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Why a Wire Gets Hot: Joule's Heating Effect
When current flows through a conductor, moving electrons keep colliding with the atoms of the metal and hand over energy to them. The atoms vibrate harder and the wire warms up. This is the heating effect of electric current, and it is the working principle of the electric iron, geyser, toaster, room heater, bulb and the fuse.
James Prescott Joule measured it, so the rule carries his name: H = I²Rt. The heat produced depends on three things: the square of the current, the resistance of the wire, and the time for which the current flows.
This short gives you the formula family, the bulb-and-heater logic, and the unit-conversion step (watt-hour to kWh to joule) where most exam marks are lost.
1. If the current through a resistor is doubled while its resistance and the time stay the same, the heat produced becomes:
2. An electric iron of resistance 50 Ω draws 4 A for 2 minutes. The heat produced is:
3. The resistance of a bulb marked 100 W, 220 V is:
4. A 2 kW heater runs for 3 hours a day for 10 days. At ₹5 per unit the bill is:
5. A 60 W bulb and a 100 W bulb (both rated 220 V) are connected in series to a 220 V supply. Which glows brighter?
6. Which material is used for fuse wire?
7. Which of the following is a unit of energy, not of power?
8. The filament of an electric bulb is made of tungsten mainly because tungsten: