A gas contained in a cylinder is filled with a frictionless piston expands against a constant pressure 1 atmosphere from a volume of 4 litre to a volume of 14 litre. In doing so, it absorbs 800J thermal energy from surroundings. Determine ΔU for the process.
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Solution
Given, q=800J ΔV=(14−4)=10 litre w=−P×ΔV=−1×10=−10litre−atm But 0.082 litre-atm=1.987cal So, w=10×1.9870.082=−242.3cal But 1 calorie=4.184J So, w=−242.3×4.184=−1013.7J Substituting the values in the equation ΔU=q+w=(800−1013.7)=−213.7J