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If you do a number of things to a sample of gas, one after the other you could end up with a graph like this:


To deal with this and work out what's going on, consider each process separately.

For example: process 1 is constant pressure expansion (as its volume goes from small to large).

Process 2 is constant volume cooling (its temperature drops - see constant temperature lines).

Process 3 is constant pressure contraction (volume decrease).

A cycle of processes is one in which you eventually returns to the starting point. For example:


In a cycle of processes, the following rules apply:

  1. For the complete cycle, ΔU = O

    i.e. there is no overall change in internal energy. From moment to moment you get changes, but when you return to the start point, the internal energy returns to its original value.

  2. the net work done = area enclosed by the cycle.

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