**dT/dt = k(T**_{surr}-T)
Says that the

rate at which the

temperature of a

body changes is

proportional to the

difference between the body's temperature and that of the

surroundings. When

**T**_{surr} is taken to be a

constant (and not a

function of time), this law is only an

approximation, since the temperature of the body effects that of the surroundings, not just the other way around. The latter effect is usually

neglected, since the 'surroundings' are generally much larger than the body in question. (The surroundings act as a '

sink'.)

But, nevertheless, this

law is only correct for substantial temperature differences if the

heat transfer is by forced

conduction or

convection.

As with all members of this family of

differential equations, it leads to some sort of

exponential function. In this case, it's the temperature difference which falls

exponentially to

zero.