Question

According to first law of thermodynamics, heat supplied is used to do work and to change internal energy. But for Carnot engine, we see the heat supplied us used to do work and the remaining is discarded to the sink.Then what about heat being used to do internal energy?

Answer 1

The processes in Carnot cycle are isothermal process and adiabatic process.

In Isothermal process the temperature is constant.

The internal energy is a state function dependent on temperature. Hence, the internal energy change is zero.