Mains Electricity » The Heating Effect, Fuses and Circuit Breakers

Why a current makes things hot

Touch the charger of a laptop after an hour and it feels warm. Whenever there is a current in something that has resistance, that thing heats up. This is called the heating effect of a current, and it happens in every resistor, including an ordinary metal wire.

To see why, picture the inside of a metal wire. The metal is made of ions held in fixed positions, with electrons that are free to move between them. A current in the wire is a flow of these electrons. The explanation has four steps:

1. The supply makes the electrons move along the wire.
2. The moving electrons collide with the ions of the metal.
3. In each collision the electron transfers energy to the ion, so the ions vibrate more.
4. The energy in the thermal store of the wire increases, so its temperature rises.

In energy terms, energy is transferred electrically from the supply to the thermal store of the resistor. The hot resistor then heats whatever is around it.

A larger current gives more heating, because more electrons pass each second, so there are more collisions.

Key terms:

• Heating effect: the rise in temperature of a resistor when there is a current in it.
• Heating element: a length of wire with a high resistance, designed to get hot when there is a current in it.
• Fuse: a thin piece of wire that melts and breaks the circuit if the current is too large.
• Circuit breaker: an automatic switch that opens and breaks the circuit if the current is too large.
• Live wire: the mains wire that brings the current from the supply to the appliance.

Putting the heating effect to work

Many appliances at home use the heating effect. Most contain a heating element. The same current passes through the connecting cable and the element. The cable has a low resistance and stays cool, but the element has a high resistance, so that is where the energy is transferred.

Appliance	What the heating effect does
Kettle	An element in the base heats the water
Toaster	Wires glow red hot and brown the bread
Electric heater	A hot element warms the air in the room
Oven	Elements heat the air inside to cook food
Hairdryer	A fan blows air over a hot element
Iron	An element heats the flat metal plate
Filament lamp	A very thin wire gets so hot that it glows and gives out light

In an exam, the same four-step explanation works for all of them. Only the name of the appliance changes.

When the heating is not wanted

The heating effect does not switch off in the places where we do not want it. Cables, plugs and sockets all have some resistance, so they warm up a little too. Normally this is too small to notice. It becomes a danger when the current is too large.

So every mains circuit needs something that cuts the current off before the cables overheat. That is the job of fuses and circuit breakers.

Fuses

A fuse uses the heating effect to do its job. Inside a small tube is a thin piece of wire, and the whole current of the appliance passes through it. Here is what happens when something goes wrong:

1. A fault or an overload makes the current too large.
2. The thin fuse wire heats up.
3. The wire melts.
4. This leaves a gap, which breaks the circuit.
5. The current stops, so the cable and the appliance cannot overheat or catch fire.

Each fuse has a rating, such as 3 A or 13 A. This is the current it can carry before it melts. How to pick the right rating is covered in Electrical Power and Choosing Fuses.

The fuse is always fitted in the live wire, the wire that brings the current from the supply. When the fuse melts, the appliance is cut off from the supply. If the fuse were in another wire, the appliance would stop working but would still be connected to the supply, which is dangerous for anyone who touches it.

A fuse works only once. After it has melted ('blown') it must be replaced with a new one, and the fault should be found first.

Circuit breakers

A circuit breaker does the same job in a different way. It is an automatic switch. If the current gets too large, the switch opens ('trips') and breaks the circuit. Modern homes have a row of them in a box near the electricity meter, one for each circuit in the house.

Circuit breakers have two big advantages. They switch off very quickly, much faster than a fuse wire can melt. And they can be reset: once the fault is fixed, you just push the switch back on.

	Fuse	Circuit breaker
What it is	A thin wire	An automatic switch
What happens when the current is too large	The wire heats up and melts	The switch opens (trips)
Speed	Slower: the wire needs time to heat and melt	Very fast
Afterwards	Must be replaced	Can be reset
Cost	Cheap	Costs more

Both protect the device, because its cable and parts cannot overheat. Both protect the user, because they prevent fires. The circuit breaker protects the user better because it acts faster. Fuses and circuit breakers also work together with the earth wire, which you will meet in Insulation, Double Insulation and Earthing.

Exam-style question

A toaster is connected to the mains supply. Its plug contains a fuse.

(a) Explain why the wires inside the toaster get hot when there is a current in them. [3 marks]

(b) State which wire in the plug the fuse is connected to. [1 mark]

(c) A fault makes the current in the toaster much larger than normal. Describe how the fuse protects the toaster and its cable. [3 marks]

(d) The circuit for the kitchen sockets is also protected by a circuit breaker. Give two advantages of a circuit breaker compared with a fuse. [2 marks]