4.4 The Heating Effect of Current
While building the electromagnet, you may have noticed the wire ends getting warm. That is the second hidden power of electric current at work.
Activity 4.5 — Feel the wire warm up Stretch a thin nichrome wire between two nails in a simple circuit. Touch it with the switch OFF — cool. Switch ON for about 30 seconds, switch OFF, and touch it briefly: it is now distinctly warm. (Never hold a current-carrying heating wire — touch only for a moment, under a teacher’s supervision.)
Why does the wire heat up?
Resistance Turns Electricity into Heat As current flows through a conductor, it meets some opposition called resistance. Pushing current against this resistance converts some electrical energy into heat energy, so the conductor gets hot — the heating effect of electric current. Different materials resist differently: nichrome offers much higher resistance than a copper wire of the same size, so for the same current it gets far hotter. That is why heaters use nichrome, not copper.
How much heat is produced depends on several things: the material of the wire, its thickness and length, the amount of current (more cells → more current → more heat), and the duration the current flows. (A 2-cell battery heats the same wire more than a single cell in the same time.)
Where we use it
Many everyday appliances run entirely on the heating effect. Each contains a coil or rod of high-resistance wire called a heating element — sometimes visible, glowing red-hot.
| Appliance | What the heating element does |
|---|---|
| Electric room heater | Warms the air |
| Electric stove / kettle | Cooks food / boils water |
| Electric iron | Presses clothes |
| Immersion rod | Heats water |
| Hair dryer | Warms the airflow |
| Incandescent lamp | Filament glows white-hot to give light |
Exam Tip & Safety The heating effect can be a problem too: current wastes energy as heat in transmission wires, and overheating can melt plugs and sockets or start fires. That is why household circuits use correctly-rated wires, plugs and sockets and safety devices like fuses. In industry, the same effect powers high-temperature furnaces that melt and recycle scrap steel.
Two effects, two families of devices. Sort each device or phenomenon below by the effect it relies on — the magnetic effect or the heating effect.