Thermal Physics

Question 1

If you increase the temperature, how does the distribution of molecules’ molecular speeds change? (2)

Answer 1

Peak number of molecules (i.e. with the most frequent speed) decreases as the most frequent speed increases
Since molecules in a gas travel at a range of speeds whereas solids have much similar speeds, the variation in molecular speeds also increases

i.e. graph of number of molecules against speed gets wider and moves to the right

Question 2

Explain how and why a curve of number of molecules against molecular speed changes shape when a substance is heated. (3)

Answer 2

Graph of moves to the right and gets wider but peak decreases.
This happens because the area under the graph represents the number of molecules in the substance, and this is constant asusming no molecules enter or leave the system.

Question 3

For an ideal gas, internal energy is equal to what energy of the atoms?

Answer 3

Kinetic energy - 0 potential energy is assumed between the particles (unless they are colliding?

Question 4

State six assumptions you make when defining an ideal gas.

Answer 4

• Particle collisions with themselves/container wall are elastic (no kinetic energy is lost) and the particles are continuingly colliding with the container walls
• Collisions against the container walls takes a negligible amount of time
• No force of attraction between the molecules (no force unless they are colliding)
• Mass of particles is negligible compared to the mass of the container
• Particles are moving in Brownian motion (i.e. move randomly)
• Gas particles are modelled as point masses that take up no volume
• All gas molecules are identical, and there are a lot of them

Question 5

What sort of value is the root mean square (c_rms)? Why is this relevant?

Answer 5

It’s a mean. When calculating the kinetic energy using c_rms, you are calculating the AVERAGE energy (E-bar)

Question 6

You have a gas of different particles at constant temperature. Compare their kinetic energies. Explain your answer.

Answer 6

All types of particles in a container have the same Ek (may have different m and so v) because they’re all at the same temperature

I think this is correct.