Read Chapter 7; pages:

Transcription

1 Forces Read Chapter 7; pages: Objectives: - Describe how electrical charges exert forces on each other; Compare the strengths of electric and gravitational forces; Distinguish between conductors and insulators; Explain how objects become electrically charged; Describe how voltage difference causes current to flow; Explain how batteries produce a voltage difference in a circuit; List the factors that affect an object s electrical resistance; Define Ohm s Law; Describe the difference between series and parallel circuits; Recognize the function of circuit breakers and fuses; Calculate electrical power; Calculate the electrical energy used by a device; PSc.3.3 Understand electricity and magnetism and their relationship PSc Summarize static and current electricity. PSc Explain simple series and parallel DC circuits in terms of Ohm s law. PSc Explain how current is affected by changes in composition, length, temperature, and diameter of wire. PSc Explain magnetism in terms of domains, interactions of poles, and magnetic fields. PSc Explain the practical applications of magnetism. Vocabulary: Static electricity law of conservation of charge conductor Insulator charging by contact charging by induction Electrical current voltage difference circuit Resistance Ohm s law series circuit Parallel circuit electrical power

2 Chapter 7 Electricity

3 Electric Charge Conservation of Charge According to the law of conservation of charge, a charge can be transferred from one object to another object, but it cannot be created or destroyed. Whenever an object becomes charged, electric charges have moved from one place to another.

4 Electric Charge Electric charge: an electrical property of matter that creates electric and magnetic forces and interactions; An object can have a negative charge, a positive charge, or no charge at all.

5 Electric Charge, continued Like energy, electric charges are never created or destroyed. Like charges repel, and opposite charges attract. The electric charge, depends on the imbalance of protons (positive charge) and electrons (negative charge) and neutral (no charge). Negatively charged objects, have more electrons than protons; Where as, positively charged objects, have fewer electrons than protons (NOT more protons). The SI unit of electric charge is the coulomb, C (koo lum);

6 Charging by Contact Objects can be charged by contact. The transfer of electrons from one object to another can charge objects. Objects charged by touching a charged object to a neutral object are said to be charged by contact. Electrons can be transferred from one material to the other, when different materials are rubbed together. The transfer of electrons to the metal doorknob gives the doorknob a net negative charge.

7 Visual Concept: Charging by Contact

8 Objects can be charged by friction. Charging by friction occurs when one material gains electrons and becomes negatively charged, and the other loses electrons and becomes positively charged. Your clothes are charged by friction as they rub against each other inside the dryer, and stick together because of static electricity. When insulators are rubbed together, one gives up electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.

9 Induced Charges A surface charge can be induced by an insulator. When a charged object is brought near an insulator, the positions of the electrons within the individual molecules of the insulator change slightly. One side will be slightly more positive or more negative than the other side.

10 Visual Concept: Electrical Conductors and Insulators

11 Electric Charge Static Electricity and Lightning Static electricity is the accumulation of excess electric charge on an object; and lightning is a large static discharge, which is a transfer of charge between two objects because of a buildup of static electricity. A thundercloud is a mighty generator of static electricity. As air masses move and swirl in the cloud, areas of positive and negative charge build up. Eventually, enough charge builds up to cause a static discharge between the cloud and the ground; any object connected to Earth by a good conductor will transfer any excess electric charge to Earth.

12 Detecting Electric Charge The presence of electric charges can be detected by an electroscope. One kind of electroscope is made of two thin, metal leaves attached to a metal rod with a knob at the top. The leaves are allowed to hang freely from the metal rod. If a negatively charged object was to touch the knob at the top, electrons would travel down the rod into the leaves. Both leaves become negatively charged as they gain electrons and because they have similar charges the leaves would repel each other.

13 Electric Charge Detecting Electric Charge If a glass rod is rubbed with silk, electrons move away from the atoms in the glass rod and build up on the silk. When the positively charged glass rod is brought into contact with the metal knob of an uncharged electroscope, electrons flow out of the metal leaves and into the glass rod. The leaves repel each other because each leaf is positively charged. Electroscope leaves: A - hang down when uncharged, B - repel when negatively charged, C repel when positively charged;

14 Electric Force The electric force at the atomic and molecular levels is responsible for most of the everyday forces that we observe, such as the force of a spring and the force of friction. The electric force is also responsible for effects that we cannot see: Bonding of atoms to form molecules is also due to the electric force. electric force: the force of attraction or repulsion on a charged particle that is due to an electric field;

15 Electric Field Electric force acts through a field. electric field: the space around a charged object in which another charged object experiences an electric force. One way to show an electric field is by drawing electric field lines. Electric field lines point in the direction of the electric force on a positive charge. The electric field lines around a positive charge point outward. The electric field lines around a negative charge point inward.

