Unit mass m of liquid with volume V1 completely changed into gas with volume V2 at constant external pressure P and Temperature T. If latent heat of gas is L then the increase in internal energy of the gas is:
Correct answer: C. L- P(V2-V1)
- A. L
- B. Zero
- C. L- P(V2-V1)
- D. P(V2-V1)
Explanation
According to the first law of thermodynamics,∆U= Q - W -------- (1)where ∆U is change in internal energyQ is heatand W is work doneSince the mass of liquid is converting into gas,Q = m x Lwhere Q is the heat energyL is the latent heatand m is the massThe formula for work done can be simplified as:W = P∆Vwhere W is work doneP is pressureand ∆V is the change in volumeW = P (V2-V1)Putting both these formulas in (1)∆U= mL - P(V2- V1)Since mass provided is one, the equation becomes:∆U= L - P(V2- V1)
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About First Law of Thermodynamics
The first law relates heat supplied, work done and the change in internal energy through energy conservation. Problems use sign conventions and apply the law to isothermal, adiabatic, isobaric and isochoric processes. Internal energy is a state function, while heat and work depend on the path followed.
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