The correct answer is A, according to the answer key. Please explain why it is A

Question

The correct answer is A, according to the answer key. Please explain why it is A and why it is not B. I had chosen choice B.

A system consists of a gas contained in a thin balloon. If the balloon deflates as the temperature of the gas changes from 90 °C to 25 °C, then (A) Heat is transferred out of the system and work is (B) Heat is t (C) Heat is transferred into the system and work is done (D) Heat is transferred into the system and work is done done on the system. done by the system. on the system. by the system. out of the system and work is

Explanation / Answer

When balloon deflates heat transferred out of the system and in this process work is done on the system, because,

Work done = PdV

Here temperature of the gas decreases from 90 to 25° C,

and since PV = nRT

So, Pressure and volume of the gas also decreases. (because Temperature, pressure and volume are directly proportional to each other).

ie dV = V2 - V1 = negative quantity (Because V2<V1)

So, W = P(-dV)

W= negative quantity...... ( because dV is negative)

since work is negative which means work is done on the system.

Therefore option A is the correct choice.

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