# Answer to Question #2978 in Molecular Physics | Thermodynamics for Jack Tredrea

Question #2978

What is the equation for work done in a reversible isothermal process (processes 1 to 3 in this case)? My course tutor wrote down (P3V3)ln(V1/V3), what is this?

Expert's answer

The expression of work for that process

where V

From the equation of state, if our substance is a gas we can get an approximate value of the expansion work using the ideal gas equation of state

Since T=const, we can say that

also from the properties of ln-function

So we have

**A=-nRT*ln(V**_{3}/V_{1})where V

_{1}-the initial volume, V_{3}-the final volume.From the equation of state, if our substance is a gas we can get an approximate value of the expansion work using the ideal gas equation of state

**pV=nRT**Since T=const, we can say that

**nRT=p**_{3}V_{3}also from the properties of ln-function

**ln(V**_{3}/V_{1})=-ln(V_{1}/V_{3})So we have

**A=p**that's exactly what ur teacher wrote._{3}V_{3}*ln(V_{1}/V_{3})
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