If a current of 0.3 A is passed through a coil of resistance 418 ohms for 1 min, find the heat energy generated.

The correct answer is B, 540 cal. The heat produced by a current is given by Joule's law of heating, H equals I squared into R into t. Here the current is 0.3 A, the resistance is 418 ohms and the time is 1 minute, which is 60 seconds. So H = 0.09 into 418 into 60 = 2257.2 joule. One calorie equals 4.18 joule, so 2257.2 divided by 4.18 gives 540 calorie. Joule's law says the heat produced is proportional to the square of the current, to the resistance and to the time, which is why doubling the current gives four times the heat. The same law explains the electric iron, the heater, the geyser, the filament of a bulb and the fuse that melts to protect a circuit. A is wrong because 360 calorie would need a current of about 0.245 A for the same resistance and time. C is wrong because it is one tenth of the correct value. D is wrong because 36 calorie would need a current of only about 0.077 A. Exam tip: use H equals I squared Rt, then divide the joule figure by 4.18 to get calories.