Melt some ice with the help of an electrical heater, and collect the water over a measured length of time. Calculate the power input from the voltage and current and so calculate the latent heat of fusion. Repeat the experiment at a different power rate to cancel out errors due to heat absorbed from the surroundings.
Electrical methods to measure the latent heat of melting of solids are the easiest because the heater can be controlled very accurately.
Notice that both the ice and the water collected below are at 0°C. The energy input from the heater (and the surroundings) is only being used to melt the ice, not to warm the water up. Heat in + gains = L m where L is the latent heat and m is the mass melted (gains (or little g for short) is the heat gained from room)
(Power × time) + g = L m or V I t + g = L m
Dividing both sides by t we get (VI) + G = L (m/t). m/t could also be written a dm/dt, it is equal to the rate of melting. G is the rate of heat gained from room (= g/t) which should be roughly constant.
In this last equation we have two unknowns: L and G. We wish to find L but do not normally care about G. If we repeat the experiment twice, we can eliminate G and find L.
Better still: If we repeat the experiment for many different power inputs, we can plot a graph of Power (Y axis) against dm/dt (X axis). The gradient will be a more accurate value for L, and the intercept will be the negative of G.
Heat 15.10
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