 # Electricity

2. (a) The particles are negatively charged electrons.

(b) Electrons are negatively charged particles that orbit the positively charged nucleus of an atom. Because the electrons are not fixed in the nucleus but rather in motion around the nucleus, they can be rubbed off by the action of feet on the carpet.

3. (a) The law of attraction states that opposite charges attract.

(b) The law of repulsion states that similar charges repel.

4. The charge on an object can be detected by observing the interaction between it and an object of known charge. An electroscope is typically used for this purpose. First, the electroscope is charged with an object of known charge, and then the object with unknown charge is brought near the electroscope. If the leaves of the electroscope separate from one another further, then the unknown charge is the same as that which was used to charge the electroscope. If the leaves of the electroscope approach one another, then the unknown object has the opposite charge to the charge of the known object.

5. Students’ diagrams should indicate a charged object inducing charge separation in a neutral object, causing an attractive force between the charged object and the locally charged area of the neutral object.

6. There may be no effect if the materials do not have mobile electrons or if they are very close to one another in the triboelectric series.

7. (a) Balloon B is also negatively charged.

(b) If the balloons attract one another, then balloon B could be positive or neutral, since negatively charged balloon A will attract both neutral and positively charged objects.

8. (a) Object D has a positive charge.

(b) Object C has a negative charge.

9. Students’ Venn diagrams should show that both types of charging occur due to the movement of electrons, but that charging by conduction requires contact between the charging item and the item to be charged. Charging by induction does not require contact between the charging item and the item to be charged, but does require a third object to either provide electrons or remove excess electrons from the object being charged.

10. Students’ diagrams should be well labelled and indicate the process of lightning forming, specifically showing charge separation in the clouds, the induction of a locally positive area on Earth, the formation of a channel made of charged particles, and the transfer of excess electrons from the cloud to the ground. See Figure 10.22 on student book page 410 for an example.

11. (a) Clothing from the clothes dryer often sticks together because of having been rubbed against other clothing made of different materials. This friction causes electrons to be transferred and charges to be accumulated.

(b) This effect can be reduced by removing clothing from the dryer before it is fully dry, drying only articles made of the same material together, and/or using anti-static sheets in the dryer.

12. The metal rods in the photograph are lightning rods, designed to create a safe pathway for lightning to travel to ground if it were to strike the building.

13. Static electricity controls have helped in the development of technologies that depend on the attraction and repulsion of particles to other particles and surfaces.

14. (a) Fuelling airplanes would be safer if static electricity were eliminated.

(b) A photocopier would not function at all if static electricity were eliminated.

17.(a) The four factors affecting resistance in a wire are material, temperature, length, and cross-sectional area.

(b) Both increased length and temperature increase the resistance of a wire. Increased cross-sectional area decreases resistance. The material the wire is made from also affects the resistance of the wire depending on how good a conductor the material is.

18. The light bulb lights up immediately because electrons in the wire are pushed into the light bulb; they do not need to travel all the way from the switch.

20. (c) More current flows in the parallel circuit.

(d) Both bulbs will go dark if one of the bulbs in circuit (a) is removed.

(e) There will be no difference in brightness in the remaining bulb in circuit (b) if one of the bulbs is removed.

21. (a) Voltage V1 in the circuit is 6.0 V.

(b) Current A1 in the circuit is 2.0 A.

(c) It is a series circuit.

22. (a) Voltage V1 in the circuit is 3.0 V.

(b) Current A1 in the circuit is 1.0 A.

(c) It is a parallel circuit.

23.Students’ diagrams may vary. The following circuit diagram is one of many possibilities:

24. Ohm’s law states that the voltage drop across a resistor is equal to the current flowing through the resistor multiplied by the resistance of the resistor.

25. If the resistance of a load becomes larger, less current will flow through the load. Resistance and current are inversely proportional.

26. Homes are wired for safety with circuit breakers or fuses, grounded plugs, and ground fault circuit interrupters.

27. Fused safety power bars help prevent damage to sensitive electronics due to power surges or voltage spikes from lightning strikes or the electrical grid.

35. Electrical energy consumption is typically measured in kilowatthours

(kW•h).

36. The efficiency of a device is the ratio of its useful energy output to the total energy input.

37. An EnerGuide label states how much energy the appliance uses on a monthly or annual basis and  compares the usage of the appliance to other appliances in the same class.

38. The Energy Star label on an appliance indicates that the appliance is one of the most efficient units in its class.

39. The EnerGuide and Energy Star labels could be used to help determine which appliances are the most efficient and will result in the lowest long-term energy cost of operation.

40. A front-loading washing machine uses much less water than a toploading unit. This results in the use of much less energy to heat the water and significantly less energy consumption overall.

41. Any appliance that stays in “standby” mode consumes electricity even when not in use. Students’ answers may vary but could include a computer, printer, modem, DVD player, television, answering machine, microwave oven, clock radio, cordless phone, or garage door opener.

42. Lowering energy demands results in lower personal energy costs, reduced adverse environmental effects, and reduced social cost of construction and operation of energy generating plants.

43. (a) If lightning strikes a car, it is safer to be inside the car (and touching nothing metal) than outside the car and touching it. This is because the metal body of the car provides a path for current to flow to the ground.

