ELECTRICITY. Chapter ELECTRIC CHARGE & FORCE

Transcription

1 ELECTRICITY Chapter ELECTRIC CHARGE & FORCE Essential Questions: What are the different kinds of electric charge? How do materials become charged when rubbed together? What force is responsible for most everyday forces? Electric Charge Video Clip 1

2 If your car gets struck by lightning in a thunderstorm, will you be safe. Why? Answer: You will be safe because your cars metal chassis acts like a Faraday Cage. The charged particles travel around the outside of the car and into the ground. Video 2

3 Copyright 2010 Ryan P. Murphy What would life be like without it electricity? Copyright 2010 Ryan P. Murphy 3

4 Much different than it is for most of us. ELECTRIC CHARGE Electricity is a variety of independent science concepts all with one single name. Copyright 2010 Ryan P. Murphy 4

5 ELECTRIC CHARGE Electricity is related to charges, and both electrons (-) and protons (+) carry a charge. Electric charge is an electrical property of matter. All matter is made of atoms that contain electrons, neutrons, and protons An object can have a negative charge, positive charge or no charge. Copyright 2010 Ryan P. Murphy ELECTRIC CHARGE Charge of proton Positive Charge of electron Negative Charge of neutron NONE Atoms have no charge because the charges of the protons and electrons cancel each other out. Atoms become charged by gaining or losing electrons 5

6 Lightning is a big spark that occurs when electrons move from one place to another very quickly because of the unequal distribution of electrons. Copyright 2010 Ryan P. Murphy ELECTRIC CHARGE Charges in objects can produce a force between the objects Objects are forced together or attracted when their charges are different Opposite charges attract Same electric charges they push apart Like charges repel Like charges REPEL Opposites attract! 6

7 Like Charges Repel Opposite Charges Attract Which one is right and which is wrong? Answer: Now they are both right. Answer: They are both wrong. Copyright 2010 Ryan P. Murphy 7

8 ELECTRIC CHARGE Electrons can be moved around When different materials are rubbed together, electrons can be transferred from one material to the other. 8

9 ELECTRIC CHARGE The SI Unit of electric charge is the coulomb (C). STATIC ELECTRICITY Static electricity- buildup of excess negative charge on an object Excess electrons on an object Very short electric discharge Static electricity is electricity at rest Friction can cause it Law of conservation of charge- charge may be transferred from object to object, but it cannot be created or destroyed Opposite Charges attract, and like charge repel 9

10 STATIC ELECTRICITY Why you end up with a static shock after walking across the carpet: Atoms in the carpet hold their electrons more loosely than atoms in your shoes Shoes gain electrons from the carpet, becoming negatively charged Carpet loses electrons & becomes positively charged Shock occurs when electrons are suddenly transferred from one object to another- this appears as a spark TRANSFER OF ELECTRIC CHARGE Different materials are rubbed together Electrons can be transferred from one material to the other The direction depends on the material Conduction: Electrons move more easily through conductors, like metals Metals conduct well because: atoms in metals have electrons that move easily through the material 10

11 TRANSFER OF ELECTRIC CHARGE Electrical conductors, such as copper wiring, allow electrons to move freely. TRANSFER OF ELECTRIC CHARGE Insulation: Insulator- a material that doesn t allow electrons to move through it easily Occurs because electrons are held tightly to the atoms in insulating materials like wood, plastic, glass Charging by contact- the process of transferring charge by touching or rubbing two objects together Charging by friction- rearrangement of electrons on a neutral object by a nearby charged object 11

12 We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you can not build up as big a static charge. ELECTROSCOPE Used to detect static electricity Electrons are transferred to the metal ball and down to the foil Foil becomes negative and repels 12

13 ELECTROSCOPE No Chargeleaves hang straight down INDUCTION Rod with negative charge 13

14 INDUCTION Rod with negative charge Pushes electrons in electroscope down INDUCTION Rod with negative charge Pushes electrons in electroscope down Extra negative charge 14

15 INDUCTION Rod with negative charge Pushes electrons in electroscope down Extra negative charge Leaves move apart Induction Remove rod everything returns 15

16 Conduction Rod with negative charge Conduction Rod with negative charge 16

17 Conduction Rod with negative charge Transfers electrons Conduction Rod with negative charge Transfers electrons Extra negative charge 17

18 Conduction Rod with negative charge Transfers electrons Extra negative charge Moves leaves apart. Conduction Remove rod leaves stay apart. 18

19 Electric Force Electric force: the force of attraction or repulsion on a charged particle that is due to an electric field. Electric Force Electric force depends on charge and distance. Electric force acts through a field 19

