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4.4

## University of Liverpool

Skip to 0 minutes and 4 secondsClausius defined change in entropy as being equal to heat supplied reversibly divided by temperature. When we talk about heat being supplied reversibly, we simply mean that there's an infinitesimal difference in the temperature of the system and the surroundings. So we only move a small amount of heat, and that makes it reversible. If we remove that little bit of reversibility, we can express the second law of thermodynamics as being the change in entropy being greater than or equal to the heat supply divided by the temperature at which it's supplied. We can think of entropy as being associated with disorder. So a high entropy environment would be, for instance, a disorderly arrangement of matter and energy in a gas.

Skip to 0 minutes and 47 secondsWhereas a low entropy environment would be the orderly arrangement of matter in a solid or in a crystal. Let me give you an analogy for the entropy change that might occur in different environments. So we think of a sneeze in a quiet library like this, where it's nice and quiet and peaceful. Then the sneeze has a big impact on the environment. [SNEEZE]

Skip to 1 minute and 11 secondsWhereas if we were to sneeze out in the busy street, it would hardly be noticed.

Skip to 1 minute and 21 secondsSo if we think of the sneeze as being the disordered heat transfer, and the noise level in the library being a low temperature environment, compared to the noise level in the street being a high temperature environment, then the in change entropy is our change in heat transfer, which is constant, that's the sneeze, divided by the temperature. Small t for the library, so you get a big disturbance. Large t for the street, and so you get a small disturbance. So you can think of the sneeze in a library and in a high energy environment outside as being an analogy for the entropy change that occurs when we move heat around.

# Sneezing in the library

In thermodynamics when heat transfer occurs entropy is created and is:

the amount of heat transfer that occurs divided by the temperature at which it happens.

Eann uses an analogy of sneezing in the library or a busy street to explain entropy.