Pressure (OCR GCSE Physics A (Gateway)): Flashcards

True.

This is the relationship pV = constant. It means pressure and volume are inversely proportional: when one increases, the other decreases, as long as temperature and mass are unchanged.

When a gas is compressed, the volume decreases and the pressure also increases. When the gas expands, the pressure decreases. Temperature is assumed constant.

pV = constant — pressure and volume are inversely proportional at constant temperature. Doubling the pressure halves the volume, and vice versa, for a fixed mass of gas.

Compressing a gas increases pressure because the molecules are forced into a smaller volume. They collide with the container walls more frequently, creating a greater net force per unit area.

False.

Compression at constant temperature does not change the speed of the molecules, so individual collisions are no more forceful. Instead, the molecules collide with the walls more frequently because there is less space, which increases the pressure.

The table shows how compressing or expanding a gas affects pressure and volume at constant temperature.

The equation P₁V₁ = P₂V₂ links the initial and final states of a gas. Here P₁ and V₁ are the initial pressure and volume, and P₂ and V₂ are the final values, assuming constant temperature.

Doing work on a gas involves transferring energy to the gas by applying a force. This can increase its internal energy and may raise its temperature.

When a gas is compressed, work is done on the gas. This increases its internal energy, which raises the temperature of the gas.

True.

When a gas is compressed, work is done on it, increasing its internal energy and raising its temperature.

(Higher Tier Only)

When a piston is pushed into a container, the volume of the gas decreases, the molecules move around faster, and the temperature of the gas increases.

Internal energy is the total energy stored by the molecules of a substance. It can be increased by doing work on the gas or by heating it.

When a gas expands, the gas does work rather than having work done on it. This means the gas loses energy, causing its temperature to decrease.

False.

When a gas expands, the gas does work (not the other way around). This means the gas loses energy, causing its temperature to fall, not rise.

(Higher Tier Only)

The table shows what happens when work is done on or by a gas.

The atmosphere is a thin layer of air surrounding the Earth. It exerts a pressure of about 101 kPa at sea level.

Atmospheric pressure decreases with altitude because there are fewer air molecules above a surface at greater heights. Less air above means less weight pressing down, so the pressure is lower.

True.

The Earth's atmosphere exerts a pressure of about 101 kPa at sea level, created by the weight of air above pushing down on the surface.

The number of air molecules above a surface decreases as altitude increases. This reduces the weight of air above, which decreases the atmospheric pressure.

A fluid is any substance that can flow — either a liquid or a gas. Both exert pressure on objects immersed in them.

The atmosphere exerts pressure because air molecules constantly collide with surfaces, exerting a force per unit area. This creates atmospheric pressure acting in all directions.

False.

An object underwater experiences pressure from both the water and the atmosphere. At 10 m depth, the total pressure is the water pressure plus the atmospheric pressure of about 101 kPa.

The table shows how atmospheric conditions change with altitude.

An object submerged in water experiences pressure from two sources: the liquid pressure due to the depth of water above it, and the atmospheric pressure of the air above the water's surface.

Upthrust is the upward force exerted by a fluid on an object submerged in it. It always acts in the opposite direction to the object's weight.

An object floats when the upthrust acting on it is equal to or greater than its weight. This happens when the object's density is less than the density of the fluid.

True.

If an object has a greater density than the fluid, it cannot displace enough fluid to generate sufficient upthrust to support its weight, so it sinks.

(Higher Tier Only)

Upthrust always acts upwards on a submerged object, opposite to the direction of its weight. The size of upthrust depends on the density of the fluid and the volume of fluid displaced.

Whether an object floats or sinks depends on the comparison between the object's density and the density of the fluid. If the object's density is less than the fluid's, it floats; if greater, it sinks.

Wood floats because its density (about 0.9 g/cm³) is less than water (1.0 g/cm³), so upthrust equals its weight. Iron sinks because its density (about 7.9 g/cm³) is much greater than water, so upthrust cannot support it.

False.

Polystyrene floats in water. Its density (about 0.05 g/cm³) is much less than the density of water (1.0 g/cm³), so the upthrust equals or exceeds its weight.

(Higher Tier Only)

The table shows three objects placed in water (density 1.0 g/cm³).

The pressure due to a column of liquid is given by p = hρg, where p is pressure in Pa, h is the height of the column in m, ρ is the density of the liquid in kg/m³, and g is the gravitational field strength in N/kg.

The pressure at a point in a liquid increases with greater depth (height of liquid above that point) and with greater density of the liquid. Both factors increase the weight of liquid pressing down.

True.

As depth increases, the amount of liquid above a point increases. This increases the weight of liquid pressing down, which raises the pressure at that depth.

Pressure in a liquid is caused by the weight of liquid pushing against objects. A more dense liquid exerts a higher pressure because it has a greater weight for the same volume.

Upthrust is caused by the difference in pressure between the bottom and top of a submerged object. The bottom experiences greater pressure than the top, creating a resultant upward force.

Using p = hρg:

p = 5 × 1025 × 10

p = 51 250 Pa

False.

Pressure in a liquid increases with depth. The bottom of a liquid column is under more weight of liquid, so the pressure there is greater than at the top.

The table shows how changes to depth or density affect pressure in a liquid.

Liquid pressure is exerted across the whole surface of a submerged object and acts at right angles to every part of that surface. This means the pressure acts in all directions, not just downwards.