20 Electric Force Electric field: the space around a charged object in which another charged object experiences an electric force. Electric fields surround charged objects. Any charged object that enters a region with an electric field experiences an electric force. Electric field lines never cross each other Electric fields 20

21 SECTION 2: CURRENT VOLTAGE & CURRENT An electric charge has electrical potential energy that depends on its position in an electric field. A negative charge that is close to another negative charge has the potential to move away. The 2 negative charges repel each other as a result of their electric fields being squished together. 21

22 ELECTRICAL POTENTIAL ENERGY The electrical potential energy between two negative charges decreases as the distance between them increases. Voltage: difference in energy per unit charge as the charge moves between two points in the path of a circuit Higher voltage, the more work the electrons can do. Voltage can vary 22

23 VOLTAGE AND CURRENT It is more practical to consider potential difference than electrical potential energy. Potential difference is the voltage difference in potential between two points in a circuit. Also called voltage This change occurs as a charge moves from one place to another in an electric field. SI Unit for Potential difference is Volts (V) Example: a AA battery has a potential difference of 1.5 V between the two ends (the positive and negative terminals). VOLTAGE AND CURRENT There is a Voltage across the terminals of a battery. The potential difference or voltage across the two ends, or terminals, of a battery ranges from about 1.5 V for a small battery to about 12 V for a car battery. 23

24 VOLTAGE AND CURRENT Most batteries are Electrochemical Cells (or groups of connected cells) that convert chemical energy into electrical energy. VOLTAGE AND CURRENT Electrochemical cells contain Electrolyte- a solution that conducts electricity, two electrodes- each a different conducting material. 24

25 THE CELL WHAT IS A CURRENT? Current: the rate at which charge passes a given point A flow of electrons, or individual negative charges. Made by electrons moving in a wire 25

26 ELECTRIC CURRENT Electric current- The flow of electrons through a wire or any conductor. Measured in units of Amperes (A) Different from static electricity because it lasts longer Charges flow from High voltage to Low voltage Voltage difference- push that causes charges to move For charges to flow, the wire must always be connected in a closed path, or circuit ELECTRIC CURRENT Ampere: A measure of how much current moves through a wire in one second. The larger the size of wire, the greater the ampere capacity. Copyright 2010 Ryan P. Murphy 26

27 Where do your see these plugs? Why are they larger? Answer: The Plug to a dryer or stove is much thicker than a standard outlet to account for extra amps. Copyright 2010 Ryan P. Murphy ELECTRIC CURRENT There are two main kinds of electric current, Direct current (DC) Alternating current (AC). Explains how current gets moved 27

28 ELECTRIC CURRENT DIRECT CURRENT (DC) Direct current is a flow of charge always in one direction. From batteries ALTERNATING CURRENT (AC) -Alternating current is a flow of charge back and forth, changing its direction many times in one second. (Plugs and outlets / household) Advantages of AC Voltage can be raised or lowered More efficient over long distances From Generators Used in your home ELECTRIC CURRENT Transformers change AC to DC 28

29 Is this (AC) Alternating Current, or (DC) Direct Current? AC DC AC? AC DC AC ELECTRICAL RESISTANCE Resistance- the tendency for a material to oppose the flow of electrons, changing electrical energy into Thermal energy and light Opposition to the flow of charge All materials have some electrical resistance. Making wires thinner, longer, or hotter increases the resistance As resistance increases current decreases!!!! Measured in ohms: omega 29

30 OHM S LAW Ohm s law states that the current in a circuit is equal to the voltage divided by the resistance I = V R The relationship among current, voltage, and resistance. I = Current V = Voltage R = Resistance Measured as Amperes (A) Volts ( V) Ohm (Ω) I V R PRACTICE PROBLEMS: RESISTANCE 1. A car has a 12 volt system. The headlights are on a 10 amps circuit. How much resistance do they have? V= 12 volts I= 10 amps R=? R= V/I R= 12 volt/10 amp R= 1.2 Ohm (Ω) 2. Your house uses 120 volts. What amount of current would flow through a 20 ohm resistor? V= 120 volts I=? R= 20 ohm I= V/R I= 120 volts/20 ohm I= 6 amps 30

31 PRACTICE PROBLEMS: RESISTANCE 3. A refrigerator s circuit has a current equal to A in it when the voltage across the circuit equals 116 V. What is the resistance of the circuit? V= 116 volts I= amps R=? R= V/I R= 116 volts/0.647 amps R= 179 ohms 4. The resistance of a wire in a hair dryer is 7.7 Ω. If the current through the wire equals 15.6 A, what is the voltage across the terminals of the hair dryer? V=? I= 15.6 amps R= 7.7 Ω V= I x R V= 15.6 amps x 7.7 Ω V= 120 Volts ELECTRICAL RESISTANCE Conductors have low resistances. Insulators have high resistances. Semiconductors conduct under certain conditions. materials that have electrical properties between those of insulators and conductors Some materials can become superconductors. Some metals and compounds have zero resistance when their temperature falls below the critical temperature. Once a current is established in a superconductor, the current continues even if the applied voltage is removed. Superconductor clip 31

