Water Equivalent of a given body is the mass of water for which the rise in temperature is the same as that for the body, when the amount of heat supplied is the same for both.
Let, the mass of a body be m and its specific heat is s.
Hence, the amount of heat required to raise the temperature of the body by t,
H = mst
If this amount of heat is also required for a mass W of water for the same rise in temperature, then
H = mst = W x sw x t
∴ W = ms/sw
So, water equivalent of the body, W = ms/sw
In SI system, the value of sw are 4186 J.kg-1.K-1, so the expression of W in SI unit are:
W = ms / 4186
Hence, heat gained or lost by a body,
H = Wswt
So, heat gained or lost by a body = water equivalent of the body x specific heat of water x rise or fall in temperature
The water equivalent of a body is 10g means that heat required to raise the temperature of the body by 1°C can raise the temperature of 10g of water by 1°C.