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lecture 4

# lecture 4 - Work(path dependent quantities and functions of...

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Work (path dependent quantities and functions of state) From physics you know that mechanical work = force × displacement Consider gas in equilibrium with the weight of the piston F. Now consider a process where the piston goes down by -dh. The work done by the force F is δ W = -F dh = -p A dh = -p dV The units of work are Joules (1J = 1 N × m). The work done on the system by the medium (me pushing the piston down) has a positive sign. The work done by the system (the gas) is negative and equal to δ W gas = p dV If the gas is expanding, it is doing positive work and the medium negative work. Work is an extensive quantity: δ W = -p dV = -p n d The total work done in a process is: W = - Geometric meaning: the area under the curve p(V). This also shows that the work depends on the path of transformation (i.e., how one goes from V 1 to V 2 ). Unlike this, some quantities do not depend on the path of the process. For example, the change in the volume, V 2 – V 1 depends only on the initial and the final state and not on what one does in between them. This kind of quantities is known as

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