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OCR GCSE Physics A · J249
OCR J249 · P1.3d Check the specification (PDF) (opens in a new tab)
A gas consists of particles in constant, rapid and random motion. The particles travel through the empty space between them and repeatedly collide with the walls of their container.
During a collision, a particle changes direction and momentum. It therefore exerts a force on the wall. A single impact produces a tiny effect, but an enormous number of impacts produces a measurable force. Gas pressure is this force per unit area of the container wall.
Consider a fixed amount of gas in a sealed container with a movable wall. Moving the wall outwards gives the gas a larger volume. The particles are now, on average, further from the walls and must travel further before striking them.
Consequently, each unit area of wall receives fewer collisions per second. The total force exerted per unit area decreases, so the gas pressure decreases.
At constant temperature, increasing the volume increases the distance travelled between wall collisions. The collision rate per unit area falls, so the pressure falls, while typical particle speed remains unchanged.
This explanation depends on the temperature remaining constant. Temperature is related to the particles’ average kinetic energy. At constant temperature, their typical speeds do not decrease when the volume increases. Their velocities still vary and point in random directions, but their overall speed distribution remains the same. The pressure falls because wall collisions become less frequent per unit area, not because the particles move more slowly or strike with less momentum.
The reverse also applies: decreasing the volume at constant temperature shortens the typical distance between wall collisions. Collisions occur more frequently per unit area, so the pressure increases.
For the same enclosed gas at constant temperature, pressure and volume change in opposite directions. This is an inverse relationship: increasing volume lowers pressure, while decreasing volume raises pressure. Under the usual Boyle’s law model,
so two states of the same gas can be represented by . This mathematical relationship only applies to the comparison when the amount of gas and its temperature remain constant.
A marshmallow contains many pockets of trapped gas within a flexible structure. If it is placed in a vacuum chamber and air is pumped out, the pressure outside it decreases. The trapped gas pushes the flexible structure outwards and occupies a larger volume. As that trapped gas expands, its pressure decreases until the pressure difference is reduced.
Eventually some gas may escape and the marshmallow can deflate. When air is allowed back into the chamber, the greater external pressure compresses it. In a demonstration, the marshmallows should not be eaten because vacuum equipment may contain chemical residues.
For a fixed amount of gas at constant temperature:
Compression reverses the chain: smaller volume → more frequent wall collisions per unit area → greater pressure.
Link every step in the explanation: larger volume → particles travel further between wall collisions → fewer collisions per second per unit area → lower pressure.
State that the temperature is constant. Do not claim that the particles slow down when the volume increases.
When comparing pressures, assume the same fixed amount of gas is enclosed.
Do not say that particles themselves expand or become further apart because they grow; the empty space between them increases.
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Last updated
Volume
The amount of three-dimensional space occupied by a gas or its container.
Pressure
The force exerted normally on each unit area of a surface.
Constant temperature
The temperature condition in which the average kinetic energy, and therefore the typical speed, of the gas particles does not change.
Collision rate
The number of particle impacts on a surface during a given time.
Put your knowledge into practice — try past paper questions for Physics A
Volume
The amount of three-dimensional space occupied by a gas or its container.
Pressure
The force exerted normally on each unit area of a surface.
Constant temperature
The temperature condition in which the average kinetic energy, and therefore the typical speed, of the gas particles does not change.
Collision rate
The number of particle impacts on a surface during a given time.