SBI 3AI - Mrs. Van Es
|SNC 1DI: Physics (Electricity) Exam Review |
The electrostatic series will be given on the exam, along with the formulas in the box below:
I = V/R E = V x I x Δt P = E/Δt P = I x V Cost = E x rate % eff = output energy x 100
Input energy
I - __________________ V - ____________________ R- __________________
Δt - _________________ E - ____________________ P - __________________
Equation Triangles (not given on exam):
Major Topics to Know:
• Charging by friction, contact, and induction
• Circuit Diagrams
• Electricity problems (using formulas above)
|Charging by Friction (see p. 275) |Charging by Contact |Charging by Induction |
|Glass window is rubbed with a cotton cloth. |Neutral pith ball touched with negative rod. |Positive rod is held near a neutral pith |
| | |ball that is being grounded. |
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|Charge on glass: __________________ |Charge on pith ball: ______________ | |
|Charge on cotton: _________________ |Charge on rod: __________________ |Charge on pith ball: ______________ |
| | |Charge on rod: __________________ |
Circuit Diagram Example:
Draw a circuit diagram of a 4 cell battery (cells in series) and two light bulbs connected in series. A motor and a resistor are connected in parallel with the bulbs and with each other. Add two closed switches. One switch will turn the entire circuit on and off, and another switch will only turn the light bulbs on and off. Add an ammeter to measure current through the resistor, and a voltmeter to measure the voltage drop across the motor.
|You try! |Right answer: |
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Practice Questions – Electricity
1. To protect against electrocution, copper wiring should be covered with:
a. aluminum c. carbon
b. rubber d. nickel
2. Static electrical charges are placed on an object when it is:
a. rubbed with another object made of the same material
b. held near a magnet
c. connected to a battery in a circuit
d. 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. neutral
b. positive d. it depends on how many electrons move
5. When a positive rod is used to charge a neutral ball by induction there is a flow of:
a. electrons from the + rod into the ball
b. electrons from the ball into the + rod
c. electrons from the ground into the ball
d. electrons from the ball into the ground
6. Which two units are both used to measure electrical energy?
a. watt and ohm c. ampere and volt
b. 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 resource
b. radioactive waste d. both B and C are correct
Practice Problems – Electricity
1. State the Law of Electric Charges.
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?
b. What is the charge on the piece of wool?
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)?
d. What is the charge on the ebonite rod after contact?
e. What is the charge on the pith ball after contact?
f. What would happen if you grounded the charged pith ball?
3.
See handout for diagrams
a. What meter do you use to measure voltage? ________________________
Show how you would connect this meter to measure the voltage drop across bulb A in the series circuit.
b. What meter do you use to measure current? ________________________
Show how you would connect this meter to measure the current passing through bulb B in the parallel circuit.
c. Refer to the circuits above to fill in the following chart. Voltage at battery is 6 V.
| |Voltage at bulb A |Voltage at bulb B |Current at bulb A |Current at battery |
|Circuit #1 | |___ | |0.5A |
|Circuit #2 | |4 V | |0.25A |
|Circuit #3 | |3 V | |1A |
4. Fill in the following table:
| |Symbol |Unit(s) |Definition |
|Current | | | |
|Voltage | | | |
|Resistance | | | |
|Electrical Energy| | | |
|Power | | | |
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 turned on.
6. A coffee grinder has a resistance of 100Ω and a current of 1.20 A flowing through it. What is the voltage?
7. 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?
b. What is the power of the flashlight?
c. How much energy does the flashlight use if it is turned on for 2 hours?
8. Calculate the energy released by a hair dryer when it is used for 0.5 hour. It was plugged in to a 120V outlet and draws a current of 15.0 A.
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?
b. 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)
c. 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.
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.
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