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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?

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  • $\begingroup$ To achieve this one needs the reservoir to be arbitrarily large in size and the process to proceed arbitrarily slowly (often termed "quasi-static"). $\endgroup$ – dmckee Jul 8 '13 at 15:10
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The gas temperature is the same as the temperature of the reservoir

This is exactly why there is energy exchange.

You should think in small steps: The volume is expanding, but the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange.

See Isothermal process for more details.

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  • $\begingroup$ thank you. When I asked the question I thought that the heat exchange is what makes the gas to expand. As I understand it now the gas expansion is what causes the heat exchange. Am I correct now? $\endgroup$ – Molecular Man Jul 9 '13 at 11:08
  • $\begingroup$ 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. $\endgroup$ – Zorich Jul 9 '13 at 12:20

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