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(a) Power is the rate at which heat is transferred and has units of watts (joules per second). In our analogy this is represented by the flow of water (litres per second)

(b) the temperature is the parameter which is adjusted to control the heat flow. In our analogy this is represented by the height of the water in the bath (the height controls the pressure of water and hence the rate at which water leaves the bath through the plug hole).

(c) the internal heat is a measure of the stored energy. In the case of the bath this is represented by the amount of stored water (litres).

We see from this that we are equating joules with litres.

Question 3: In terms of heat (rather than water) how are:


thermal equilibrium


thermal steady state


thermal transient

represented in this analogy?


The University of Birmingham 

Physics and Astronomy Department, The University of Birmingham