Covers: Gas laws, heat transfer, the first law and entropy. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaTemperature measures the average translational kinetic energy per molecule: Kavg = (3/2)kBT. At the same temperature, lighter molecules move faster on average.
Common mistakeThinking that at the same temperature all gases have the same speed, or that heavier molecules have more kinetic energy.
Key ideaPV = nRT, with T in kelvins. Use ratios when the amount of gas stays fixed.
Common mistakeUsing Celsius instead of kelvins.
Key ideaHeat flows from hot to cold by conduction, convection or radiation. The conduction rate is kAΔT/L.
Common mistakeConfusing heat (energy in transfer) with temperature.
Key ideaΔU = Q + W, where W is the work done on the gas. When the gas expands, W is negative (minus the area under the P–V curve); when it is compressed, W is positive.
Common mistakeMixing up the sign of W (work done on the gas versus by it), or forgetting that ΔU = 0 for an isothermal process.
Key ideaThe total entropy of an isolated system never decreases, and heat flows spontaneously from hot to cold.
Common mistakeThinking the entropy of every part must increase. Only the total must not decrease.
This is one of the 26 questions in the Unit 9 quiz.
On the molecular level, why does heating a gas in a rigid container increase its pressure?
Pressure comes from molecules hitting the walls. How does heating change the way they hit?
Answer: A. The molecules hit the walls more often and with more momentum
Pressure comes from molecules colliding with the walls. Hotter molecules move faster, so each collision delivers more momentum and collisions happen more often.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
