is considerably less than the current at point A since charge is being used up in the light bulbs. 4. Journey of a Typical Electron An electrochemical cell supplies energy to move a charge from its low energy, low potential terminal to the high energy, high potential terminal. In this sense, the cell supplies the energy to establish an electric potential difference across the two ends of the external circuit. Charge will then flow through the external circuit in the 26

35 same manner that water will flow from an elevated position to a low position. It is the difference in potential that causes this flow. In the wires of electric circuits, an electron is the actual charge carrier. An electron's path through the external circuit is far from being a straight path. An electron's journey through a circuit can be described as a zigzag path that results from countless collisions with the atoms of the conducting wire. Each collision results in the alteration of the path, thus leading to a zigzag type motion. While the electric potential difference across the two ends of a circuit encourages the flow of charge, it is the collisions of charge carriers with atoms of the wire that discourages the flow of charge. Different types of atoms offer a different degree of hindrance to the flow of the charge carriers that pass through it. In all cases, the collisions of charge carriers in an electric circuit with the conducting elements of that circuit result in a loss of energy. While most the electrical energy possessed by a charge carrier is lost when it passes through an electrical device (often referred to as the load), even the wires of the circuit themselves act to remove energy from a charge. It is because of this energy loss in the load and in the wires themselves that the electric potential of a charge carrier is decreased as it traverses the external circuit. The electric energy supplied by the electrochemical cells becomes entirely used up in the external circuit. In an electric circuit with several electrical devices, there may be multiple stepwise losses of electric potential as the charge traverses the circuit. Regardless of the way in which the devices are wired, the total loss of electric potential of a single charge as it passes through the external circuit is equal to the gain in electric potential that it experiences in the battery. The journey of an electron through an external circuit involves a long and slow zigzag path that is characterized by several successive losses in electric potential. Each loss of potential is referred to as a voltage drop. Accompanying this voltage drop is a voltage boost occurring within the internal circuit - for instance, within the electrochemical cell. 27

36 4.1 Resistance An electron traveling through the wires and loads of the external circuit encounters resistance. Resistance is the hindrance to the flow of charge. For an electron, the journey from terminal to terminal is not a direct route. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance that discourages it. The rate at which charge flows from terminal to terminal is the result of the combined affect of these two quantities. Variables Affecting Electrical Resistance The flow of charge through wires is often compared to the flow of water through pipes. The resistance to the flow of charge in an electric circuit is analogous to the frictional affects between water and the pipe surfaces as well as the resistance offered by obstacles that are present in its path. It is this resistance that hinders the water flow and reduces both its flow rate and its drift speed. Like the resistance to water flow, the total amount of resistance to charge flow within a wire of an electric circuit is affected by some clearly identifiable variables. Activity 4 [on the Net] Learners can perform this activity on their own to understand the effect of length and area of cross section of a wire on its resistance. interactive website First, the total length of the wires will affect the amount of resistance. The longer the wire, the more will be its resistance. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions mean more resistance. Second, the cross-sectional area of the wires will affect the amount of resistance. Wider wires have a greater cross-sectional area. Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe. This can be attributed to the lower amount of resistance that is present in the wider pipe 28

37 A third variable that is known to affect the resistance to charge flow is the material that a wire is made of. Not all materials are created equal in terms of their conductive ability. Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors but is never used in wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with suitable conducting ability to permit their use in wires of household circuits. The conducting ability of a material is often indicated by its resistivity. The resistivity of a material is dependent upon the material's electronic structure and its temperature. For most (but not all) materials, resistivity increases with increasing temperature. The table below lists resistivity values for various materials at temperatures of 20 degrees Celsius. Material Resistivity (ohm meter) -8 Silver 1.59 x 10-8 Copper 1.7 x 10-8 Gold 2.4 x 10-8 Aluminum 2.8 x 10-8 Tungsten 5.6 x 10-8 Iron 10 x 10-8 Platinum 11 x 10-8 Lead 22 x 10-8 Nichrome 150 x 10 5 Carbon 3.5 x Polystyrene Polyethylene Glass Hard Rubber 10 29

