Science & Technology in Action

11th Edition

Understanding electricity


Volts, amperes and watts are related as follows: watts = volts × amperes. This lesson explains the meaning of these units and why they are related in this way. It also highlights how the unit of electric charge (the coulomb) relates to volts and amperes.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Common units 
The labels on electrical devices usually show one or more of the following symbols: W, V, A and maybe Hz. But what do they mean and what information do they provide? Manufacturers use these symbols to inform users so that they can operate appliances safely. In this lesson we will explore the meaning of the symbols and the quantities they represent and show how to interpret them correctly. 

Watts, volts and amperes

The labels on typical domestic appliances show values such as the following:

Hair drier 230 V, 750 W

Bulb 230 V, 40 W

Phone charger 100 – 240 V, 200 mA, 50 – 60 Hz

Jug kettle 230 V, 2300 W

The plugs on domestic appliances are commonly marked ‘13 A’.

This lesson will explain what these units mean and how they are related. 

How are the units related?
Volts, amperes (colloquially known as ‘amps’) and watts are related as follows: watts = volts × amperes 

Let’s take the example of the jug kettle listed above; the label shows ‘230 V, 2300 W’. We can use these values to find the electric current (in amperes).

watts = volts × amperes
2300 = (230) (x) 

By re-arranging we can find x.

 x = 2300 / 230 
= 10 amperes 

This indicates that the electric current in the jug kettle is 10 amperes (10 A). So why is the plug on the kettle marked ‘13 A’? 

Safety, fuses and 13 A
Electric current produces a heating effect and excessive currents can cause insulation on wires to melt. In Ireland and many other European countries the maximum current that should be drawn from mains electrical sockets is 13 amperes. In order to prevent damage a fuse is fitted in the plugs of all standard electrical appliances. The current rating of the fuse should exceed the anticipated maximum current for the appliance but must not exceed 13 amperes. If for any reason the current rises above 13 A, then a thin wire in the fuse melts and cuts off the electric current to the appliance. 

What are watts?
Domestic light bulbs are manufactured in a variety of specifications. A bulb marked ‘80 W’ would be expected to be brighter than a bulb of the same kind that is marked ‘40 W’. The letter ‘W’ stands for ‘watt’ and the value indicates how much energy the bulb radiates every second. The watt is the unit of power; one watt means one joule per second.

Lamps of different types, but with the same power, may have different light outputs. For example, a 40 W fluorescent lamp generally gives out more light than a 40 W incandescent bulb. In other words, some devices are more efficient than others.

Quiz questions

  1. Electric charge is measured in coulombs. true
  2. Electric potential is measured in joules. false
  3. One ampere means ‘one coulomb per second’. true
  4. The power of an electric appliance is measured in watts. One watt means one joule per minute. false
  5. The power of an electric appliance can be found by multiplying the electric current by the voltage. true
  6. The maximum current that should be drawn from a domestic mains electric outlet is 13 amperes. true
  7. In Ireland the voltage used for domestic mains is 120 V. false
  8. Electric potential is also known as ‘voltage’. true
  9. The heat transferred to a room by a radiator depends on the rate of flow of water and the temperature difference between the inlet and outlet pipes. true
  10. The heat transferred to a room by an electric heater depends on the rate of flow of charge and the potential difference between the inlet and outlet conductors. true