Joulette

Meet Joulette, a hot wire who glows because current pushing through her turns into heat.

Electrons moving through a conductor keep colliding with its atoms, handing over energy that becomes random thermal motion. James Prescott Joule found in the early 1840s that the heat produced per second is proportional to the resistance and to the square of the current: P = I²R.

The square matters. Double the current through the same resistor and the heating goes up four times. A 2,000 watt kettle on a 230 volt supply draws about 8.7 amperes, so its element has a resistance of about 26 ohms.

Joule heating is useful in toasters, kettles, electric heaters, hair dryers and fuses, which melt when the current gets too high. It is also a loss: in power lines it wastes energy, which is why electricity is sent at high voltage and low current.

For a component that obeys Ohm's law, I²R equals V²/R and VI. Resistance usually rises as a metal heats, so the power changes as the element warms up.