# Entropy and heat transfer

1. Mar 6, 2014

### kittu1421

hi to everybody out there,
entropy as i know of now is associated with heat which is basically energy in transit(for a rev. process it is indicative of unavailable part of energy) having said that what is meant by entropy change for a system as heat flows into or out of it(for instance consider a thermal reservoir if Q heat leaves the reservoir at constant temp T the change in entropy for the reservoir is Q/T what is the meaning of this in terms of heat that has left the reservoir and why is it called the entropy change for "reservoir".

2. Mar 6, 2014

### ShayanJ

Entropy itself is usually interpreted as a measure of disorder in the system. Taking into account the fact that giving heat to the system means increasing its particles' kinetic energies which makes them jiggle more, you can think of an entropy transfer as an increase of order in one system at the cost of a decrease in order in another system.

3. Mar 6, 2014

### dauto

Also note that if heat leaves a reservoir, the entropy of the reservoir decreases. Q < 0, and Q/T < 0. Entropy measures the number of microscopic states available for a system in a logarithmic scale (sort of) (consistent with its macroscopic description: Temperature, pressure, etc). When heat leaves a reservoir, the number of microscopic states available to the reservoir decreases.