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Unformatted text preview: le process. Equation 7–6 is particularly useful for determining the entropy changes of thermal energy reservoirs that can
absorb or supply heat indefinitely at a constant temperature.
Notice that the entropy change of a system during an internally reversible
isothermal process can be positive or negative, depending on the direction of
heat transfer. Heat transfer to a system will increase the entropy of a system,
whereas heat transfer from a system will decrease it. In fact, losing heat is the
only way the entropy of a system can be decreased.
EXAMPLE 7–1 Entropy Change during an Isothermal Process A piston-cylinder device contains a liquid–vapor mixture of water at 300 K. During a constant-pressure process, 750 kJ of heat is transferred to the water. As a
result, part of the liquid in the cylinder vaporizes. Determine the entropy change
of the water during this process. T = 300 K = const. SOLUTION We take the entire water (liquid ∆Ssys = vapor) in the cylinder as the system (Fig. 7–4). This is a closed system since no mass crosses the system
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This document was uploaded on 11/28/2012.
- Spring '09