38 4.2 Mathematical Expression of Resistance Resistance is a numerical quantity that can be measured and expressed mathematically. The standard metric unit for resistance is the ohm, represented by the Greek letter omega - Ù. An electrical device having a resistance of 5 ohms would be represented as R = 5 Ù. The equation representing the dependency of the resistance (R) of a wire shaped conductor upon the variables that affect it is R = p L A where L represents the length of the wire (in meters), A represents the cross-sectional area of the wire (in meters2), and represents the resistivity of the material (in ohmometer). This equation shows that the resistance of a wire is directly proportional to the length of the wire and inversely proportional to the cross-sectional area of the wire. Knowing the length, cross-sectional area and the material that a wire is made of (and thus, its resistivity) one can determine the resistance of the wire. Check Your Understanding Worksheet 4 1. Household circuits are often wired with two different thickness of wires: 12-gauge and 14-gauge. The 12-gauge wire has a diameter of 2.1 mm while the 14-gauge wire has a diameter of 1.8 mm. Thus, 12-gauge wire has a wider cross section than 14- gauge wire. A 20-Amp circuit used for wall receptacles should be wired using 12- gauge wire and a 15-Amp circuit used for lighting and fan circuits should be wired using 14-gauge wire. Explain the physics behind such an electrical code. 2. Based on the information stated in the above question, explain the risk involved in using 14-gauge wire in a circuit that will be used to power an 16-ampere power saw. 3. Determine the resistance of a 1 Km length of 12-gauge copper wire of diameter 2.1 mm. 4. Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the diameter of wire B than wire A? 30

39 Student Teacher Material 5. Ohm's Law Ohm's Law gives the relation between the potential difference across the two terminals of a conductor and the current flowing through it. It states that the potential difference across the two terminals of a conductor is directly proportional to the current passing through it. V I V = RI where R(resistance) is the proportionality constant A very well known equation which pervades the study of electric circuits, is the equation V = I R In words, the electric potential difference between two points on a circuit (V) is equivalent to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R). Often referred to as the Ohm's law equation, this equation is a powerful predictor of the relationship between potential difference, current and resistance. Ohm's Law as a Predictor of Current The Ohm's law equation can be rearranged and expressed as I = V R This equation can be used for, calculating the current if the electric potential difference and the resistance are known. This equation indicates the two variables that would affect the amount of current in a circuit. The current in a circuit is directly proportional to the electric potential difference impressed across its ends and inversely proportional to the total resistance offered by the external circuit. The greater the battery voltage (i.e., electric potential difference), the greater the current. And the greater the resistance, the less the current. The table below illustrates this relationship both qualitatively and quantitatively for several circuits with varying battery voltages and resistances. 31

40 Circuit Battery Total Current Diagram Voltage Resistance (Amps) (V) (? ) V 3? 0.50 Amp V 3? 1 Amp V 6? 0.25 Amp V 6? 0.5 Amp Because the current in a circuit is affected by the resistance, resistors are often used in the circuits of electrical appliances to affect the amount of current that is present in its various components. By increasing or decreasing the amount of resistance in a particular branch of the circuit, one can increase or decrease the amount of current in that branch. Kitchen appliances such as electric mixers and light dimmer switches operate by altering the current at the load by increasing or decreasing the resistance of the circuit. The diagram below depicts a couple of circuits containing a voltage source (battery pack), a resistor (light bulb) and an ammeter (for measuring current). In which circuit does the light bulb have the greatest resistance? 32

41 Ohm's law can be verified in the laboratory using a resistor, a battery pack, an ammeter, and a voltmeter. An ammeter is a device used to measure the current at a given location. A voltmeter is a device that can be touched to two points on a circuit to determine the electric potential difference across those points. By altering the number of cells in the battery pack, the electric potential difference across the external circuit can be varied. The voltmeter can be used to determine this potential difference and the ammeter can be used to determine the current associated with this?v. The process can be repeated several times to yield a set of I-?V data. A plot of I versus?v is seen to be a straight line with a slope that is equivalent to the reciprocal of the resistance of the resistor. Ohm's law equation thus gets checked. Activity 5 Verification of Ohm's Law Learning Objectives: The learners will be able 2to perform an experiment to verify Ohm's Law 2to practice constructing electric circuits 2to practice using an ammeter and a voltmeter The teacher may give the instructions before hand. Discussion: In the lab, you will construct a simple circuit using a single known resistance, R. Then you will use an ammeter to measure the current, I, through the resistance and a voltmeter to measure the potential difference, V, across the resistance. With this data, you can check the validity of Ohm's Law (V = IR) in the circuit. Equipment: 1.5 or 2 V power supply switch resistor 0-1 A ammeter 0-3 V voltmeter connecting wires 33

