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Practice – Ohm’s Law and Electrical Power
Directions: For each problem show all work on a separate sheet of paper. This means write the givens, formula,
plug in, answer, units, and circle your answer. Answers are in bold so you can check your work. Assume 120 volts for
anything plugged into a wall.
1) What voltage is required to make 0.25 A flow against 8 Ω of resistance? 2 v
2) What current flows in a circuit with a potential difference of 12v and a resistance of 25 Ω? 0.48 A
3) What is the resistance of a circuit with a voltage of 45v and a current of 5 A? 9 Ω
4) A circuit has 80 Ω of resistance. What voltage is required to push 25 mA through it? (convert mA to A) 2 v
5) A 40v circuit has a resistance of 80 Ω. How much current flows through it? How much power does the circuit use?
a) 0.5 A, b) 20 W
6) 4 A flows through a circuit with a resistance of 8 Ω. What’s the potential difference across the circuit? How much
power is in the circuit? a) 32 v, b) 128 W
7) How much current flows through the filament of a 75 W light bulb plugged in at home? 0.625 A
8) What is the resistance of a 600 W drill plugged into an outlet at home? 24 Ω
9) A walkman uses a standard 1.5v battery. How much resistance is in the circuit if it uses 0.01 A of current? 150 Ω
10) What current goes through a hair dryer plugged in at home if it has a resistance of 25 Ω? How much power does it
use? a) 4.8 A, b) 576 W
11) How much current does a 12 v car battery push through a circuit with a resistance of 10 Ω? 1.2 A
12) How much current does a 900 W microwave use? How much resistance does it have? a) 7.5 A, b) 16 Ω
13) Your fuse panel at home has a 15 A circuit breaker. If more than 15 A try to flow in that circuit at one time, it will
pop the breaker and shut down the circuit. You are cooking hot pockets in a microwave, and your mom turns on the
vacuum cleaner. The microwave uses 1200 watts and the vacuum cleaner has 18 Ω of resistance. If the vacuum
cleaner and microwave are turned on at the same time, will they pop the circuit breaker? What if it’s a 900 watt
microwave? a) yes, b) no
14) A home stereo uses 120 volts to push 5.5 A to run properly. What is the resistance of the stereo? 21.8 Ω
15) If a kitchen appliance uses 3 A, how much power does it need? 360 W
16) What is the current in a microwave that requires 700 W of power? 5.83 A
17) What is the voltage in a circuit that uses 2640 W of power if 11 A of current flows in the circuit? 240 v
18) How much energy is used when a 120 kW appliance is used for 3 hours? (unit for energy is kW-hr) 360 kW-hrs
19) A TV has a power rating of 210 W. If it uses 1.68 kW-hrs of energy, how long has the TV been on? 8 hrs
20) How much does it cost to light six 100 W light bulbs for eight hours if the price of electricity is $0.09 per kilowatt
hour? $0.054
21) An electric clothes dryer uses 4kW of electric power. How long did it take to dry the clothes if electric power costs
$0.09 per kWh and the cost of running the machine was $0.27? 3 hrs
22) What is the resistance of a light bulb that draws 0.5 A of current? What is its power rating? a) 240 Ω, b) 60 W
23) How much current flows through a 100 W light bulb at home? 0.83 A
24) 8 amps of current flow through a hair dryer plugged in at home. How much electrical power does it use? 960 W
25) Compare the energy used by a 100 W light bulb for 10 hours and a 1200 W hair dryer used for 15 minutes?
1000 W/300 W
26) A toaster oven uses 10 A when plugged in at home. How much power does it use? 1200 W
27) A DVD player still uses 10 W of power even when not in use. What current still flows through it? 0.08 A
28) A potential of 25 v is applied to a circuit with a resistance of 25 Ω. How much current flows in the circuit? How
much power is used? How much power would be used if the potential was 100 v? a) 1 A, b) 25 W, c) 100 W
29) A circuit with 47 Ω is connected to a 20 v battery. How much current flows in the circuit? How much power does the
circuit use? a) 0.43 A, b) 8.51 W
30) A car power window draws 1.5 A when connected to a 12 v car battery. Calculate the resistance in the motor.
Calculate the power used by the motor. a) 8 Ω, b) 18 W
31) How much current flows through a 1200 W toaster? How much resistance does it have? a) 10 A, b) 12 Ω
32) An RC car battery is 8.4 v and runs a 20 W motor. Calculate the current and resistance in the motor. 2.38 A, 3.53 Ω