Pressure in gases (physics only) — AQA GCSE Physics
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Pressure in gases (physics only) explained
When a molecule hits a wall it bounces off, and the change in its motion is perpendicular to the surface.
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Averaged over the huge number of impacts, the sideways contributions cancel and what is left is a net force acting at right angles to the wall, whatever the wall's orientation. Pressure is that force divided by the area it acts on, measured in pascals, where 1 Pa is 1 N/m². Because the force is always normal to the surface, a gas pushes outwards on every part of a container equally. Raise the outside pressure above the inside pressure and the gas is compressed into a smaller volume; lower it and the gas expands, as a balloon does when it is carried up a mountain.
Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.
Let a fixed mass of gas expand into a bigger container while keeping the temperature the same. The number of molecules has not changed and, because the temperature is constant, neither has their mean speed or mean kinetic energy. What changes is the geometry: the molecules are now further apart and travel further between hits, so they strike the walls less often. Fewer collisions per second, each no harder than before, means a smaller total force, and that force is now shared over a larger wall area. Both effects reduce the force per unit area, so the pressure falls. Compression is the same argument in reverse, which is why a pushed syringe plunger raises the pressure inside.
For a fixed mass of gas held at a constant temperature: pressure × volume = constant p V = constant pressure, p, in pascals, Pa volume, V, in metres cubed, m Students should be able to calculate the change in the pressure of a gas or the volume of a gas (a fixed mass held at constant temperature) when either the pressure or volume is increased or decreased.
For a fixed mass of gas at a constant temperature the product of pressure and volume does not change, so p₁V₁ = p₂V₂. Pressure and volume are inversely proportional: halve the volume and the pressure doubles. A gas at 100 kPa occupying 2.0 × 10⁻⁴ m³, squeezed to 5.0 × 10⁻⁵ m³, ends at p₂ = (100 000 × 2.0 × 10⁻⁴) ÷ (5.0 × 10⁻⁵) = 400 kPa. The units do not have to be pascals and cubic metres, but both pressures must share one unit and both volumes must share another, because the conversion factors cancel. Plotted against volume the relationship is a curve; plotted against 1/V it is a straight line through the origin. It only holds while the temperature and the mass of gas stay fixed.
Your focus
- Explain why the net force a gas exerts on a container wall acts at right angles to that wall.
- Calculate the pressure produced by a stated force acting on a stated area, giving the unit.
- Describe what happens to the volume of a gas when the pressure acting on it is increased at constant temperature.
Show all 9 objectives
- Explain, using the particle model, why the pressure of a fixed mass of gas falls when its volume is increased at constant temperature.
- Explain why the pressure in a syringe rises when the plunger is pushed in slowly, without referring to a change in kinetic energy.
- Identify which quantities stay constant during an isothermal expansion and use them to justify each step of the explanation.
- Calculate the new pressure of a fixed mass of gas when its volume is changed at constant temperature.
- Calculate the new volume of a gas when its pressure is changed, checking the answer against the inverse relationship.
- State the two conditions that must hold for p V = constant to be used, and identify a situation in which it does not apply.
Pressure in gases (physics only) exam tips
Marking Points
- one mark for stating that the force produced by the gas acts at right angles, or normal, to the surface
- one mark for stating that this force is the result of molecules colliding with and rebounding from the wall
- one mark for stating that pressure is the force divided by the area on which it acts, in Pa or N/m²
- one mark for stating that a gas is compressed into a smaller volume when the pressure on it is increased
- one mark for stating that the molecules are further apart in the larger volume
- one mark for stating that the frequency of collisions between molecules and the walls decreases
- one mark for stating that the force is spread over a larger surface area, or that the force per unit area falls
- one mark for stating that the mean speed, or mean kinetic energy, of the molecules is unchanged because the temperature is constant
- one mark for writing p₁V₁ = p₂V₂, or for stating that pressure × volume is constant
- one mark for substitution with both pressures in the same unit and both volumes in the same unit
- one mark for the calculated value with a correct unit
- one mark for stating that the mass of gas and the temperature must remain constant for the relationship to hold
Examiner Tips
- 💡On a diagram, draw the force arrows perpendicular to each surface, including the sides and the top.
- 💡Quote the unit as Pa, and remember that 1 Pa is 1 N/m² if a question mixes units.
- 💡If the question says net force, make it clear the sideways effects of individual collisions cancel out.
- 💡Say explicitly that the temperature is constant so the mean kinetic energy does not change; it protects you from the commonest lost mark.
- 💡Keep the answer to collision frequency and area; do not mention speed or energy increasing.
- 💡The same three sentences, reversed, answer a compression question, so learn one argument and turn it round.
- 💡Write p₁V₁ = p₂V₂ and label which numbers are the before values before you substitute anything.
- 💡Sense check the answer: a smaller volume must give a larger pressure, so if it does not, you have divided the wrong way round.
- 💡Only convert units if the two sides disagree; matching units cancel and save time.
Common Mistakes
- drawing or describing the force as acting along the wall rather than at right angles to it
- saying a gas presses only downwards because of its weight
- confusing pressure measured in pascals with force measured in newtons
- assuming only the molecules nearest the wall contribute, so a bigger container has a lower pressure automatically
- saying the molecules slow down as the gas expands, which the mark scheme refuses because the temperature is constant
- linking the pressure change to a change in kinetic energy, which is not credited in a constant temperature question
- claiming there are fewer molecules in the larger volume
- describing collisions between molecules rather than collisions with the container walls
- adding pressure and volume, or dividing one by the other, instead of multiplying
- mixing kPa on one side of the equation with Pa on the other
- using p V = constant for a gas that has been heated or cooled
- quoting one volume in cm³ and the other in m³ within the same calculation