42 1.5 or 2 V DC Procedure: IMPORTANT: In this lab you will use ONLY the "COMMON" and "1.5 or 2 V DC" terminals on the power supply. Connecting the circuit to any other terminals will certainly result in destruction of equipment and might well be risky for the user. Set up the circuit shown in the diagram shown using the resistor of known value. a. Screw one end of the resistor to the 1.5/2 VDC terminal of the power supply. b. Using one of the connecting wires, connect the other end of the resistor to the red terminal of the ammeter. c. Be sure that the switch is open. d. Using another wire, connect the black terminal of the ammeter to either side of the switch. Notice that when the switch is closed, current will flow through the resistor, the ammeter, and the switch in this circuit. e. Connect the other switch terminal to the COMMON terminal on the power supply using a wire. f. Connect a wire from the red terminal of the voltmeter to the 1.5/2 VDC terminal of the power supply. g. Connect a wire from the black terminal of the voltmeter to the red terminal of the ammeter. After the completion of the circuit carefully read the voltage across the resistor and the current through the resistor and record the readings in a tabular form. Repeat the experiment for different values of voltage. 34

43 Results: For each set of values of the voltage drop and the current, the calculated resistance remains constant, hence verifying Ohm's law. Check Your Understanding 1. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. a. decrease the voltage b. decrease the resistance c. increase the voltage d. increase the resistance 2. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply. a. increase the voltage of the battery by adding another cell b. decrease the voltage of the battery ( by removing a cell) c. decrease the resistance of the circuit d. increase the resistance of the circuit 3. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause. a. the voltage of the circuit to be higher b. the voltage of the circuit to be lower c. your resistance to be higher d. your resistance to be lower Worksheet 5 35

44 4. If the resistance of a circuit were increased three times, then the current through the circuit would be. a. one-third as much b. three times as much c. unchanged 5. If the voltage across a circuit is increased four times, then the current through the circuit would be. a. one-fourth as much b. four times as much c. unchanged 6. A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 ma (milliamps). Determine the new current if the voltage of the power supply was... a.... increased by a factor of 2 and the resistance was held constant. b.... increased by a factor of 3 and the resistance was held constant. c.... decreased by a factor of 2 and the resistance was held constant. d.... held constant and the resistance was increased by a factor of 2. e.... held constant and the resistance was increased by a factor of 4. f.... held constant and the resistance was decreased by a factor of 2. g.... increased by a factor of 2 and the resistance was increased by a factor of 2. h.... increased by a factor of 3 and the resistance was decreased by a factor of 2. i.... decreased by a factor of 2 and the resistance was increased by a factor of Use Ohm's law equation to provide numerical answers to the following questions: a. An electrical device with a resistance of 3.0? will allow a current of 4.0 amps to flow through it if a voltage drop of Volts is impressed across the device. 36

45 b. When a voltage of 120 V is maintained across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is?. c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60? will have a current of Amps. 8. Use Ohm's law equation to determine the missing values in the following circuits. 9. Refer to question 8 above. In the circuits of diagrams A and B, what method was used to control the current in the circuits? And in the circuits of diagrams C and D, what method was used to control the current in the circuits? 37

46 6. THE SERIES CIRCUIT ACTIVITY 6 After reading this section, students will be able to do the following: 2Define a series circuit, and list the components needed to make it. 2Construct a simple and complex series circuit. 2Define what a load is. Try building this simple series circuit The circuit we see above is something called a series circuit. This is called a series circuit because there is only one path for the electrons to take between any two points in this circuit. In other words, the components, which are the battery, the switch, the ammeter, and light, are all in "series" with each other. Notice that when we close the switch to complete the electrical circuit, the electrons start moving and the ammeter indicates that there is current flowing in this circuit. Also notice that the light bulb begins to glow. Load defined The light bulb is considered a load in this circuit. We might think of a load as anything that is using the energy that is being delivered by the electric current in a circuit. It could be anything from a light bulb to a computer to a washing machine and so on. Try building a series circuit with resistors Let's build another series circuit, but this time we will use some resistors instead of a light bulb. Resistors are components that are used to control that amount of current flowing in a circuit. If there are no resistors to control the flow of electrical current, too much current may flow through the circuit and damage its components or wires. 38

