In spite of the simplicity of this derivation, the result for td deviates by less than 20% from
the exact solution, except for the asymptotic behaviour at long times and the initial transient
(see figure 2). These greater discrepancies can be understood as follows. Our approximate
solution considers that the heat flux is constant (equation (14)), which results in a constant rate
of temperature diminution (equation (16)). However, since the temperature is uniform inside
the plate at t D 0 and t D1, no heat arrives at the centre and the rate of temperature change is,
consequently, zero at these times. Now, one may generalize equation (16) to other geometries