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since the volume V is fixed in a canonical ensemble I'm a bit confused about the fact, that the pressure is calculated as the derivation of the internal energy U with respect to the volume V.

Sure, P = dU/dV comes from dU = dQ + dW = tdS - pdV + .... But what does it mean to derivate with respect to the volume, when the volume of the system can't be changed, since it is fixed. (by the way: when the volume is fixed, pdV= p*0 =0, so p is "undefined", since it can take any value.)

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derivator

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# On the notion of pressure in the canonical ensemble

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