Topic 9 - Thermodynamics

Question 1

How are temperature and average kinetic energy mathematically related?

Answer 1

KE = (3/2)kT

Question 2

Explain why the pressure of the air inside the scuba tank increases as the temperature increases. Your answer should refer to the motion of the air molecules. (6 marks)

Answer 2

• As the temperature rises, the (average) kinetic energy of the (air) molecules increases.
• Hence, the mean/average speed of the air molecules increases.
• The (average/mean) change of momentum of air molecules upon colliding with the tank/walls increases.
• The rate of collision of air molecules with the tank/walls rises.
• The rate of change of momentum increases, and thus the force on the tank/walls increases.
• Consequently, the pressure (exerted by the gas) rises, since p = F/A.

Question 3

A teacher used liquid nitrogen to demonstrate low-temperature physics.

The teacher dropped a peeled banana at room temperature into a flask of liquid nitrogen.

Explain why the liquid nitrogen boiled when the banana was submerged in the liquid nitrogen. (2 marks)

Answer 3

• Energy is transferred from banana to liquid nitrogen (Cold -> Hot)
• The molecular potential energy of nitrogen molecules increases (as the
  nitrogen boils)