Electrical energy

Since:

{Power} =\frac{\text{energy~transferred}}{\text{time~taken}}

Then:

Energy = power x time

Hence:

Electric energy = electric power x time (t)

Electric energy = VI x t

Electric energy = VI t

This is usually remembered in the form:

Electric energy = ItV

Where:

Electric energy is in joules, J

I = current in amperes, A

t = time in seconds, s

V = voltage in volts, V

Key point

  • Electric power P = VI
  • Electrical energy = ItV

Example

What is the power of a small electric motor if a current of 2 A flows when connected to a 12 V power supply?

P = VI

I = 2 A

V = 12 V

P = 2 A x 12 V

P = 24 W

The power of the electric motor is 24 W.

Question

A light bulb is rated 60 W, 240 V. What does that mean?

When the voltage across the bulb is 240 V the bulb has a power of 60 W.

It converts 60 J of electrical energy into light and heat energy every second.

Question

A kettle has a power of 2.2 kW and is connected to mains voltage of 240 V.

  1. What current flows when the kettle is operating normally?
  2. What is the resistance of the kettle’s heating element?

1.

{I} =\frac{\text{P}}{\text{V}}

P = 2.2 kW = 2200 W

V = 240 V

{I} =\frac{\text{2200 W}}{\text{240 V}}

I = 9.17 A

2.

{R} =\frac{\text{V}}{\text{I}}

V = 240 V

I = 9.17 A

{R} =\frac{\text{240 W}}{\text{9.17 A}}

R = 26.2 \Omega

The resistance of the kettle’s heating element is 26.2 \Omega

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