Pressure — OCR GCSE Physics

This Higher tier, separate-science relationship gives the pressure created at the base of a vertical column of liquid: p = h × ρ × g. Height h is measured in metres, density ρ in kilograms per cubic metre and gravitational field strength g in newtons per kilogram, so the product comes out in pascals (N/m²). A worked case: a column 2.0 m deep of water with density 1000 kg/m³ and g = 9.8 N/kg gives p = 2.0 × 1000 × 9.8 = 19 600 Pa, about 2.0 × 10⁴ Pa. Notice that the pressure depends on depth, density and g, not on the cross-sectional area or the total volume of the container. Convert depths to metres and densities to kg/m³ before multiplying, and keep units consistent throughout.

In a gas the particles move randomly at high speed. Raising the temperature increases their average kinetic energy, so they move faster. In a closed system the number of particles and the volume are fixed, so faster particles strike the container walls more often and with greater force, raising the pressure. Conversely, compressing a fixed mass of gas at constant temperature crowds the particles so collisions with the walls become more frequent and pressure rises. A worked case: warming a sealed rigid can raises the temperature, the particles speed up, collisions with the walls become harder and more frequent, and the pressure inside increases. Keep the reasoning to closed systems, where mass and often volume stay constant.

At constant volume, the number of gas particles and the space they occupy stay fixed. Heating the gas transfers energy to the particles, so their mean kinetic energy rises and they move faster. Faster particles strike the container walls more often and with greater force per collision, so the total force on a fixed area of wall increases and the pressure rises. Cooling reverses this: particles move more slowly, collide less often and less forcefully, and the pressure falls. The relationship is qualitative here, so you explain the direction of change and the particle reason rather than calculating values. A sealed rigid metal can heated on a bench is a useful picture: its volume cannot change, yet the pressure inside climbs as the temperature climbs.

This statement is assessed only in the physics separate science qualification, so treat it as separate-science content even though it sits in a section shared with the combined route. Gases are compressible: applying extra pressure squeezes the particles into a smaller volume, and reducing the external pressure lets a fixed mass of gas expand into a larger volume. In both cases the particles keep moving and colliding with the container walls. Each collision exerts a tiny push on the wall, and because the particles arrive from all directions inside the gas, the combined effect is a net force acting at right angles (perpendicular) to every surface the gas touches. That perpendicular force spread over an area is what we call gas pressure, which is why a balloon pushes evenly outward on all of its skin.

At constant temperature the gas particles keep the same average speed, so their average velocity magnitude does not change when the container is enlarged. What changes is the space available. In a larger volume the particles are spread more thinly, so the number of particles per unit volume falls. Each particle still travels at the same typical speed, but it now has further to go between collisions with the walls, and there are fewer particles near any given area of wall. Fewer collisions per second with each unit area of wall means a smaller force per unit area, and pressure is force per unit area, so the pressure decreases. A useful check is to imagine doubling the volume: the same particles now occupy twice the space, so roughly half as many wall collisions occur per second per unit area, and the pressure roughly halves.

Doing work on a gas transfers energy to it. When a gas is compressed quickly, for example by pushing the piston of a bicycle pump, the moving piston does work on the gas. The particles gain kinetic energy, so their average speed increases and the temperature of the gas rises. In the bicycle pump the air inside is compressed as the piston moves in, the air warms up, and the pump barrel feels warm to touch. The key idea is energy transfer by work: a force moves through a distance on the gas, energy is transferred to the particles, and the rise in average kinetic energy shows as a rise in temperature. If the compression is slow, energy can be transferred to the surroundings and the temperature rise is smaller, but the work still transfers energy to the gas.

This separate-science item asks you to describe a simple model, not to name atmospheric layers. In the model, treat the atmosphere as a single uniform layer of air of constant density sitting on the Earth's surface, rather than as a stack of named regions. Atmospheric pressure at the surface is then explained as the weight of this air column pressing down on each unit of area, giving a pressure that acts in all directions on any surface within the air. Because density is assumed uniform, the pressure falls steadily with height in the model, which is a simplification of the real gradual thinning of air. Use the model to predict that pressure decreases as you rise, and recognise that the uniform-density assumption is what makes the simple calculation possible.

This separate-science item asks you to explain, not merely state, why atmospheric pressure changes with height. As you move higher above the surface, there is less air above you, so the weight of the air column pressing down is smaller and the atmospheric pressure is lower. Near the surface the column of air is longest, so pressure is greatest; at high altitude the column is short and pressure is reduced. The explanation must connect height to the amount (and therefore weight) of air above the point, and hence to the pressure exerted. A useful check is to compare a point at sea level with a point high on a mountain: the mountain-top point has less air above it, so the pressure there is lower.

An object floats when the upthrust on it balances its weight; it sinks when its weight exceeds the maximum upthrust the fluid can supply. Upthrust equals the weight of fluid displaced, so the outcome depends on the object's weight compared with the weight of fluid it could displace. Density is the key factor: an object less dense than the fluid floats, one more dense sinks. Shape matters because it sets the volume displaced when fully submerged, so a hollow steel hull floats while a solid steel block sinks. Overall density, not material alone, decides the result.

Pressure in a liquid increases with depth because more liquid lies above and its weight presses down; it also increases with density because a denser liquid has more mass in the same volume. The pressure at a point is given by p = hρg, where h is depth, ρ is density and g is gravitational field strength. On a partially submerged object, the pressure on the lower surface exceeds the pressure on the upper surface, so the liquid pushes up more strongly than it pushes down. This net upward force is upthrust, and it equals the weight of fluid displaced.

This Higher tier separate physics criterion asks you to calculate how pressure changes between two depths in a liquid. The pressure difference depends on the liquid's density, the gravitational field strength g and the vertical depth difference: Δp = ρ × g × Δh. Near the Earth's surface g is the strength of the gravitational field and equals 10 N/kg, so a liquid of density 1000 kg/m³ gives an extra pressure of 1000 × 10 × 1 = 10 000 Pa for each extra metre of depth. Work out the pressure at each depth, or the difference directly from the depth gap, and keep units consistent: density in kg/m³, depth in m, pressure in Pa (N/m²).