Heat Transfer
(a) Heat transfer occurs spontaneously from a hot object to a cold one, consistent with the second law of thermodynamics. (b) A heat engine, represented here by a circle, uses part of the heat transfer to do work. The hot and cold objects are called the hot and cold reservoirs. Qh is the heat transfer out of the hot reservoir, W is the work output, and Qc is the heat transfer into the cold reservoir.
Source
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"OpenStax College, College Physics. February 13, 2013."
http://cnx.org/content/m42234/latest/?collection=col11406/latest
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