Question

Why is Q = ∆U (heat = change in thermal energy)?

Answer 1

From the first law of thermodynamics,

`delQ = delU + delW`

Where `delQ`, `delU` and `delW` are differentials in heat, internal energy and work respectively.

Then if there is no work done by/upon the system, `delW = 0`

Then the above relationship reduces to `delQ = delU` and heat transfer is equal to change in internal energy of the gas.