16 Electric Field, continued Electric field lines never cross one another. Field lines near two like charges - point away from each other b/c like charges repel each other. Whereas, electric field lines of unlike charges connect to each other. Field lines show both the direction of an electric field and the relative strength of the charge. More lines are drawn for greater charges to indicate a greater force.

17 Electrical Potential Energy Electrical potential energy: the ability to move an electric charge from one point to another; just as a ball rolls downhill; A negative charge will move away from another negative charge; the electric potential energy of the moving charge decreases because the electric field does work on the charge. An electric charge also has potential energy-electrical potential energy that depends on its position in an electric field; electrical potential energy between two like charges decreases as the distance between the charges increase.

18 Potential Difference (Voltage) Potential difference (often called voltage): is the change in the electrical potential energy of a charged particle divided by its charge; also known as the voltage difference in potential between two points in a circuit; The SI unit for potential difference is the volt (V), which is equivalent to one joule per coulomb (1J/C). For this reason, potential difference is often called voltage.

19 Electric Current Batteries To keep an electric current continually flowing in the electric circuit a voltage difference needs to be maintained in the circuit. A battery can provide the voltage difference that is needed to keep current flowing in a circuit. Current flows as long as there is a closed path that connects one battery terminal to the other battery terminal.

20 Electric Current Dry-Cell Batteries A cell consists of two electrodes surrounded by a metal paste material called an electrolyte. The electrolyte enables charges to move from one electrode to the other. One electrode is the carbon rod (+), and the other is the zinc (-) container. The electrolyte is a moist paste, not a liquid (reason it is called a dry-cell) containing several chemicals.

21 Electric Current Wet-Cell Batteries A wet cell contains two connected plates made of different metals or metallic compounds in a conducting solution. A wet-cell battery contains several wet cells connected together. In this wet cell, chemical reactions transfer electrons from the lead plates to the lead dioxide plates.

22 Electric Current Lead-Acid Batteries Most car batteries are lead-acid batteries. A lead-acid battery contains a series of six wet cells made up of lead and lead dioxide plates in a sulfuric acid solution. The chemical reaction in each cell provides a voltage difference of about 2 V, giving a total voltage difference of 12 V.

23 AC/DC Current Current is the rate of charge movement. The SI unit of current is the ampere, A. electric current: the rate at which charges pass through a given point; Direct current (DC) the charges always move from one terminal to the other in the same direction. Alternating current (AC) an electric current which periodically reverses direction, in contrast to DC which flows only in one direction.

24 Current, continued Electron current flow: the flow of electrons flow from the negative terminal to the positive terminal. Conventional current or simply current, behaves as if positive charge carriers cause current flow, and flows from the pos. terminal to the neg. terminal, opposite to the direction that electrons flow (neg to pos).

25 Galvanometer: used to measure the amount and direction of small electric currents. (Ex: current from a potato cell) Ammeter: used to measure the amount of larger electric currents. (Ex: circuit in your house) When using these devices you connect negative to positive in series with the object being measured.

26 Electrical Resistance Electrical resistance is caused by internal friction, which slows the movement of charges through a conducting material. Resistance: the opposition presented to the current by a material or device and can be calculated if current and voltage are known. Ohms law: The SI unit of resistance is the ohm (Ω): Ohm s Law equations: V= IR R= V I [Resistor : special type of conductor used to control current.]

27 Resistance Math Skills The headlights of a typical car are powered by a 12 V battery. What is the resistance of the headlights if they draw 3.0 A of current when turned on? 1. List the given and unknown values. Given: current, I = 3.0 A voltage, V = 12 V resistance, R =? Ω R = 4.0 Ω

28 Electrical Resistance, continued Conductors have low resistances, where as, insulators have high resistances. Semiconductors conduct under certain conditions. semiconductors: 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.

