# Thermodynamic Cycles and Thermal Reservoirs

In Lesson 4F we learned that thermodynamic cycles consist of a system, a cold reservoir, and a hot reservoir.
Reservoirs are regions outside the system that are so large that their intensive properties remain constant.
Thermal Reservoirs are bodies that can exchange an infinite amount of heat with the system. The temperature of a thermal reservoir never changes.
The system transfers heat to a thermal reservoir called a heat sink.
A heat source is a thermal reservoir that transfers heat to the system.
Examples:
Earth's atmosphere, large bodies of water, vapor condensing at a constant pressure
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### Ch 6, Lesson B, Page 2 - Thermodynamic Cycles and Thermal Reservoirs

• Here is a quick review of some key concepts from lesson 4F.
• We begin with our heat engine, or tie-fighter, diagram.
• Recall that reservoirs can give and receive heat, but their temperatures never change.
• A reservoir that provides heat is often called a heat source.
• A reservoir that absorbs heat is often called a heat sink.
• The hot reservoir in our tie-fighter diagram is a heat source and the low temperature reservoir is a heat sink.
• Reservoirs are usually LARGE objects such as the earth’s atmosphere or a lake or ocean.
• But some small systems can act like reservoirs as well.
• Consider saturated steam flowing through a pipe.
• We can remove heat from the steam, but its temperature doesn’t change.
• The energy comes from the latent heat of vaporization.
• We can imagine other thermal reservoirs that use melting and sublimation to maintain a constant temperature as well.
• But, they are not as common.
• OK, now we are up to speed on thermal reservoirs, so let’s get to the interesting part.
• What is going on inside the system ?!