First slide

First law of thermodynamics

Question

Heat of 30 kcal is supplied to a system and 4200 J of external work is done on the system so that its volume decreases at constant pressure. What is the change in its internal energy. (J = 4200 J/kcal)

Easy

A

1.302 × 10⁵ J
B

2.302 × 10⁵ J
C

3.302 × 10⁵ J
D

4.302 × 10⁵ J

Solution

$\Delta Q = 30kcal:\Delta W = - 4200J:\Delta U = ?$
J = 4200J/kcal
$\Delta Q = 30 \times 4200 = 126000J$
Now, $\Delta Q = \Delta U + \Delta W$
$\Rightarrow \Delta U = \Delta Q - \Delta W = 126000 - \left( { - 4200} \right)J$
$\Rightarrow \Delta U = 130,200J$
$= 1.302 \times {10^5}J$

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