Where the energy goes
Show on a big screenHow much heat does a current make?
- Two threads, each on its own 6 V cell.
- The 6 Ω thread carries 1 A.
- In 10 s it makes H = 60 J. The 3 Ω thread carries 2 A and makes H = 120 J.
Step through 1 A, 2 A and 3 A. Watch the heat.
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The heat is current squared times resistance times time. Squared means multiplied by itself. A thread here is a thin heating wire.
New word
Heating effect — energy turns to heat as current flows.
The rule
A current heats a wire as it flows. Heat equals current squared times resistance times time. Keep resistance and time the same. Double the current and the heat is four times as much.
H = I² × R × t
H is the heat, in joules (J). I is the current, in amperes (A). R is the resistance, in ohms (Ω). t is the time, in seconds (s).
The 6 Ω thread: H = 1² × 6 × 10 = 60 J. The 3 Ω thread: H = 2² × 3 × 10 = 120 J.
Both cells push 6 V. The 3 Ω thread has the smaller resistance. So the same push drives a bigger current through it. That thread makes more heat. Compared with the 6 Ω thread, the current doubled. The resistance halved. So the heat doubled, not four times.
A fuse is a wire placed in series with a device. Its metal melts at a low temperature. Too much current heats it until it melts. The circuit breaks. The extra current stops.
Try the lab to finish this lesson.