# Where does additional heat energy come from in isothermal step of Carnot cycle

According to widipedia:

During step 1 (isothermal heat addition or absorption) the gas is allowed to expand and it does work on the surroundings. The temperature of the gas does not change during the process, and thus the expansion is isothermal. The gas expansion is propelled by absorption of heat energy Q1.

The gas temperature is the same as the temperature of the reservoir. But where may heat energy Q1 come from as there is no temperature gradient between gas and reservoir?

• To achieve this one needs the reservoir to be arbitrarily large in size and the process to proceed arbitrarily slowly (often termed "quasi-static"). – dmckee Jul 8 '13 at 15:10

• Yes. But note that your system just passes through some thermodynamic states, and because you control it by volume, You can say that it is the main cause. Physically, all You must have is an isothermal process (at $T_1$), with initial volume $V_1$ and final volume $V_2$, and all events (heat exchange, volume expansion,etc) are equivalently a part of this process. – Zorich Jul 9 '13 at 12:20