18 Thermodynamics

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18 Thermodynamics

• 1st 2nd laws of thermodynamics
• heat engines
• Lab Quiz Wednesday energy, rotational
  equilibrium
• archimedes principle heat labs covered on final

2
Processes

• processes change characteristics of a system,
  e.g. heat added, temperature rises.
• reversible process (ideal) exactly repeatable
  process e.g. slow heating followed by slow
  cooling.
• irreversible process cannot reverse in exact
  same path, e.g. we cant unburn a match.

3
First Law of Thermodynamics

• heat added to a system can be stored and/or used
  to do work on its environment
• Q DU W

Example 300 J of heat are added to a system and
the system does 100J of work (e.g. a heat
engine). internal energy increases by 300J
100J 200J
4
Second Law of Thermodynamics

• Heat flows spontaneously from a substance at a
  higher temperature to a substance at a lower
  temperature.
• The reverse situation does not occur
  spontaneously (hot drink cools but does not
  reheat)

5
Heat Engines

• partially convert heat into work
• Hot Reservoir (e.g. steam)
• Cool Reservoir (e.g. pool of water)
• Efficiency is work done per unit of input heat (e
  W/QH)
• Ex. A heat engine does 100J of work when given
  400J from the hot reservoir. The efficiency is
  100J/400J 0.25 25.

6
Entropy

• Entropy is a measure of the disorder of a system
• Low entropy (ordered)
• High entropy (disordered)
• Reversible Process Entropy is constant
• Irreversible Process Entropy increases
• 2nd Law Thermodynamics entropy increases for any
  spontaneous processes

7
Summary

• 1st Law is energy conservation
• 2nd Law dictates the direction of natural
  processes, which is equivalent to saying entropy
  always increases
• Heat engines accept heat and output work and
  heat, always increasing entropy