Question
State first law of thermodynamics for isothermal process.
Solution
The first law of thermodynamics states that, when some quantity of heat (dQ) is supplied to the sytem, then the quantity of heat absorbed by the system (dQ) is equal to the sum of increase in the internal energy of the system (dU) due to rise in temperature and the external work done by the system (dW) in expansion.
i.e., dQ = dU + dW
For isothermal process,
dQ = 0 + dW
This implies,
dQ = dW
i.e., dQ = dU + dW
For isothermal process,
dQ = 0 + dW
This implies,
dQ = dW