47 Characteristics of 'SERIES CIRCUITS In a Series Circuit the charge carriers [electrons] have only one path to flow. Hence the electrons must go through each component to complete the flow. When the loads are placed in series, we would observe the following Review 1. An open in the circuit will disable the entire circuit. 2. The voltage gets divides (shared) between the loads. 3. The current flow is the same throughout the circuit. 4. The resistance of each load can be different. 1. When all the components are in line with each other and the wires, a series circuit is formed. 2. A load is any device in a circuit that is using the energy that the electron current is delivering to it. 6.1 SERIES CIRCUIT CALCULATIONS If, for example, two or more lamps (resistances R and R, etc.) are connected in a 1 2 circuit as follows, there is only one route that the current can take. This type of connection is called a series connection. The value of current I is always the same at any point in a series circuit. The combined resistance RO in this circuit is equal to the sum of individual resistance R 1 39

48 and R. In other words: The total resistance(ro) is equal to the sum of all resistances 2 (R + R + R +...) R = R + R Therefore, the strength of current (I) flowing in the circuit can be found as follows: Resistance R0 (a combination of resistances R1 and R2, which are connected in series in the circuit as illustrated) and current I flowing in this circuit can be determined as follows: 40

49 7. THE PARALLEL CIRCUIT ACTIVITY 7 After reading this section, students will be able to do the following: 2Define a parallel circuit and explain how it compares to a series circuit. 2Construct a parallel circuit. 2Explain what a voltmeter does and how it is different from an ammeter. Like the series circuit, parallel circuits also contain a voltage (current) source as well as wires and other components. The main difference between a series circuit and a parallel circuit is in the way the components are connected. In a parallel circuit the electricity has several paths that it can travel. Try building this simple parallel circuit Notice that when you closed the switch, the electrons flowed through both loads at the same time. In our series circuit, all the electrons flowed through all the components in order. With the parallel circuit, some electrons go through one load and some go through the other load, all at the same time. At point A, the total current splits up and takes different paths before the circuit joins back together again at point B. A parallel circuit exists whenever two or more components are connected between the same two points. Those two points in this circuit are points A and B. Both resistors connect to both points A and B. Each parallel path is called a branch of the parallel circuit. We will now try building this parallel circuit, including a voltmeter 41

50 The parallel circuit shown here, contains 3 branches (two resistors and a voltmeter), which means the electron current goes through all three branches at the same time. We put a voltmeter on this second circuit to show an important fact. In the last 4 circuits we made, we included an ammeter into them. Ammeters must always be placed in series in a circuit, otherwise they will not work. The voltmeter we added in the last circuit has a different requirement in order to work. Voltmeters must be placed in parallel with the circuit in order to work. This is because voltage meters measure the difference in potential from one point to another. CHARACTERISTICS OF PARALLEL CIRCUITS A Parallel Circuit has multiple paths or branches to ground. Therefore: 1. In the event of an open in the circuit in one of the branches, current will continue to flow through the remaining. 2. Each branch receives the same source voltage. 3. Current flow through each branch can be different. 4. The resistance of each branch can be different. 42

51 Review 1. When some of the components are connected parallel with each other, they form a parallel circuit. 2. A voltmeter must be wired in parallel in a circuit in order to measure the difference in potential from one point to another. 7.1 PARALLEL CIRCUIT CALCULATION In parallel connection, two or more resistances (R1, R2, etc.) are connected in a circuit as follows, with one end of each resistance connected to the high (positive) side of the circuit, and one end connected to the low (negative) side. Full battery voltage is applied to all resistances within a circuit having a parallel connection. Resistance R (a combination of resistances R1 and R2) in a parallel connection can be 0 determined as follows: 43

