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NEET levelSingle correct optionDifficulty 1 of 3td001

For the reversible isothermal expansion of one mole of an ideal gas, which pair of relations is correct? (IUPAC convention: $w$ is work done on the system.)

A $Δ U = 0$ and $q = - w$
B $Δ U = 0$ and $q = w$
C $q = 0$ and $Δ U = w$
D $Δ U = q$ and $w = 0$

Answer: A

For an ideal gas

U

depends only on

T

, so at constant

T

,

Δ U = 0

. The first law

Δ U = q + w

then gives

q = - w

. The gas absorbs heat equal to the work it does.

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