First slide

First law of thermodynamics

Question

A gas is at one atmospheric pressure with a volume 800 cm³ When 100 J of heat is supplied to the gas , it expands to 1 litre at constant pressure. The change in its internal energy is

Easy

A

80 J
B

- 80 J
C

20 J
D

- 20 J

Solution

$\begin{array}{l} P_{s} = 10^{5} N / m^{2} \\ W = P_{a} (V_{2} - V_{1}) = 10^{5} [1000 - 800] \times 10^{- 8} J = 20 J \end{array}$
By first law of thermodynamics,
$ΔU = Q - W = (100 - 20) J = 80 J$

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