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In thermodynamics a heat reservoir is considered as a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it. A thermal(heat) reservoir is that part of environment which can exchange heat energy with a system. It has sufficiently large heat capacity and its temperature is not affected by the quantity of heat transferred to or from it. A thermal reservoir is thus characterized by its temperature which remains constant. The changes that do take place in the thermal reservoir as heat enters or leaves are so slow and so small that processes within it are quasi-static. The reservoir which is at high temperature and supplies heat is known as heat source.