Electricity Unit Test Review Answers 463k2c

Sc 9:

Electricity REVIEW Answers

1

1. Compare and contrast current and static electricity. What type of electricity is in the wires in our homes? What type of electricity is lightening? Static electricity is an excess of positive OR negative charges on an object. These charges are STATIC –( they don’t move) for a relatively long period of time. Lightening is one example of STATIC electricity that has built up such an excess of charge that it then moves through the atmosphere (or you if you are in the way)to get neutralize itself. Contrast that with CURRENT electricity: the MOVEMENT of ELECTRONS through a CONDUCTOR. Current electricity is the type of electricity we use in our homes to power our appliances. 2. What causes static electricity (in most cases)? Static electricity is usually caused by friction between two objects. One object literally rubs the electrons off of another one. The objects that pick up the extra electrons becomes negative; the object that loses its electrons becomes posititve. 3. Describe the difference between an insulator and a conductor. What types of objects are good insulators? Good conductors? Insulators do not allow electrons to flow through easily, while conductors DO allow electrons to flow easily. The best conductors are objects that have densely packed atoms so that the electrons can move easily between each of the atoms. Metals are usually good conductors of electricity. Insulators usually have loosely packed atoms, where the electrons have a hard time being ed from atom to atom. Gases are TERRIBLE conductors (therefore the are usually good insulators) because their atoms have so much space between them. Rubber and Styrofoam are good insulators. 4. What is a Coulomb? A Coulomb is the amount of charge created by 6.24 x 1018electrons. This is a very large measurement of charge! The charges you typically receive from the static in your carpets are 0.00001C. Lightening (VERY STRONG!) is about 5-25 C of charge. 5. Why does your hair stick up in the air when you are holding a Van de Graaff Generator?

Sc 9:

Electricity REVIEW Answers

2

The Van de Graaff generators moves a belt around a track that has rubs against some brushes. The brushes are connected by wires to the dome of the generator. This rubs electrons off, creating static charge. These generators can either rub electrons off of the belt and onto the dome, or off of the dome and onto the belt. For our purposes we will describe the kind that rub the electrons off the belt and onto the dome. That makes the dome negatively charged. Since negatively charged electrons repel one another, the electrons will move all through the persons body to get as far away from each other as possible. When they get to the hair, they will move out to the very ends of the hair, filling the hair with electrons. Since the hair is not very heavy, the repulsion of the electrons in the hair cause the hair to move away from one another. The hair will lie back down if the person is “grounded” – allowing the electrons to move back to the protons and returning the body and the hair to neutral. 6. What happens when a positive electric charge is created? A negative? What is a neutral object? A positive charge is created when an object looses some electrons. (therefore the number of electrons is less than the number of protons). A negative charge is created when an object gains an excess of electrons (#electrons is greater than # protons). A neutral object has the same number of protons and electrons. 7. What is a lightening rod and what does it protect against? A lightening rod is a good conductor that connects an object to the “ground” in order to prevent the buildup of charges on that object. The idea is that if the electrons can move through the conductor, they will (electrons are lazy). Otherwise if lightening or some other source of charge were to try to move through the object that you were trying to protect (for example, your house!) they might move so fast, they will cause a fire! The grounding wire is trying to protect against the event that you are struck by lightening. 8. What is the third peg in an electrical plug for? This is a separate grounding wires in appliances –it protects against electrocution in case a circuit is somehow leaking current into the appliance itself. 9. How do charged object react to a. other like charges b. other unlike charges c. neutral objects. Like charges attract one another; unlike charges attract one another. Neutral objects are attracted to both negative and positive charged objects due to charge induction. 10. a. What are the parts of an atom and how are they arranged? b. How do they compare as far as their mass and size are concerned? c. What charges do each of the parts of the atom carry? a. An atom is made up of a nucleus of protons and neutrons. Around the nucleus in

Sc 9:

Electricity REVIEW Answers

3

a “cloud” (or orbital )are circling electrons. b. Protons are MASSIVE compared to electrons, and therefore are responsible for the majority of the measurable mass of an atom. c. The protons carry a positive charge, electrons carry a negative charge and neutrons are neutral. 11. What happens to the distribution of charges if you bring the rod close to the ball? a.

This is called INDUCED CHARGE – the objects do not touch! b.

This is called CONDUCTION – the charge is transferred when the objects touch. 12. What happens if these two objects come close to one another? They will repel one another because they are both positively charged.

