Show that the external work done by a gas in an isothermal expansion is equal to the heat supplied to the gas.

Explanation

For an ideal gas, the internal energy depends only on its temperature. For a real gas also, the change in internarial energy at constant temperature is negligible. So for an isothermal expansion (T = constant), `U_(f) = U_(i)`. Then, from the first law of thermodynamics,
`U_(f)-U_(i) = Q-W or, 0 = Q-W or, W = Q` This means that the work done by the gas is equal to the heat supplied to it.