Question

Question #f5afc

Answer 1

When a gas undergoes an expansion without energy being supplied from outside, the energy required for the expansion to take place comes at the expense of the internal energy of the gas.

From the first law of Thermodynamics,

`dQ = dU + dW`

For no external energy supplied `dQ = 0`

Thus, `dW = -dU`

Hence, as the gas performs work, there is a decrease in internal energy of the gas.

Now the internal energy is temperature dependent as such that increase of either increases the other.
Hence, a decrease in internal energy means a decrease in temperature and hence a cooling.