29 What Are Circuits? An electric circuit: a set of electrical components connected such that they provide one or more complete paths for the movement of charges; A closed circuit is the conducting path produced when a load, such as a string of light bulbs, is connected across a source of voltage; An open circuit is an interrupted path or open path in a circuit, resulting in no charge flow and no current flow; like a switch open or series bulb burned out; Voltage source, whether a battery or an outlet, is always part of the conducting path of a closed circuit;

30 What are Circuits?, continued Switches interrupt the flow of charges in a circuit. You can use a switch to open and close a circuit. Schematic diagrams are used to represent circuits. schematic diagram: a graphical representation of a circuit that uses lines to represent wires and different symbols to represent components All electrical devices can be described by schematic diagrams. Schematic diagrams use standard symbols.

31 Electrical Energy Series and Parallel Circuits One kind of circuit is called a series circuit. In a series circuit, the current has only one loop to flow through. Series circuits are used in flashlights and some holiday lights, when one bulb burns out, the entire string is out.

32 Electrical Energy Series and Parallel Circuits, cont. Houses are wired with parallel circuits. Parallel circuits contain two or more branches for current to move through. The current can flow through both or either of the two branches; when one bulb burns out, the other will still be lit.

33 Electrical Energy Series and Parallel Circuits, cont. Parallel circuits have several advantages. When one branch of the circuit is opened, such as when you turn a light off, the current continues to flow through the other branches.

34 Series and Parallel Circuits, cont. In this diagram, we see a combination of both a series and parallel circuit. The series circuit is the battery, R1, and R2 or the battery R1 and R3. The parallel is thru both R2 and R3. What would happen if R1 opened? What would happen if R2 or R3 opened?

35 Finding Series Resistance: Find the total resistance for three resistors: 60 Ω, 30 Ω, and 20 Ω, in series circuit: 60 Ω + 30 Ω + 20 Ω = 110 Ω Finding Parallel Resistance: Find the equivalent resistance for three resistors: 60 Ω, 30 Ω, and 20 Ω, in a parallel circuit: 1/60 Ω + 1/30 Ω + 1/20 Ω = 10 Ω

36 Electrical Energy Household Circuits The wiring in a house must allow for the individual use of various appliances and fixtures. This wiring is mostly a combination of parallel circuits connected in an organized and logical network.

37 Electrical Energy Household Circuits, cont. The main switch and circuit breaker or fuse box serve as an electrical headquarters for your home. Parallel circuits branch out from the breaker or fuse box to wall sockets, major appliances, and lights. To protect against overheating of the wires, all household circuits contain either a fuse or a circuit breaker.

38 Electrical Energy Household Circuits, cont. A circuit breaker contains a piece of metal that bends when the current in it is so large that it gets hot. The bending causes a switch to flip and open the circuit, stopping the flow of current. Circuit breakers usually can be reset by pushing the switch to its "on" position.

39 Electrical Energy Fuses An electrical fuse contains a small piece of metal that melts if the current becomes too high. When it melts, it causes a break in the circuit, stopping the flow of current through the overloaded circuit. To enable current to flow again in the circuit, you must replace the blown fuse with a new one. Car fuses

40 Electrical Energy Overheating Electrical Circuits Too many appliances in use at the same time is the most likely cause for the overheating of the circuit.

41 Electrical Energy Electric Power The reason that electricity is so useful is that electrical energy is converted easily to other types of energy. Some of the energy in a circuit, is energy transformed into useful work, such as the turning of a motor, and/or is lost as heat. The rate at which electrical energy is converted to another form of energy is the electric power: the rate at which electrical energy is converted into other forms of energy; Electric power is calculated by multiplying the total current, I, by the voltage, V, in a circuit. Power = current x voltage P = IV the SI unit for power is the watt (W). 1 W = 1 A 1 V

42 Electric Power Math Skills When a hair dryer is plugged into a 120 V outlet, the hair dryer has a 9.1 A current in it. What is the hair dryer s power rating? 1.List the given and unknown values. Given: voltage, V = 120 V current, I = 9.1 A Unknown: electric power, P =? W P = (9.1 A)(120 V) P = 1,092 W P = 1.1 kw P = IV

43 Electrical Energy Electrical Energy Electric companies charge by the amount of electrical energy used, rather than by the electric power used. Electrical energy usually is measured in units of kilowatt hours (kwh) and can be calculated from this equation: Electric energy (in kwh) = electric power (in kw) x time (hrs) E = PT

