Gas pressure and temperature GCSE physics lesson: particles, kelvin and p V = constant

Gas pressure: your turn

Show your working on paper. K = °C + 273. For p₁ V₁ = p₂ V₂ the temperature stays the same; use the same units on both sides.

1. Gas particles move fast in ___ directions. Each hit on a wall pushes on it; the force on each square metre of wall is the ___. Heat the gas and its particles move ___ on average.
2. Turn 37 °C, body temperature, into kelvin.
3. A gas is at 200 K. What is that in °C?
4. A sealed bottle of air is moved from a warm room into a fridge. What happens to the pressure inside?
   1. It goes down: the particles move more slowly, so they hit the walls less often and less hard
   2. It goes up: cold air is denser
   3. It stays the same: no air got in or out
   4. It goes down: the particles shrink
5. 1.5 litres of gas at 200 kPa expands, at the same temperature, to 6.0 litres. What is its pressure now, in kPa?
6. A balloon holds 3.0 litres of air at 100 kPa. It is squeezed, at the same temperature, until the pressure is 120 kPa. What is its volume now, in litres?

Gas pressure and temperature: practice

Show your working on paper. K = °C + 273. In p₁ V₁ = p₂ V₂ the mass of gas and its temperature stay the same. Questions marked (Higher) are for Higher; the stretch is optional.

Diagnostic question: choose and explain

1. A sealed, rigid can of air is heated. Why does the pressure inside rise?
   1. The particles get bigger
   2. They move faster: more hits, and harder
   3. Heating makes more particles
   4. They vibrate faster in fixed places

Practice questions

1. Turn 120 °C into kelvin.
2. What is absolute zero?
   1. 0 °C, where water freezes
   2. −273 °C, where the particles have the least energy they can
   3. −100 °C, the coldest a gas can be
   4. 273 K, where a gas turns into a liquid
3. A gas is heated in a sealed, rigid box. What happens to the particles' average speed, and to the pressure?
   1. Both go up
   2. The speed goes up; the pressure stays the same
   3. The speed stays the same; the pressure goes up
   4. Both stay the same
4. A winter night in the Arctic is −40 °C. What is that in kelvin?
5. A syringe holds 250 cm³ of air at 100 kPa. Its end is sealed and it is pushed in slowly, so the temperature stays the same, to 100 cm³. What is the pressure now, in kPa?
6. A gas is squeezed into a smaller volume at the same temperature. Why does its pressure rise?
   1. The particles move faster
   2. The particles hit the walls more often: they have less far to go
   3. The particles are squashed smaller
   4. There are more particles in the box
7. Ali heats a sealed, rigid can of gas from 20 °C to 40 °C. He says the pressure doubles. Is he right?
   1. Yes: the temperature doubled
   2. No: in kelvin it goes from 293 K to 313 K, much less than double
   3. No: heating makes the pressure fall
   4. Yes: twice as many particles hit the walls
8. (Higher) Pump up a tyre quickly and the air in the pump warms up. Why?
   1. Pushing the piston does work on the air: its particles bounce off the moving piston faster
   2. The particles are squashed together and rub against each other
   3. Squeezing the air makes more particles
   4. The air warms because there is less room for the heat to spread out
9. A diver breathes out a bubble of 3.0 cm³ where the pressure is 200 kPa. It rises to the surface, where the pressure is 100 kPa. If its temperature stays the same, what is its volume at the surface, in cm³?
10. A gas cylinder holds 0.040 m³ of helium at 1500 kPa. What volume would that helium take up at 100 kPa and the same temperature, in m³?
11. Stretch: the helium fills balloons of 0.010 m³ each, at 100 kPa. When it stops, the cylinder is still full of helium at 100 kPa. How many balloons does it fill?

Support: Draw the box with a few particles and arrows. Hotter: longer arrows. Smaller box: shorter trips to the wall. Then say what happens to the hits: how often, and how hard.

Gas pressure: exit

Decide first whether each answer should be bigger or smaller.

1. A gas is at 57 °C. What is its temperature in kelvin?
2. A gas at 90 kPa fills 4.0 litres. At the same temperature it is squeezed into 1.5 litres. What is its new pressure, in kPa?
3. The squeezed gas is then heated, its volume kept the same. What happens to its pressure, and why?
   1. It goes up: the particles move faster, so they hit the walls more often and harder
   2. It stays the same: the volume didn't change
   3. It goes up: the particles get bigger and fill the space
   4. It goes down: a hot gas spreads out

Exit check

A kelvin conversion, a squeezed gas and a heated one show who adds 273, who divides the right way and who explains with particles.

A gas is at 57 °C: its temperature in kelvin? Then 90 kPa in 4.0 litres, squeezed into 1.5 litres at the same temperature: the new pressure?

Gas pressure and temperature

K = °C + 273. Explain with particles where a question asks why. For questions 4 and 5 write p₁ V₁ = p₂ V₂ first, then the numbers. Question 6 is Higher.