Ohm’s Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions, such as temperature, remain constant.

Part of

Since:

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.

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

1.

P = 2.2 kW = 2200 W

V = 240 V

I = 9.17 A

2.

V = 240 V

I = 9.17 A

R = 26.2

The resistance of the kettle’s heating element is 26.2