32 SECTION 3: CIRCUITS 32

33 CIRCUITS For current to flow there must be a complete loop Electric circuit: complete, a closed path through which electrons travel. Electrons flow from negative to positive terminal Work is done if there is a resistance in the wire. CIRCUITS Open Circuit- if any part of the circuit is disconnected, no current flows Example- old style Christmas lights Closed Circuit- A closed-loop path for electrons to flow through, creating a current. Closed Open 33

34 CIRCUITS Resistance is supplied by a resistor. A resistor is a device that uses electric energy to do work. A wire connected from the resistor to the positive terminal completes the circuit. An open switch breaks the circuit. DRAWING A SCHEMATIC DIAGRAM A schematic diagram is a model of an electric circuit with standard symbols for the electrical devices. 34

35 SCHEMATIC DIAGRAM SYMBOLS TWO TYPES OF CIRCUITS Series circuits: A circuit with only one path. All the resistors in a series circuit lie along a single path. The amount of current in a series circuit is the same at all parts of the circuit. Resistance in the circuit changes if resistors are added or taken away. 35

36 Series Circuits Series Circuits 36

37 Series Circuits Series Circuits Break in the wire turns off all the lights 37

38 TWO TYPES OF CIRCUITS Parallel circuits: The electrons in a parallel circuit can travel through more than one path, each path is separate. If there s a break in one path in the circuit, electrons can still flow through the other paths and maintain a complete circuit. Parallel circuits in your home allow each light or appliance to use the amount of current it needs to work. A parallel circuit prevents all the lights or appliances from shutting off when one of them stops working. Parallel Circuit 38

39 Parallel Circuit Series vs. Parellel Circuit Video ELECTRIC POWER Electric Power is the rate at which electrical energy is used in a circuit. When a charge moves in a circuit, it loses energy. This energy is transformed into useful work (like turning a motor) and is lost as heat in the circuit. 39

40 ELECTRIC POWER Power: The rate at which electricity does work or provides energy The amount of electric power a device uses to do work is determined by its resistance. P = V x I P V I (P) power = (V) voltage x (I) current in the circuit. Electrical power is expressed in watts (W) P (watts) = V (volts) x I (amperes) PRACTICE PROBLEMS: ELECTRICAL POWER 1. An electric mixer draws W of power. If the mixer is plugged into an outlet across a voltage of 115 V, what current is in the mixer s circuit? P= Watts V=115 Volts I=? I=P/V I=200.0 Watts/115 Volts I= 1.7 amps 2. A nightlight uses 4.00 W of power when plugged into an outlet. Assume that the only resistance in the circuit is provided by the light bulb s filament. The current in the circuit is 3.40 x 10-2 A. What is the voltage across the filament? P= 4.00 Watts V=? I= 3.40 x 10-2 A V=P/I V=4.00 Watts/ 3.40 x 10-2 A V=117 volts 40

41 PRACTICE PROBLEMS: ELECTRICAL POWER 3. A current of 5.83 A is used to produce the microwave radiation in a microwave oven. If the voltage across the oven is 120 V, how much power does the use? P=? V=120 Volts I= 5.83 A P= V x I P= 120 V x 5.83 A P= watts 4. A refrigerators uses a current of 0.62 A and a voltage of 116 V. How much power does the refrigerator use? P=? V=116 Volts I= 0.62 A P= V x I P= 116 V x 0.62 A P= watts ELECTRIC SAFETY Fuses and circuit breakers protect against overloaded circuits. Fuses- contain a small piece of metal that melts if the current becomes too high, opening the circuit and stopping the flow of current Circuit breakers- contain a small piece of metal that bends when it gets hot, opening the circuit and stopping the current Circuit breakers are often used in place of fuses. 41

42 ELECTRIC SAFETY Short Circuits Broken wires or water can cause electric appliances to short-circuit. A short circuit occurs when electricity takes a short path and bypasses the resistors in the circuit. Because of this the resistance of the circuit is less and the circuit wire increases. The increased current can produce enough heat to melt wires and start a fire, or cause serious electric shock. Don t over connect outlets because they could short circuit. Copyright 2010 Ryan P. Murphy 42

43 CLASSWORK 1. Identify the components, and the number of each in this diagram. 2. Draw a schematic diagram with 4 lights in parallel. 3. Draw a schematic diagram of 2 lights in series. 4. Draw a schematic diagram with 2 lights in parallel, and one in series. 43