52 From the above, the total current I flowing in this circuit can be determined from Ohm's law as follows: The total current I is also equal to the sum of currents I1 and I2 flowing through individual resistances R1 and R2 I = I + I 1 2 Since battery voltage V is applied equally to all resistances, the strength of currents I1 and I2 can be determined from Ohm's law as follows: Resistance R (a combination of resistances R1 and R2, which are connected in parallel 0 in the circuit as shown below), the total current I flowing in the circuit, and currents I1 andi2 flowing through resistances R1 and R2, can be determined respectively as follows: 44

53 8. THE SERIES/PARALLEL CIRCUIT ACTIVITY 8 After reading this section, students will be able to do the following: 2Explain what a series/parallel circuit is and what components are needed to complete it. 2Construct a series/parallel circuit. When we have a circuit in which some of the components are in series and others are in parallel, we have a series/parallel circuit. Try building a series/parallel circuit 45

54 Notice in this series/parallel circuit that the resistors R1, the switch, the battery, and the ammeter are in series with each other while resistors R2 and R3 are in parallel with each other. We might also say that the R2/R3 combination is in series with the rest of the components in this circuit. By applying Ohm's law, to a given series or parallel or a combination of the two circuits, we can calculate the current flowing at any point in such circuits. The combined resistance R02 in this series-parallel connection can be determined in the following order: a. Determine combined resistance R01, which is a combination of resistances R2 and R3 connected in parallel. b. Then, determine resistance R02, which is a combination of resistance R1 and combined resistance R01 connected in series. Total current I flowing in the circuit can be determined from Ohm's law as follows: The voltage applied to R2 and R3 can be found by the following formula: 8.1 SERIES PARALLEL CALCULATIONS Currents I1, I2 and I flowing through resistances R1, R2 and R3 in the series-parallel connection, as shown below, can be determined as follows: 46

55 NOTE: The Powerpoint Presentation attached here can be used for recapitulating the concepts dealt till this section of the chapter. 47

56 9. Heating effect of current ACTIVITY 9 Class experiment Illustrates two ideas: electric current causes heating effect; temperature affects the resistance of a wire. Apparatus and materials For each student group: Safety 2Cells, 1.5 V, with holders, 3 2Lamp with holder 2Crocodile clips, 2 2Ammeter (0-1 amp), DC 2Leads, 4 mm, 5 2Eureka wire 34 SWG, 15 cm length Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can Procedure 48

57 a b c Set up a series circuit of three cells and a lamp. Include two crocodile clips in the circuit. Wind the length of bare Eureka wire into a coil (using, say, a pencil). Clamp the ends of the wire into the two crocodile clips. (Make sure that the turns of wire do not touch each other.) Stand back! Carefully, hold your hand above the wire coil. Can you feel hot air rising? Teaching notes 1 When an electric current passes through a metal, it warms up. The open coil of wire will be warm to the touch. Blowing on the wire will reduce its temperature and the lamp will glow brighter. Try using an electronics freezer spray to reduce the temperature of the coil even more and the lamp will glow brighter still. 2 It is important that the coils must not touch each other, or the coil will become a short circuit. 3 This experiment can be demonstrated in order to explain how a filament lamp works. The filament is just a short piece of wire which gets so hot that it glows red for low currents, becoming whiter as the current increases staying within its permitted [safe] limits, of course. 4 Careful and observant students are likely to have noticed that when a circuit consisting of a cell, a lamp and an ammeter is connected, the current is momentarily greater when the connection is made, and then the current settles down to a steady lower value. This is because the resistance of the cold wire is less than the resistance of the hot wire. 9.1 Applications of 'Heating Effect of Current' Heating effect of electricity is one of the widely used effects in the world. When electric current is passed through a conductor, it generates heat due to the resistance it offers to the current flow. The work done in overcoming the resistance is generated as heat. This is studied by James Prescott Joule and he enunciated various factors that affect the heat generated. The heat produced by a heating element is directly proportional to the square of the electric current (I) passing through the conductor, directly proportional to the resistance (R) of the conductor, time (t) for which current passes through the conductor. It is given by the expression H = I2Rt and is well known as Joule's Law. 49

