First slide

First law of thermodynamics

Question

In a thermodynamic process, a system absorbs 2 kilo calorie of heat and at the same time does 500 J of work. What is the change in internal energy of the system :

Easy

A

–500 J
B

500 J
C

7900 J
D

8900 J

Solution

Since $dQ = dU + dW$
$∴ dU = dQ - dW .$
Here $dQ = 2000 cal. = 2000 \times 4 .2 = 8400 J$ and $dW = 500 J$
$∴ dU = 8400 - 500 = 7900 J$
Thus, correct choice is (3).

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