Which one of the following equations does not correctly represent the first law of thermodynamics for the given processes involving an ideal gas? (Assume non -expansion work is zero)
A
Adiabatic process: ΔU=−w
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B
Isochoric process: ΔU=q
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C
Cyclic process: q=−w
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D
Isothermal process: q=−w
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Solution
The correct option is A Adiabatic process: ΔU=−w The first law of thermodynamics states that, ΔU=q+w a) In an adibatic process q=0, so ΔU=w. b) In an isochoric process ΔV=0⇒W=PΔV=0. So, ΔU=q c) For a cyclic process ΔU=0. So, q=−w d) For an isothermal process, ΔT=0⇒ΔU=0 Hence, q=−w