Sc 9:

Electricity REVIEW Answers

4

⇔ 13. Why are charged objects attracted to neutral objects? This happens because of induction. When the charged object approaches the neutral object, the electrons will move away. This creates a temporary charge in the neutral object that will be the opposite charge to the charged object. That will cause the two to attract. (see figure 7.13, page 260, BC Science 9. 14. Compare electricity to water flowing through a pipe. The water moving through the pipe is like the electrons flowing through a wire. (except that it doesn’t take any time for the electrons to “fill” the wire). The pump that pushes the water through the pipe is like the battery in a circuit. The speed of the water moving through the pipe is like current in a circuit. If you have a bigger pump, it will move the water faster, just like if you have a battery that has more voltage, the current will be greater. If you fill the pipe with rocks, it will slow down the flow of the water, just like resistors slow down the current. The lower the resistance, the faster the current (and visa versa). You can make the water only follow one path, like a series circuit, or you can have branches off the main line (alternative paths) like a parallel circuit. As long as the pump is strong enough, the water will be able to flow faster if there are more pathways it can take – just like the current in parallel circuits. 15. What is the difference between a chemical cell and a battery? A chemical cell is one unit that provides electrical energy to a circuit. Two or more chemical cells ed in series or parallel make up a battery. Unfortunately, when we are outside of the physics room, the English language uses the two as if they were identical. 16. What does a chemical cell do to “create” the electricity? Energy cannot be created or destroyed. It can only be transformed from one type to another. The chemical cells (batteries) convert the stored chemical energy into electrical potential energy. 17. What is a volt and what is it measuring? A volt is one Joule of energy for each coulomb of charge. It measures the amount of electrical potential energy is supplied to the coulombs of electrons in a chemical cell. It can be thought of as the “strength” of the electrons in that particular circuit. A high voltage packs a lot of punch!

Schematic Diagrams!

Electricity REVIEW Answers

Sc 9:

3

1

2

V 5

18.

6

5

4

A 7

Identify the above symbols: 1. one chemical cell 4. lLight bulb 7. ammeter

2.a battery of 2 cells 5. switch

3. resistor 6. voltmeter

19. a) Draw a circuit with four chemical cells in series, a resistor and an ammeter. With arrows, indicate the direction of the flow of electrons in the circuit. b) if the chemical cells are 1.5 V each, what is the total voltage of this battery? c) If you took 2 of the chemical cells away, how would the current in the circuit change? What if your added four cells instead?

b. The total voltage in this battery is 4 x 1.5V = 6 V c. If you half the voltage, you also half the current. If you double the voltage, you would also double the current. (I = V/R) 20. a. Draw a circuit with 4 chemical cells in parallel, 2 light bulbs in series, and a switch. With arrows, indicate the direction of the electron flow in the circuit. b. If the chemical cells are all 3 volts each, what is the total voltage of this battery? c. If you added another light in series to the circuit, how would the brightness of the lamps be affected? d. How could you make the light bulbs light up brighter? (2 ways) e. Explain what happens to the current in a circuit as you add more light bulbs in parallel, and then again in series. f. What would happen to the light bulbs (in series) if one of the lights burned out?

Sc 9:

Electricity REVIEW Answers

6

b)The total voltage would be 3 V. c) The lights would be dimmer because adding another light will decrease the current running through the circuit (they offer more resistance). d) You could make the light bulbs brighter by connecting them in parallel, or by adding more voltage. Both of these solutions will increase the current, which will cause the lights to glow brighter. e. Adding light bulbs in parallel will increase the current in a circuit. However, you might also need to add more chemical cells, since you have divided the voltage by the number of pathways you have provided. There might not be enough “juice” (voltage) to light up a whole bunch of lights in parallel without adding more chemical cells. Adding more light bulbs in series will decrease the current since light bulbs offer a great deal of resistance in a circuit. f. If the light bulbs are connected in series and one of them burns out your have broken the circuit, so all of the lights would go out until you fixed the light or reconnected the circuit. 21. Make a parallel circuit that has the following parts: 1.5V cell, 2 light bulbs, a switch that turns off both bulbs and a 2nd switch that turns off the 2nd bulbs.

22. You are given the following ingredients: six 1.5 volt cells, 3 switches, three light bulbs, plus all the connecting wire you need. Using all the materials listed, design a circuit that has a total of 4.5 volts and three light bulbs that can be turned on and off independently from each other.

Sc 9:

Electricity REVIEW Answers

7

23. What is an ampere? (amp) and what is it measuring? An ampere is one Coulomb per second. I.e. 6.24 x 1018 electrons ing in one second. It measures current: how fast the electrons are travelling through the circuit. 24. What is the formula to calculate resistance? Current? Voltage? R=V/I I = V/R V=IxR 25. Calculate the voltage of a battery that provides a current of 10 A through a circuit with a resistance of 20 Ω. V = IR = 10 A x 20Ω = 200V 26. Calculate the resistance of a circuit that has a battery with 9 V and a current of 3A. R = V/I = 9V/3A =3Ω 27. Calculate the current of a circuit that has 120V and a resistance of 12 Ω. I = V/R = 120 V/12Ω = 10 A 28. Metric Conversions: 1000mA = 1 A 2 A = 2000 mA 29. What happens to the amount of current that will flow if you increase the resistance in a circuit? How would that affect the brightness of a light bulb if it was also in that circuit? Increasing the resistance in a circuit will decrease the current. That will cause lights to be less bright in the circuit. 30. What happens to the total resistance in a circuit as more bulbs are added in parallel? As you add more bulbs in parallel, the total resistance will decrease. (and the total current will increase!) 31. In the following circuit a. What is the total voltage of the battery? 18 V b. What would happen if switch #1 was left open, but switch #2 was closed?

