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OCR GCSE Physics A · J249
OCR J249 · P1.3b Check the specification (PDF) (opens in a new tab)
Gas particles move rapidly and randomly in every direction. When they collide with the walls of their container, their motion changes and they exert forces on the walls. The combined effect of enormous numbers of collisions produces . Because particles travel in every direction, the gas pushes on every wall—not only on the bottom.
Consider a fixed amount of gas in a sealed, rigid container. Being rigid means that the container cannot expand, so its volume remains constant.
Transferring energy to the gas raises its temperature. A higher temperature means that the particles have greater average kinetic energy, so they move faster on average. This changes their collisions with the walls in two ways:
The total force on each unit area therefore increases, so the pressure rises.
For the same amount of gas at constant volume, raising the temperature makes particles collide with the walls harder and more frequently, increasing the pressure.
The qualitative relationship is therefore:
“Qualitative” means that you must explain the direction of the change and its particle mechanism; this outcome does not require a numerical temperature–pressure equation.
The conclusion only applies directly when the volume is fixed and the same gas sample is being considered. In a flexible balloon, heating may make the balloon expand. The particles then have farther to travel between walls, so the pressure change is not described by the constant-volume situation alone.
Likewise, adding more gas particles can raise the pressure even if the temperature does not change. To test the effect of temperature fairly, keep both the volume and the amount of gas constant.
A safe school demonstration can use a sealed, rigid vessel connected to a pressure sensor. The gas is warmed or cooled using water baths while its pressure is recorded. The vessel's volume and the amount of gas remain fixed. The readings should show a higher pressure at a higher temperature; time should be allowed for the gas to reach each new temperature before taking a reading.
OCR also suggests an exploding-can demonstration. If a sealed, rigid can of gas is heated, increasingly energetic particle collisions raise the internal pressure. If the pressure produces forces greater than the can can withstand, it may rupture, so this is a teacher demonstration with suitable protection rather than a pupil procedure.
An Alka-Seltzer film-canister rocket also demonstrates rising gas pressure, but for a different reason: the reaction produces additional gas particles inside the closed canister. Collisions become more frequent until the pressure force ejects the lid. It demonstrates the effects of gas pressure, but it does not by itself isolate the relationship between temperature and pressure.
For a fixed amount of gas at constant volume:
This relationship is required qualitatively, not as a numerical equation.
Higher temperature means greater average kinetic energy. The particles move faster, strike the walls more frequently and produce a larger force in each collision. The force per unit area—and therefore the pressure—increases.
The container must be rigid so that its volume remains constant, and no gas should enter or escape. A flexible container may expand, while adding particles can raise pressure without raising temperature.
An Alka-Seltzer canister gains pressure mainly because the reaction produces more gas; heating a sealed rigid can raises pressure because the existing particles move faster.
For an explanation, give the whole chain: higher temperature → greater average kinetic energy → faster particles → harder and more frequent wall collisions → higher pressure.
Always state that the volume is constant. If the container can expand, heating need not produce the same pressure change.
Do not say that gas pressure is caused by the weight of the gas. It results from particle collisions with the container walls.
When interpreting an experiment, identify temperature as the independent variable and pressure as the dependent variable; the volume and amount of gas must be controlled.
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Gas pressure
The force exerted normally on each unit area of a surface, caused in a gas by particles colliding with that surface.
Temperature
A measure related to the average kinetic energy of the particles in a substance.
Kinetic energy
The energy an object has because it is moving.
Constant volume
A condition in which the space occupied by the gas does not change.
Closed gas sample
A fixed quantity of gas from which no particles enter or escape.
Put your knowledge into practice — try past paper questions for Physics A
Gas pressure
The force exerted normally on each unit area of a surface, caused in a gas by particles colliding with that surface.
Temperature
A measure related to the average kinetic energy of the particles in a substance.
Kinetic energy
The energy an object has because it is moving.
Constant volume
A condition in which the space occupied by the gas does not change.
Closed gas sample
A fixed quantity of gas from which no particles enter or escape.