Electrical Power Examples: 46 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Electrical Power.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept 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.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Electrical Power asks students to follow the circuit path and identify what quantity changes at each component.

Common stuck point: Students often know a formula related to electrical power but skip the recognition step: Can I identify the circuit path, what quantity is flowing or changing, and which electrical rule links the quantities? That leads to a correct-looking substitution attached to the wrong physical model.

Sense of Study hint: Ask: Can I identify the circuit path, what quantity is flowing or changing, and which electrical rule links the quantities?

Worked Examples

Example 1

easy

A device draws

3 \text{ A}

from a

120 \text{ V}

outlet. What is the electrical power consumed?

Answer

P = 360 W

First step

Use the electrical power formula:

P = VI

Full solution

2
Substitute the given values: $P = 120 \times 3$ .
3
$P = 360 W$

Electrical power is the rate at which electrical energy is consumed. It can be calculated as voltage times current.

Example 2

medium

60 \text{ W}

light bulb operates at

120 \text{ V}

. What is its resistance and the current it draws?

Example 3

medium

1500\ \text{W}

microwave runs on

120\ \text{V}

. Find (a) the current it draws and (b) its resistance.

Example 4

medium

60\ \text{W}

bulb is rated for

120\ \text{V}

but is connected to a

60\ \text{V}

source. Assuming the filament resistance stays the same, what power does it now dissipate?

Example 5

hard

24\ \text{V}

supply drives a

12\ \Omega

resistor and a

4\ \Omega

resistor in series. What fraction of the total power is dissipated in the

4\ \Omega

resistor?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

An electric kettle rated at

2000 \text{ W}

operates for

3 \text{ minutes}

. How much energy does it use in kJ?

Example 2

medium

A kettle is rated at

2000 \text{ W}

and operates on a

250 \text{ V}

supply. (a) What current does it draw? (b) What is its resistance? (c) How much energy does it use in

3 \text{ minutes}

Example 3

easy

Write the three equivalent formulas for electrical power.

Example 4

easy

What is the SI unit of electrical power?

Example 5

easy

A device draws

2

A at

12

V. What power does it use?

Example 6

easy

3\,\Omega

resistor carries

2

A. Find the power dissipated.

Example 7

easy

6\,\Omega

resistor has

12

V across it. Find the power.

Example 8

easy

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

Example 9

easy

100

W bulb runs for

10

s. How much energy does it use?

Example 10

easy

If current through a resistor doubles, what happens to the power dissipated (

P=I^2R

Example 11

medium

9

V battery drives a

3\,\Omega

resistor. Find the power dissipated.

Example 12

medium

1500

W heater runs for

2

minutes. How much energy does it use, in joules?

Example 13

medium

Two resistors

2\,\Omega

and

4\,\Omega

are in series with a

3

A current. Which dissipates more power, and how much total?

Example 14

medium

Two resistors

2\,\Omega

and

4\,\Omega

are in parallel across

8

V. Which dissipates more power?

Example 15

medium

A motor uses

240

W at

120

V. What current does it draw?

Example 16

medium

An appliance is rated

0.5

kW. How much energy in kilowatt-hours does it use in

3

hours?

Example 17

medium

A resistor dissipates

20

W at

4

A. Find its resistance.

Example 18

challenge

12

V battery drives a

4\,\Omega

resistor in series with a

2\,\Omega

resistor. Find the power dissipated in each resistor.

Example 19

challenge

24

V source feeds

6\,\Omega

and

12\,\Omega

in parallel. Find the total power delivered.

Example 20

challenge

A heater is rated

2000

W at

200

V. If the supply voltage drops to

100

V, what power does it deliver (assume constant resistance)?

Example 21

medium

4

A current flows through a

5\,\Omega

resistor. Find the power dissipated.

Example 22

medium

A device uses

60

W at

12

V. What current does it draw?

Example 23

easy

A device draws

5\ \text{A}

from a

9\ \text{V}

source. Find the power consumed.

Example 24

easy

10\ \Omega

resistor carries

3\ \text{A}

. Find the power dissipated.

Example 25

easy

20\ \Omega

resistor has

10\ \text{V}

across it. Find the power.

Example 26

easy

100\ \text{W}

device runs at

120\ \text{V}

. What current does it draw?

Example 27

easy

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

Example 28

medium

12\ \text{V}

battery drives a

6\ \Omega

resistor in series with a

4\ \Omega

resistor. Find the power dissipated in the

6\ \Omega

resistor.

Example 29

medium

12\ \text{V}

battery drives

6\ \Omega

and

4\ \Omega

in parallel. Find the total power delivered.

Example 30

medium

An electric heater uses

1200\ \text{W}

. How much energy in kJ does it consume over

5\ \text{minutes}

Example 31

medium

5\ \Omega

resistor dissipates

45\ \text{W}

. Find the current through it.

Example 32

medium

200\ \text{W}

TV runs for

4\ \text{hours}

a day for

30\ \text{days}

. At $0.15/kWh, what is the cost?

Example 33

medium

0.5\ \text{A}

current flows through a

6\ \Omega

resistor. Find the energy dissipated in

2

minutes.

Example 34

medium

Two identical

60\ \text{W}

bulbs (rated

120\ \text{V}

) are connected in series across a

120\ \text{V}

supply. How much power does EACH bulb now dissipate (assume constant

R

Example 35

hard

9\ \text{V}

battery with internal resistance

1\ \Omega

drives an external

2\ \Omega

load. Find the power delivered to the load and the power dissipated in the internal resistance.

Example 36

hard

A heater is rated

1500\ \text{W}

230\ \text{V}

. What current does it draw, and what is its resistance?

Example 37

hard

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

Example 38

hard

Two resistors

R_1 = 3\ \Omega

and

R_2 = 6\ \Omega

are in parallel across a

12\ \text{V}

source. Which resistor dissipates more power, and what is the ratio

P_1:P_2

Example 39

hard

A motor runs at

80\%

electrical efficiency. If it draws

5\ \text{A}

240\ \text{V}

, what mechanical power does it deliver?

Example 40

challenge

A battery has EMF

\varepsilon = 12\ \text{V}

and internal resistance

r = 2\ \Omega

. What external load resistance

R

maximizes power delivered to the load, and what is that power?

Example 41

challenge

100\ \text{kW}

load is fed by a transmission line with

0.5\ \Omega

total resistance. Compare line power losses if the supply voltage is (a)

1000\ \text{V}

vs (b)

10{,}000\ \text{V}

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Related Concepts

Ohm's Law Voltage Electric Current Power

Background Knowledge

These ideas may be useful before you work through the harder examples.

ohms lawvoltageelectric currentpower