Resistance, heating and energy transfer

As electrons flow through the metal, they collide with lattice ions. These collisions oppose the electrons’ motion and transfer energy to the ions. The ions vibrate more vigorously, increasing the metal’s thermal energy and temperature. This explains how electrical resistance produces heating.

The important distinction is that electrons move through the conductor, whereas the ions vibrate about their positions. The ions do not travel along the wire with the current.

Energy spreads into the surroundings

Once a resistor is hotter than its surroundings, energy is transferred from it to the surroundings by heating. This can happen through conduction to materials touching it, convection in the surrounding air and radiation.

Electrical energy is therefore dissipated as thermal energy in the surroundings. ‘Dissipated’ means spread out, not destroyed. The energy still exists, but becomes less available for useful energy transfers.

For a given resistance, a greater current produces more heating. For the same current, a greater resistance also produces more heating. Whether this heating is useful or unwanted depends on the purpose of the device.

Useful heating and unwanted heating

In an electric kettle, heating is the intended effect: energy is transferred from the heating element to the water. A toaster uses hot elements to toast bread, while an electric heater transfers energy to warm a room. Here, electrical resistance enables a useful energy transfer.

In connecting wires, however, heating is usually unwanted. Energy transferred to the wires and their surroundings is not available for the appliance’s intended purpose, reducing its efficiency. Excessive heating can also cause burns or start a fire, particularly if too much current flows.

The same physical process can therefore be an advantage in a heating element and a disadvantage in the wires supplying it.

Reducing unwanted heating — Higher tier

Using low-resistance connecting wires reduces unwanted heating for the current they carry. Less energy is transferred to the wires and more is available for the intended component. Copper is commonly used for wiring because it has relatively low resistance and is fairly cheap. Materials with still lower resistance exist, but can be more expensive.

Reducing the current through a wire also reduces its heating. However, simply reducing the total resistance of a circuit does not guarantee less heating: this can increase the current. The comparison ‘lower resistance means less heating’ must be made for the same current.

• Useful: heating elements in kettles, toasters and electric heaters.
• Unwanted: heating in connecting wires reduces efficiency; excessive heating risks burns and fires.

Reducing heating — Higher tier

• Use low-resistance wires, such as copper: less heating for the same current.
• Reduce the current where possible.
• Lowering total circuit resistance can increase current, so it does not automatically reduce heating.