SNC 1DI: Physics (Electricity) Exam Review ANSWERS

Practice Questions – Electricity1. To protect against electrocution, copper wiring should be covered with:

a. aluminum c. carbonb. rubber d. nickel

2. Static electrical charges are placed on an object when it is:a. rubbed with another object made of the same materialb. held near a magnetc. connected to a battery in a circuitd.rubbed with another object made of different material

3. Imagine you and your partner each have an inflated balloon attached to a string. If you each rubbed your balloon against your hair and then, holding them by their strings, brought them close together, they would: a. move toward each other. c. move away from each other.b. bob rapidly back and forth. d. be unaffected by each other.

4. If you use a positive object to charge a neutral object by contact, the neutral object will become:a. negative c. neutralb. positive d. it depends on how many electrons move

5. When a positive rod is used to charge a neutral object by induction there is a flow of: a. Electrons from the strip into the objectb. Electrons from the object into the stripc. Electrons from the ground into the objectd. Electrons from the object into the strip

6. Which two units are both used to measure electrical energy?a. watt and ohm c. ampere and voltb. watt hour and joule d. kilowatt hour and ohm

7. The disadvantages of nuclear energy include:a. air pollution c. it is a non-renewable resourceb. radioactive waste d. both B and C are correct

Practice Problems – Electricity 1. State the Law of Electric Charges.

Opposite charges attract Like charges repel (same charges repel) Charged objects attract neutral objects

2. Use the Electrostatic Series on page 275 of your textbook to answer the following question.You rub an ebonite rod with a piece of wool (CHARGING BY FRICTION)

a. What is the charge on the ebonite rod?Negative (ebonite is below wool on the electrostatic series so it has a stronger hold on electrons compared to wool. When rubbed against the wool, the ebonite will steal electrons from the wool, gaining a negative charge).

b. What is the charge on the piece of wool?Positive (higher on series weaker hold will lose electrons and become positive).

You now take the charged ebonite rod and touch it to a neutral pith ball (CHARGING BY CONTACT)

c. Draw a picture of the charged ebonite rod touching the neutral pith ball. Draw an arrow to show how the charges would move. Which charge moves (negative or positive)?

Negative charges (electrons) move from the negatively charged ebonite rod into the neutral pith ball.

d. What is the charge on the ebonite rod after contact? Negative (not all of the extra electrons will move into the pith ball)

e. What is the charge on the pith ball after contact?Negative

f. What would happen if you grounded the charged pith ball?The extra electrons in the charged pith ball would move into the ground, causing the pith ball to become neutral again.

3.a. What meter do you use to measure voltage? Voltmeter

Show how you would connect this meter to measure the voltage drop across bulb A in the series circuit.

Voltmeter gets connected in parallel across a battery or load.

- - - - +- + --+ +

- + -

V

b. What meter do you use to measure current? AmmeterShow how you would connect this meter to measure the current passing through bulb B in the parallel circuit.

Ammeter gets connected in series (along the path).c. Refer to the circuits above to fill in the following chart. Assume all bulbs are identical in circuit #3.

Voltage at bulb A Voltage at bulb B Current at bulb A Current at batteryCircuit #1 6V ___ 0.5 A 0.5A

Circuit #2 2V 4 V 0.25A 0.25A

Circuit #3 3V 3 V 0.5A 1A

4. Fill in the following tableSymbo

lUnit(s) Definition

Current I Amps (A) The rate of flow of electronsVoltage V Volts (V) The amount of energy lost or

gained by electrons between 2 points in a circuit.

Resistance

R Ohms (Ω) Ability to slow down the flow of electrons.

Electrical Energy

E Watt-hour (Wh)Kilowatt-hour (kWh)Joule (J)

Electrical energy is the energy transferred to a load by moving electrons.Energy is the ability to do work.

Power P Watt (W)Kilowatt (kW)

The rate at which energy is being used.

5. Draw a circuit diagram that shows 2 lights, and 1 motor powered by a 3-cell battery. The lights are connected in series and the motor is connected in parallel to both light bulbs. There are two switches. One controls the motor and is currently in the off position. The other switch controls both lights and is currently in the on position.

Your circuit diagram must include proper symbols for closed and open switches and light bulbs, and the battery must have 3 cells (3 long bars, alternating with 3 short bars).

FOR THE PROBLEMS BELOW, PRACTICE USING THE GUESS METHOD AND SHOWING ALL WORK!

6. A coffee grinder has a resistance of 100Ω and has a current of 1.20 A flowing through it. What is the voltage?

V = 120 V Use V = I x R7. A flashlight is powered by a 6.0 V battery and draws a current of 0.25 A.

a. What is the resistance of the flashlight?R = 24 Ω Use R = V / I

b. What is the power of the flashlight?P = 1.5 W Use P = I x V

c. How much energy does the flashlight use if it is turned on for 2 hours?P = 3 Wh Use E = P x Δt or use E = I x V x Δt

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8. Calculate the energy released by a hair dryer when it is used for 30 min (0.5 hours). It was plugged in to a 120V outlet and draws a current of 15.0 A.

E = 900 Wh Use E = I x V x Δt (Δt should be in hours)

9. A microwave has a power rating of 950W and is plugged into a 120 V outlet.a. How much current does the microwave draw?

I = 7.9 A Use I = P/Vb. How much energy does the microwave use if it is turned on for 15 min in order to heat up

leftovers? (give your answer in Watt-hours and Joules)E = 237.5 Wh (Δt is 0.25 h)E = 855 000 J (Δt is 900 s)You can also use 3600 J/ Wh to convert your first answer.

Use E = P x Δt or use E = I x V x Δtc. In part b) above, you calculated how much electrical energy the microwave used to heat up the

leftovers. However, only 750 000J of the electrical energy was converted to useful heat energy; the rest of the energy was lost. Use this information to calculate the percent efficiency of the microwave.% eff = useful output energy / input energy x 100

% efficiency is 87.7% ( 750 000 J / 855 000 J x 100)10. A dishwasher has a power rating of 1500 W. A cycle takes 90 minutes. If a dishwasher is run once a

day at off-peak times, how much does it cost to wash dishes each week? The rate charged at off-peak times is about 8¢/kW·h.

P = 1500W = 1.5 kW

Δt = 90 minutes daily for one week = 1.5 hours x 7 days = 10.5 h

E = P x Δt = 15.75 kWh

Cost = E x rate = 15.75 kWh x 8¢/kW·h = 126¢ = $1.26