Sc 9:

Electricity REVIEW Answers

8

Lights A,B and C would light up; light D would not c. What would happen if switch #2 was left open, but switch #1 was closed? None of the lights would light up because the circuit is incomplete. d. What would happen if both switches were open? All lights would be out. e. What would happen if both switches were closed? All lights would be lit. f. What switches would you close to make the greatest current in the circuit? Both switches closed would produce the greatest current g. What switches would you close if you wanted to have the bulbs the brightest possible? Close both switches for brighter light bulbs. h. What would you have to do to make the bulbs even brighter? Add more chemical cells so the current would be increased. i. Is there any way in this circuit for you to have lights A and B lit, but light C not lit? No.

32. What is wrong with the following circuits? a. the batteries are b. the circuit is connected incorrectly incomplete

33. What will happen in this circuit? You will quickly drain the chemical cell till It has no electrical potential energy left. (you kill the battery!) 34. What does this tell us?  This tells us the AMOUNT of energy in kWh that the appliance will use (on average) in one year. We use this to

c. there is no source!

Sc 9:

Electricity REVIEW Answers

9

compare appliances for energy efficiency. The lower the number, the more efficient the appliance. 35. Calculate the Current for the following circuits.

Each Cell is 5V.

V R 10V I= 150Ω I = 0.07 A I=

VT = 10V

RT = R1 + R2 =50Ω + 100Ω = 150Ω

36. a. Would the current at A2 be the same as that at A1? Explain b. What would the voltmeter read at V? a. The Current will be lower at A2 then at A1. A1 will show the total current, but A2 is the partial current that moves through the 100 ohm resistor. b. The voltmeter would read 6V, because the voltage does not change in parallel circuits. (it is only in SERIES circuits that the voltage across the resistors will add up to the total resistance of the battery!)

37. Calculate a. total resistance of the circuit b. the resistance of each identical lamp.

Sc 9:

Electricity REVIEW Answers

10

c. What would the voltage across one of the lamps be? d. What would the voltage across two of the lamps be?

V I 5V VT = 5V, I = 0.5 A RT = 0.5 A RT = 10Ω The total resistance is 10Ω, therefore the resistance of each lamp is 10Ω/5 = 2Ω each. c. The voltage across one of the bulbs would be 2ohms. d. The voltage across two bulbs would be 4 ohms (2 each!) 38. Calculate the energy used by a 3000W water heater that operates continuously for 30 minutes in a. Joules b. Kilowatt hours c. The cost of the electricity at 10 cents per kWh. a)E = Pt b) E = Pt = 3000W x (30minutes x 60 seconds) = 3kW x 0.5 hours =5400000J = 1.5 kWh = 5.4 MJ RT =

d. 1.5kWh x $0.1/kWh = $0.15 39. Calculate the current required for a 40W light bulb connected to 120V. I=P/V = 40W/120V = 0.33A 40. Calculate the voltage required to light a 60W light bulb with 30A of current. V=P/I = 60W/30A =2V 41. Calculate the power rating on an appliance that uses 12000J of energy in 1minute. P = E/t = 12000J/60s = 200W

Electricity REVIEW Answers

Sc 9:

42. Calculate the amount of time it would take a 1200W appliance to use 4 kJ of energy. t=E/P = 4000J/1200W = 3.33s 43. An electrical iron uses 8.3A of current at 120V. Calculate: a. the electrical power supplied to the iron b. the electrical energy used by the iron in kilowatt.hours if it is operated continuously for 1h. c. Calculate the cost of the electricity used if energy costs $0.05/kW.h a) P=IV b) E = Pt = 8.3A x 120V = 0.996kW x 1h = 996W = 0.996 kWh c) Cost = $0.05/kW.h x 0.996kWh = $0.50 44. a) Calculate the Power of the bulb in the circuit below. Show all your work. b) Calculate the energy used by this bulb when it is left turned on for 5 hours. Show all your work.

a)P=IV V I= RT 6V 12Ω I = 0.5 A I=

P = IV

= 0.5A x 6V P= 3W

b) E=Pt = 3W x (5hours x 60 minutes x 60seconds) = 54000J E= 54kJ 45. Make a parallel circuit that has the following parts: 1.5V cell, 2 light bulbs, one switch that turns off both bulbs and a 2nd switch that turns off only the 2nd bulb.

11

Sc 9:

Electricity REVIEW Answers

12