44 Electrical Energy Electrical Energy Math Problem The cost of using the appliance can be computed by multiplying the electrical energy used by the amount the power company charges for each kwh. For example, if a 100-W lightbulb is left on for 5 h, the amount of electrical energy used is: E = Pt E = (0.1 kw)(5h) E = 0.5 kwh

45 Measuring DC Voltage with a Voltmeter Connect the RED positive lead to positive terminal and The BLACK negative lead to negative terminal, Ensure voltmeter is in the DCV setting / highest setting; Touch the two leads (red and black) across the battery terminals (+ and - ); then downrange volt setting to a lower setting until you get a reading in the digital display; V = 1.31 V

46 Measuring Resistance with a Voltmeter Connect the RED positive lead to positive terminal and The BLACK negative lead to negative terminal, Ensure voltmeter is in OHMS and on the lowest setting; touch the two leads (red and black) together, ensure a reading of zero; THEN touch the two leads across a resistor and wait for a reading, IF NO READING, Up-range the Ohms setting, until an Ohms value is displayed; Resistance is a NONpowered measurement; NEVER attempt to measure resistance when voltage is applied. R = 1.10 kω

47 Let s Review

48 Ohms Law Math Find Resistance in a Series Circuit Find the total resistance in this series circuit (hint: add all resistor values): R t = R t = 100Ω + 300Ω + 50Ω 450Ω

49 Ohms Law Math Find Current (I) in a Series Circuit Ohm s formula for current: I = V R Find the total current flowing through this series circuit, if the total resistance is 450Ω : 9V 450Ω I = 0.02A

50 Ohms Law Math Find the Voltage Drop in a Series Circuit Solve for the voltage drop across each resistor, then ADD values together (hint: if added together, equals the total voltage in the circuit): (I x R 1 ) = (0.02A)(100Ω) V 1 = 2V (I x R 2 ) = (0.02A)(300Ω) V 2 = 6V V = IR V T = 2V + 6V + 1V V T = 9V (I x R 3 ) = (0.02A)(50Ω) V 3 = 1V I = 0.02 A

51 Ohms Law Math Find Resistance in a Parallel Circuit Find the equivalent resistance of the parallel circuit (hint: add all reciprocals, then take sum s (answer s) reciprocal): 1/R t = 1/R 1 + 1/R 2 + 1/R 3 1/R t = 1/5Ω + 1/10Ω + 1/15Ω 1/R t = R t 2.73Ω

52 Ohms Law Math Find Current (I) in a Parallel Circuit Find the total current in the parallel circuit below if the total equivalent resistance is 2.73 ohms: 60V 2.73Ω I = A I 22 A I = V R

53 Ohms Law Math Find the Current Drop in a Parallel Circuit Find the current drop across each of the resistors (hint: do not take reciprocal of resistors): 60V R 1 = 5Ω 60V R 2 = 10Ω I 1 = 12 A I 2 = 6 A Sum of the current = 22 A 60V R 3 = 15Ω I 3 = 4 A

54 Ohms Law Math Find the Current in the Series Circuit I = 10V 10KΩ =.001 A Now, what would happen to the current if you doubled the resistor s value (20KΩ)? A I = 10V 20KΩ =.0005 A B In a series circuit, if you doubled the resistance, then current is cut in-half.

55 Ohms Law Math Find the Current in a Series Circuit I = 10V 10KΩ =.001 A Now, what would happen to the current if you doubled the voltage in the circuit? A I = 20V 10KΩ =.002 A B In a series circuit, if you doubled the voltage, the current in the circuit would double the current.

56 Ohms Law Math 1/R t = 1/R 1 + 1/R 2 + 1/R 3 plus series resistance Find the total resistance in circuit above: (Rt = 1 / (1/ /0.5) = (Rt = = ohms Find the total current in the circuit (hint use resistance from above: I = V R I = = Amps 0.2 Amps

57 Ohms Law Math 1) Find the equivalent resistance of the circuit: R t = 1 / (1/4 + 1/6 + 1/10) = R t = = R t = 3.94Ω 2) Find the total circuit current: I = I = V R I = 3.05 A

58 Ohms Law Math 1. Find the total resistance in the circuit below. 1/Rt = (1/ /340) = Rt = (210.11) + ( ) Rt = Ω 2. Find the total current in the circuit below = I I = A I = V R (Rt = Ω)