58 Applications of the heating effect of electric current include appliances like electric immersion water heater, electric iron box, etc. All of these have a heating element in it. Heating elements are generally made of specific alloys like, nichrome, manganin, constantan etc. A good heating element has high resistivity and high melting point. An electric fuse is an example for the application of heating effect of electric current. The rating of 3 A of an electric fuse implies the maximum current it can sustain is three ampere. Joule s law of Heating When some potential difference V is applied across a resistance R then the work done by the electric field on charge q to flow through the circuit in time t will be This work appears as thermal energy in the resistor. Heat produced by the resistance R is quite often expressed in Calorie. Since 1 calorie = 4.2 J we have 10. Electric Power The rate at which electrical energy is dissipated into other forms of energy is called electric power i.e. Units: It's S.I. unit is joule/sec or watt Bigger S.I. units are KW, MW and horse power [HP], remember that 1 HP = 746 Watt Rating values On electrical appliances (Bulbs, Heater, Geyser etc). wattage, voltage, etc. are printed. These are called its rating values. If suppose we have a bulb of 40 W, 220 V then rated power (PR) = 40 W while rated voltage (VR) = 220 V. 50

59 Resistance of electrical appliance If variation of resistance with temperature is neglected then resistance of any electrical appliance can be calculated from its rated power and rated voltage by using Power consumed V R= P An electrical appliance (Bulb, heater, etc.) consume rated power (PR) only if applied voltage (VA) is equal to rated voltage (VR) i.e. If VA = VR Z R R So So Long distance power transmission When power is transmitted through a power line of resistance R, power-loss will be i2r Now if the power P is transmitted at voltage V then P = VI, i.e. i = (P / V) So, Power loss 2 Now as for a given power and line, P and R are constant. Hence Power loss (1/V ) So if power is transmitted at high voltage, power loss will be small and vice-versa. This is why long distance power transmission is carried out at high voltage Electricity Consumption 1. The price of electricity consumed is calculated on the basis of electrical energy consumed and not on the basis of electrical power. 2. The unit joule for energy is very small. Hence a big practical unit is known as kilowatt hour (KWH) or board of trade unit (B.T.U.) is used as the practical commercial unit 51

60 3. 1 kwh or 1 commercial unit is the quantity of electrical energy which gets dissipated in one hour in an electrical circuit when the electrical power in the circuit is 1 kw thus. 4. A simple formulae to calculate the no. of consumed units is Solved example 1: Two heater wires of equal length are first connected in series and then in parallel. The ratio of heat produced in the two cases is (A) 2 : 1 (B) 1 : 2 (C) 4 :1 (D) 1 : 4 Solution: (D) Power consumed means heat produced. For constant potential difference Pconsumed = Heat Solved example 2: A wire when connected to 220 V mains supply has power dissipation P1. Now the wire is cut into two equal pieces which are connected in parallel to the same supply. Power dissipation in this case is P2. Then P2 : P1 is (A) 1 (B) 4 (C) 2 (D) 3 Solution: (B) When wire is cut into two equal parts then power dissipated by each part is 2P 1 So their parallel combination will dissipate power P = 2P + 2P = 4P, which gives

61 POST CONTENT Revision Worksheet 1 1. The main cause of resistance to the flow of charge within an electrical wire is. a. mobile charge carriers collide with atoms of the resistor b. mobile charge carriers have mass (possess inertia) which resists their motion c. the electric field which causes charge flow diminishes with distance d. charge is consumed or used up as it flows through the wire 2. Resistance is quantifiable - that is, it can be measured and calculated. The standard metric unit usedto express the amount of electrical resistance is the. a. Joule b. Watt c. Volt d. Ohm 3. For the following pairs of wire descriptions, choose the wire which has the greatest resistance.resistance to charge flow will be greatest in. (Circle the best answer.) a. a wire which is thin a wire which is thick b. a wire which is long a wire which is short c. a wire which is made of copper a wire which is made of plastic d. a wire which is made of copper a wire which is made of silver 4. The rate at which charge flows through a circuit is to the resistance. a. inversely related b. directly related c. not related 5. For the following pairs of circuit descriptions, choose the circuit which has the greatest current.given that all other factors are equal, the current will be greatest in a circuit which has. a. a high resistance a low resistance b. wires which are long wires which are short c. wires which are wide wires which are thin d. 12-gauge wires (2.1 mm diameter) 14-gauge wires (1.8 mm diameter) e. copper wiring silver wiring 53

