Electricity Review

pg. 506 -- scroll down for the answers

# 4, 5, 7, 8, 10, 11, 17, 19, 20-23, 35, 48, 51, 57-59, 62

4. Explain the steps you would take to tell the difference between a positively charged object and a negatively charged object.

5. Use a series of diagrams to explain how a charged object attracts a neutral object.

6. Suppose two different materials are rubbed together. Each one is brought near a charged electroscope with no effect on the electroscope. Explain what may be the reason that there is no effect.

7. Suppose you know that balloon A is negatively charged but you do not know the charge on balloon B.

(a) When you bring the two balloons together, they repel each other. What is the charge on balloon B?

(b) Suppose that when you bring the two balloons together they attract each other. Does this observation prove that

balloon B is positive? Explain why or why not.

8. Object C is rubbed on object D. The leaves of a negatively charged electroscope temporarily move closer together when object D is brought near.

(a) What charge does object D have?

(b) What charge does object C have?

10. Use labelled diagrams to explain how lightning occurs.

11. (a) When clothes come out of a clothes dryer, they sometimes stick to each other. Explain why.

(b) Name three different ways to reduce this effect.

17. (a) What are four factors affecting resistance in a wire?

(b) Describe how each factor affects resistance.

19. (a) Draw a circuit diagram that includes a battery, connecting wires, and a resistor.

(b) Add a voltmeter to the circuit diagram to measure the potential difference across the resistor.

(c) Add an ammeter to the circuit diagram to measure current through the resistor.

20. (a) Use circuit symbols to draw a series circuit with a battery, connecting wires, and two light bulbs.

(b) Draw a parallel circuit using the same components as (a).

(c) Describe the difference in current flowing in the two circuits (a) and (b).

(d) What will happen to the brightness of the bulbs in circuit (a) if one of the bulbs is unscrewed?

(e) What will happen to the brightness of the bulbs in circuit (b) if one of the light bulbs is unscrewed?

23. Draw a circuit that keeps two lights on at all times and can switch two other light bulbs on and off independently.

35. In what units is electrical energy consumption usually measured?

48. Explain why a cow that touches an electric fence gets a mild shock. A bird sitting on the same wire does not receive a shock. Why?

51. For the following situations, explain the safety concern.

(a) A worker carries a large aluminum ladder near overhead hydro lines.

(b) Someone takes the third prong out of a plug in order to use it with a two-prong extension cord.

(c) The washing machine electrical cord is frayed.

(d) You run out of fuses and put a piece of aluminum in place of the fuse.

57. What is the value of a resistor that transforms 2.0 mA of current when it is connected to a 6.0-V battery?

58. (a) What voltage is applied to a 5.0-Ω resistor if the current is 1.5 A?
(b) A voltage of 80 V is applied across a 20-Ω resistor. What is the current through the resistor?

(c) The current running through a starter motor in a car is 240 A. If this motor is connected to a 12-V battery, what is the resistance of the motor?

59. Copy and complete the following chart in your notebook. Use Ohm’s law to create a set of data given that there are three resistors in series and each one has a resistance of 40 Ω.
 

Voltage

(V)

 

Current

(A)

 

2.0

 
 

4.0

 
 

6.0

 
 

8.0

 
 

10.0

 
62. Copy and complete the following chart in your notebook.

Device 

Input

Energy (kJ)

 

 

Output

Energy (kJ)

 Percent

Efficiency

 

Gaspowered

SUV

 675 

 81

 
 

Gas-electric

hybrid car

 675  

195

 
 

Natural gas

furnace

 110 000 

 85 000

 
 

Electric

baseboard

heater

 9.5 

 6.0

 
 

Alkaline dry

cell

 84.52 

 74.38

 

ANSWERS

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.

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.

10. Students’ diagrams should be well labelled and indicate the process of lightning forming, specifically showing harge 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 he same material together, and/or using anti-static sheets in the dryer.

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 esistance. The material the wire is made from also affects the resistance of the wire depending on how good a onductor the material is.

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.

35. Electrical energy consumption is typically measured in kilowatthours (kW•h).

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.

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.

57. The resistance of the resistor is 3000 Ω.  

58. The voltage across the resistor is 7.5 V.

The current through the resistor is 4 A.

The resistance of the motor is 0.05 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

62.
 Device

 Input

Energy

(kJ)

 
 

Output

Energy

(kJ)

Percent

Efficiency

(%)

 
 Gas-powered SUV  675  81 12
 Gas-electric hybrid car  675 195 29
 Natural gas furnace  110 000  85 000  77
 Electric baseboard

heater

 9.5 6.0 63
Alkaline dry cell  84.52  74.38  88