Practice Electrical Power in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

The rate at which electrical energy is converted to other forms of energy (heat, light, motion). Measured in watts (W).

Power tells you how quickly a device uses energy — a 100 W bulb converts energy twice as fast as a 50 W bulb.

Showing a random 20 of 50 problems.

Example 1

medium
An electric heater uses 1200 W1200\ \text{W}. How much energy in kJ does it consume over 5 minutes5\ \text{minutes}?

Example 2

medium
A 12 V12\ \text{V} battery drives 6 Ω6\ \Omega and 4 Ω4\ \Omega in parallel. Find the total power delivered.

Example 3

challenge
A 100 kW100\ \text{kW} load is fed by a transmission line with 0.5 Ω0.5\ \Omega total resistance. Compare line power losses if the supply voltage is (a) 1000 V1000\ \text{V} vs (b) 10,000 V10{,}000\ \text{V}.

Example 4

medium
A 12 V12\ \text{V} battery drives a 6 Ω6\ \Omega resistor in series with a 4 Ω4\ \Omega resistor. Find the power dissipated in the 6 Ω6\ \Omega resistor.

Example 5

hard
If both the current and resistance through a device double, what factor does the power change by?

Example 6

easy
What is the difference between power (watts) and energy (joules)?

Example 7

easy
A 3 Ω3\,\Omega resistor carries 22 A. Find the power dissipated.

Example 8

medium
Two resistors 2 Ω2\,\Omega and 4 Ω4\,\Omega are in parallel across 88 V. Which dissipates more power?

Example 9

easy
A 6 Ω6\,\Omega resistor has 1212 V across it. Find the power.

Example 10

easy
Write the three equivalent formulas for electrical power.

Example 11

hard
A heater is rated 1500 W1500\ \text{W} at 230 V230\ \text{V}. What current does it draw, and what is its resistance?

Example 12

easy
A 20 Ω20\ \Omega resistor has 10 V10\ \text{V} across it. Find the power.

Example 13

hard
A motor runs at 80%80\% electrical efficiency. If it draws 5 A5\ \text{A} at 240 V240\ \text{V}, what mechanical power does it deliver?

Example 14

medium
An electric kettle rated at 2000 W2000 \text{ W} operates for 3 minutes3 \text{ minutes}. How much energy does it use in kJ?

Example 15

medium
A 1500 W1500\ \text{W} microwave runs on 120 V120\ \text{V}. Find (a) the current it draws and (b) its resistance.

Example 16

medium
A kettle is rated at 2000 W2000 \text{ W} and operates on a 250 V250 \text{ V} supply. (a) What current does it draw? (b) What is its resistance? (c) How much energy does it use in 3 minutes3 \text{ minutes}?

Example 17

easy
If the voltage across a resistor doubles (resistance constant), what happens to the power dissipated?

Example 18

easy
A 10 Ω10\ \Omega resistor carries 3 A3\ \text{A}. Find the power dissipated.

Example 19

medium
An appliance is rated 0.50.5 kW. How much energy in kilowatt-hours does it use in 33 hours?

Example 20

challenge
A 1212 V battery drives a 4 Ω4\,\Omega resistor in series with a 2 Ω2\,\Omega resistor. Find the power dissipated in each resistor.
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