Entropy

1
Entropy

• Physics 102
• Professor Lee Carkner
• Lecture 8

• Answers to In-class exercises

2

• When water condenses out of the air onto a cold
  surface the entropy of the water,
• Increases, since entropy always decreases
• Decreases, but that is OK since the 2nd law does
  not apply to phase changes
• Decreases, but that is OK since the entropy of
  the air increases
• Increases, since phases changes always increase
  entropy
• Remains the same

3

• Water is heated on a stove. Which of the
  following temperature changes involve the
  greatest entropy change of the water?
• T increased from 20 to 25 C
• T increased from 40 to 45 C
• T increased from 80 to 85 C
• T increased from 90 to 95 C
• All are equal

4

• Consider a box with 2 equal sides and 4 balls
  moving randomly between the two sides. Which of
  the following configurations has the highest
  probability of occurring?
• 5 balls on left side
• 3 balls on left side and 1 on right side
• 2 balls on left side and 2 on right side
• It depends on the velocity of the balls
• All have equal probability

5

• If you put a drop of ink in a bowl of water, what
  happens to the entropy?
• It increases
• It decreases
• It stays the same
• It depends on the temperature of the water
• It depends on the heat transferred

6

• If the ink in the tinted water comes together in
  a dot, what happens to the entropy?
• It increases
• It decreases
• It stays the same
• It depends on the temperature of the water
• It depends on the heat transferred

7
PAL Answers

• 1 kg block of ice (at 0 C) melts in a 20 C room
• DS Sice Sroom
• Sice Q/T mL/T (1)(330000)/(273)
• Sice 1219.8 J/K (heat gained)
• Sroom Q/T 330000/293
• Sroom -1136.5 J/K (heat lost)
• DS 1219.8 1136.5 83.3 J/K
• Entropy increased, second law holds