(b) Inside a large building is one of the safest places to take shelter during a lightning storm. However, depending on how close the lightning storm is and how far across the open field the building is located, it may not be wise to run across a field. Certainly it is not a good idea to take shelter under a tall tree during a lightning storm. Minimizing contact with the ground away from the tree might be considered a last resort.

44. A lightning bolt and a spark are both discharges of static electricity.

45. The comb is lower on the triboelectric series than hair. Therefore, the comb gains electrons from the hair when they are rubbed together. The charged comb attracts the neutral paper by inducing a charge separation. But as the paper approaches the comb, electrons are transferred from the comb to the paper, and the like charges repel.

46. (a) Grounding a charged object allows excess charge to be discharged to Earth, preventing the accumulation of large amounts of static charge.

(b) Some items need to be grounded to protect them since they are sensitive to accumulated static charge and/or static discharge.

(c) Computers and fuelling aircraft need to be grounded.

47. The charge on an electroscope is static, whereas the charge in a functioning circuit is flowing continuously (i.e., it is current electricity).

48. The bird is not grounded and since it provides no path to ground, electric charge does not flow through it and it does not experience an electric shock.

49. A voltmeter has a very high internal resistance, whereas an ammeter has a very low internal resistance.

50. Lights in a home may dim when certain high-current appliances are turned on because the large current used causes a greater than usual voltage drop in the electrical service of the home, resulting in lower voltage available for lights and other applications.

51. (a) A large aluminum ladder may provide a path for current to flow to ground from the overhead power wire through the worker carrying the ladder.

(b) Removing the ground plug from the cord of an appliance breaks the safe pathway to ground for the appliance, potentially exposing the user to unsafe electrical discharges in case of a short circuit in the device.

(c) A frayed electrical cord on a washing machine is hazardous because it may create a short circuit, potentially leading to electric shock or fire.

(d) A piece of aluminum instead of a fuse will not provide overcurrent protection to the circuit, may melt, or possibly cause a fire. No other object should ever be substituted for a fuse.

52. Replacing a cord with one that is much thinner than the original is hazardous because it may overheat. This occurs due to the higher resistance of the smaller diameter cord than the original larger diameter one.

55. A more efficient appliance that costs more than a regular model makes economic sense if the electricity cost savings of the efficient appliance can recover the initial increased purchase price within a reasonable period of time and continue producing meaningful energy savings after that.

57.

Given

Voltage V = 6.0 V

Current I = 0.0020 A (2.0 mA is 0.0020 A, since there are 1000 A in 1 A)

Required

Resistance R = ?

Analysis and Solution

The correct equation is V = IR, rearranged to R = V/I

Substitute the values and their units, and then solve the problem.

R = V/I

= 6.0 V/0.0020 A

= 3000 Ω

Paraphrase

The resistance of the resistor is 3000 Ω.

58. (a)

Given

Current I = 1.5 A

Resistance R = 5.0 Ω

Required

Voltage V = ?

Analysis and Solution

The correct equation is V = IR.

Substitute the values and their units, and then solve the problem.

V = IR

= (1.5 A)(5.0 Ω)

= 7.5 V

Paraphrase

The voltage across the resistor is 7.5 V.

(b)

Given

Voltage V = 80 V

Resistance R = 20 Ω

Required

Current I = ?

Analysis and Solution

The correct equation is V = IR, rearranged to I = V/R

Substitute the values and their units, and then solve the problem.

I = V/R

= 80V/20Ω

= 4 A

Paraphrase

The current through the resistor is 4 A.

(c)

Given

Voltage V = 12 V

Current I = 240 A

Required

Resistance R = ?

Analysis and Solution

The correct equation is V = IR, rearranged to R = V/I .

Substitute the values and their units, and then solve the problem.

R = V/I

= 12 V/240 A

= 0.05 Ω

Paraphrase

The resistance of the motor is 0.05 Ω.

59.This question contains three 40-Ω resistors in series for a total series resistance of 120 Ω. Values in the table are found from the equation I = V/R, using the voltage V from the first column of the table and the resistance R = 120 Ω.

Voltage (V) Current (A)

2.0 0.017

4.0 0.033

6.0 0.050

8.0 0.067

10.0 0.083

60. (a) 1.6 MV = 1 600 000 V

(b) 1500 Ω = 1.5 kΩ

(c) 650 mA = 0.65 A

61. This resistor does obey Ohm’s law, since current increases proportionally with increased voltage.

62.

Gas-powered SUV 12%

Gas-electric hybrid car 29%

Natural gas furnace 77%

Electric baseboard heater 63%

Alkaline dry cell  88%

69. (a) Students’ responses may vary but should include the essential concept that static electricity is a stationary charge whereas current electricity is charge flowing continuously in a closed circuit.

(b) Students’ answers should include the essential concept that the voltage drop across a resistor is directly proportional to the current flowing through the resistor, and the proportionality constant between them is the resistance of the resistor (i.e., Ohm’s law V = IR).

70. (a) Students’ answers may vary but could include statements concerning an increased awareness of their habits of energy use and a commitment to reducing their energy consumption.

(b) Students’ answers may vary but could include a decision to purchase more efficient devices when they have the responsibility to do so, walk or cycle to their destinations, when possible, or initiate a “switch it off” campaign at home or at school. Refer them to the chart created at the beginning of Chapter 12 as part of the Concepts You Will Learn discussion for other ideas.