62 6. Resistance is not the only variable which effects the current in an electric circuit. The current is also affected by the electric potential difference (? V) applied across its ends. The electric potential difference is simply the battery voltage. As the battery voltage is increased, the current is (increased, decreased). 7. A circuit is set up such that it has a current of 8.0 amps. What would be the new current if a. the resistance (R) is increased by a factor of 2? b. the resistance (R) is increased by a factor of 4? c. the resistance (R) is decreased by a factor of 3? d. the battery voltage (V) is increased by a factor of 3? e. the battery voltage (V) is decreased by a factor of 2? f. the resistance (R) is increased by a factor of 2 and the battery voltage (V) is decreased by a factor of 2? g. the resistance (R) is decreased by a factor of 4 and the battery voltage (V) is increased by a factor of 3? 8. Express your understanding of the use of the I = V / R equation by filling in the following blanks. a. An electrical device with a resistance of 2.0? has an electric potential difference of 6.0 V impressed across it; the current in the device is amperes. b. An electrical device with a resistance of 3.0? has an electric potential difference of V impressed across it; the current in the device is 4.0 amperes. c. An electrical device with a resistance of? has an electric potential difference of 120 V impressed across it; the current in the device is 6.0 amperes. 9. Resistors are electrical devices designed to have a specific resistance. They are inserted in circuits to modify the actual current flowing through the circuit. Which of the resistors in the two circuits (A or B) has the greatest resistance? Calculate the value. 54

63 10. Use arrows to show the direction of conventional current flow through the following circuits and use the I =?V / R equation to fill in the blanks. Question 1: For a given amount of water pressure, which will flow a greater rate of water: a small (restrictive) nozzle or a large (unrestrictive) nozzle? Explain how this relates to the study of voltage, current, and resistance in a simple electric circuit. Question 2: Revision Worksheet 2 Suppose you were to build this circuit and take measurements of current through the resistor and voltage across the resistor: 55

64 Recording these numerical values in a table, the results look something like this: Current Voltage 0.22 A 0.66 V 0.47 A 1.42 V 0.85 A 2.54 V 1.05 A 3.16 V 1.50 A 4.51 V 1.80 A 5.41 V 2.00 A 5.99 V 2.51 A 7.49 V Plot these figures on the following graph: 56

65 What mathematical relationship do you see between voltage and current in this simple circuit? Question 3: Explain, step by step, how to calculate the amount of current (I) that will flow through the resistor in this circuit: Question 4: Plot the relationships between voltage and current for resistors of three different values (1?, 2?, and 3? ), all on the same graph: What pattern do you see represented by your three plots? What relationship is there between the amount of resistance and the nature of the voltage/current function as it appears on the graph? 57

66 Question 5: What is the value of this resistor, in ohms? Question 6: A common saying about electricity is that ït always takes the path of least resistance." Explain how this proverb relates to the following circuit, where electric current from the battery encounters two alternate paths, one being less resistive than the other: Question 10: What would happen if a wire having no resistance at all (0? ) were connected directly across the terminals of a 6-volt battery? How much current would result, according to Ohm's Law? Suppose we were to short-circuit a 6-volt battery in the manner just described and measure 8 amps of current. Why don't the calculated figures from the previous paragraph agree with the actual measurement? Question 11: Shunt resistors can be used as a current-measuring device, By measuring the amount of voltage dropped across a shunt resistor, one can determine the amount of current going through it: 58

67 Suppose that a shunt resistance is labeled with the following rating: 75 A, 25 mv. What is the resistance of this shunt, in ohms? Question 12: One of the fundamental equations used in electricity and electronics is Ohm's Law: the relationship between voltage (E or V, measured in units of volts), current (I, measured in units of amperes), and resistance (R, measured in units of ohms): Where, V = IR I = V R R = V I V = Voltage in units of volts (V) I = Current in units of amps (A) R = Resistance in units of ohms (? ) Solve for the unknown quantity (V, I, or R) given the other two I = 20 ma, R = 5 k? ; V = I = 150? A, R = 47 k? ; V = V = 24 V, R = 3.3 M? ; I = V = 7.2 kv, R = 900? ; I = 59