59 Ohms Law Math A 20 Ω resistor and a 30 Ω are connected in series and placed across a 120 V potential difference. 1. What is the current in the circuit? Rt = = 50Ω I = 120 / 50Ω I = 2.4A I = V/R 2. What is the voltage across the 30 Ω resistor? V = (2.4A)(30Ω) V = 72V V = IR (I = 2.4A)

60 More Review

61 A 10 Ω resistor, a 15 Ω resistor, and a 5 Ω resistor are connected in series across a 90 V battery in a remote controller. R = 30Ω What is the current in the circuit? I = 90/30Ω = I = 3A I = V/R

62 A string of holiday lights has ten bulbs with equal resistance connected in series. When the string of lights is connected to a 120 V outlet the current through the bulbs is 0.06 A What is the resistance of each bulb? Answer : 200 Ω I = V/R 0.06A = 120/R R = 120/0.06A R = 2000Ω / 10

63 If the batteries in a CD player provide a terminal voltage of 12 V, what is the potential difference across the entire player? A. 3 V B. 4 V C. 6 V D. 12 v

64 Three resistors with values of 3.0 ohms, 6.0 ohms, and 12 ohms are connected in parallel. What is the equivalent resistance of this combination? A ohms B ohms C. 9.0 ohms D. 33 ohms 1/R t = 1/R 1 + 1/R 2 + 1/R 3

65 Three resistors with values of 16 ohms, 19 ohms and 25 ohms respectively, are connected in parallel. What is their equivalent resistance? 1/R t = 1/R 1 + 1/R 2 + 1/R 3

66 Three resistors with values of 4 ohms, 6 ohms and 10 ohms are connected in both a series and parallel circuit, solve for the total resistance and equivalent resistance. R t = R 1 + R 2 + R 3 Series Resistance: 1/R e = 1/R 1 + 1/R 2 + 1/R 3 Parallel Resistance: 20Ω 1.935Ω

67 A 12 V storage battery is connected to three resistors, 6.75 ohms, 15.3 ohms and 21.6 ohms respectively. The resistors are joined in series. I = V/R What is the current in the circuit? R = I = 12 / I = 0.275A

68 Answer : D Three appliances are connected in series to a 120 V potential difference: a toaster, 1200 W; a coffee pot 750 W and a microwave, 600 W. If all were operated at the same time what total current would they draw? A. 3 A B. 5 A C. 10 A D A P = I V 2550W = (I)(120V) 2550W / 120V = I I = 21.25A

69 Answer 240 ohms. I = V/R 0.5A = 120v / R R = 120v / 0.5A R = 240Ω What is the resistance of a 60 W bulb if it is rated for a voltage of 120 V? P = I V 60W = I (120v) 60W/(120v) = I I = 0.5A

70 Answer : 1 Volt Suppose that there are 10 resistors in series. Five of them have values of 10 ohms, and the other 5 have values of 20 ohms. The power source is 15 V dc. What is the voltage across one of the 10 ohm resistors? I = 15V/150Ω I = 0.1A Rt = 5(10Ω) + 5(20Ω) Rt = 50Ω + 100Ω = 150Ω V = IR V = (0.1A)(10Ω) V = 1V

71 Answer : B A 45-Ω resistor and a 65-Ω resistor are connected in series. These resistors are then connected in parallel with a 120-Ω resistor. What is the equivalent resistance for the circuit? A. 230 ohms B ohms C. 22 ohms D ohms 1/R t = 1/R 1 + 1/R 2 + 1/R 3 1/Rt = 1/110Ω + 1/120Ω Rt = 57.39Ω

72 A 2.00-ohm resistor and a 12.0-ohm resistor are connected in parallel across a 20.0-V battery. What is the current flowing through the 2.00-ohm resistor? a A c A b A d A I = V/R I = 20/2 I = 10A 1/R t = 1/R 1 + 1/R 2 Rt = Ω I = 20V/1.714 Ω I = 11.67A

73 Two resistors with values of 6.0 ohms and 12 ohms are connected in parallel. This combination is connected in series with a 2.0 ohm resistor and a 24 V battery. What is the current in the 6.0 ohms resistor? A A B A C. 6.9 A D. 12 A Answer A Rt = 4 Ω + 2Ω Rt = 6Ω V = 24/6Ω I = 4A V = 4A 4Ω V = 16V I = V/R I = 16/6 I = 2.27A