Electricity: Magnetic and Heating Effects
Current makes a wire hot
Investigate why a wire can get warm when current flows through it.
In the electromagnet activity, did the ends of the wire feel warm? Why would that happen?
Try it (Activity 4.5). Fix two nails about 5 cm apart on cardboard and tie a nichrome wire (about 0.3 mm thick and 10 cm long) between them. Complete the circuit with a cell and a switch, switch OFF. Touch the wire. Now switch ON for about 30 seconds, switch OFF, and touch the wire only for a moment: don't hold it. It feels warm.
Why? Current flowing through any conductor meets some opposition, called resistance. The wire pushes back against the flow, a little like a crowd in a narrow lane. Different conductors resist by different amounts: a nichrome wire resists much more than a copper wire of the same size and length.
That resistance turns some of the electrical energy into heat energy. So a conductor carrying current gets hot. This is the heating effect of electric current.
Repeat with 2 cells (only with your teacher watching) and the wire gets hotter: the heat depends on how much current flows.
| The heat in a wire depends on | |
|---|---|
| the material | nichrome heats far more than copper |
| the current | more current, more heat |
| the length | a longer wire resists more |
| the thickness | a thinner wire resists more, and heats more |
| the time | the longer the current flows, the more heat |
This explains the Grade 7 torch bulb that felt warm: an ordinary (incandescent) bulb glows because its thin filament is heated by the current.
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