(a)[2]
Two metal spheres are in thermal equilibrium. State and explain the meaning of thermal equilibrium.
(b)
In an electric water heater, water flows through a tube at a constant rate. A heating coil warms the water as it moves along the tube, as shown in Fig. 3.1. The inlet water temperature is $18\,^{\circ}\text{C}$. When the heater power is $3.8\,\text{kW}$, the outlet water temperature is $42\,^{\circ}\text{C}$. The specific heat capacity of water is $4.2\,\text{J g}^{-1}\,\text{K}^{-1}$.
(b(i))[3]
Calculate the flow rate, in $\text{g s}^{-1}$, of water through the tube using the data provided.
(b(ii))[2]
State and explain whether your answer in (i) is more likely to be an overestimate or an underestimate of the flow rate.