(a)[2]
Two metal spheres are in thermal equilibrium. State and explain the meaning of thermal equilibrium.
(b)
An electric water heater has a tube through which water flows at a steady rate. The water in the tube passes over a heating coil, as shown in Fig. 3.1. Water enters the tube at a temperature of $18^\circ\text{C}$. When the heater power is $3.8\ \text{kW}$, the water leaves at $42^\circ\text{C}$. The specific heat capacity of water is $4.2\ \text{J g}^{-1}\ \text{K}^{-1}$.
(b(i))[3]
Use the data to calculate the water flow rate through the tube, in $\text{g s}^{-1}$.
(b(ii))[2]
State and explain whether your answer in (i) is likely to be an overestimate or an underestimate of the flow rate.