Edexcel IGCSE Physics · Spec 5.5-5.7
Pressure and Pressure in Liquids
Covers calculate pressure from force and area, Explain why liquid pressure increases with depth and Use p = rho g h with all three factors.
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Pressure and Pressure in Liquids
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Explained
Pressure, and why it rises as you go deeper
Pressure is the force acting at right angles to a surface divided by the area it acts over. The unit is the pascal, and one pascal is one newton per square metre.
Because area is on the bottom of that fraction, the same force spread over a smaller area gives a larger pressure. A drawing pin has a broad head for your thumb and a fine point for the wood, so the same push produces a gentle pressure on you and an enormous one on the wood. Skis work the other way round, spreading weight over a large area so the pressure is low enough not to sink into snow.
Pressure in a liquid
Pressure in a liquid increases with depth, because the deeper you go the more liquid there is above you, and the weight of that liquid presses down.
It also acts in all directions, not just downwards, which is why a hole in the side of a container squirts water sideways and why the jet is faster from a hole nearer the bottom. A dam is built thicker at its base for the same reason.
The equation
Pressure difference equals height multiplied by density multiplied by gravitational field strength, usually written p equals rho g h.
Read each symbol carefully, because each is a separate mark. The height is the vertical depth below the surface of the liquid, not the width of the container and not the distance along a sloping side. The density is the density of the liquid, which is the same throughout it. And this equation gives the pressure due to the liquid alone, so to find the total pressure on a submerged object you add atmospheric pressure.
The shape of the container makes no difference at all. Only the depth matters, which surprises people and is worth remembering.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics notes that most responses correctly stated the link between pressure and depth, though fewer gave full explanations. Some candidates referred to the formula involving density, gravitational field strength and height but did not define the height or explain how it related to the water in the container, and a few wrongly suggested that the density of the water varied throughout it.
The same report observes that the best answers to a related question began with the relevant formula and used it to structure the explanation, while weaker responses were repetitive or vague, with statements such as the pressure being spread out over the nails.
A later report describes a manometer question where references to the given relationship, pressure difference equals height times density times g, were rare despite it being printed in the question stem. Very few learners could link a change in density to a change in height.
The pattern is consistent. Start with the equation, name what each quantity refers to in the situation described, and the explanation writes itself.
Atmospheric pressure
The atmosphere is a fluid too, so the same reasoning applies. Pressure falls with altitude because there is less air above you, which is why aircraft cabins are pressurised and why water boils at a lower temperature on a mountain.
At sea level the atmosphere presses on every square metre with roughly one hundred thousand newtons. You do not feel it because it pushes equally in all directions, including from inside your body.
Spec 5.5-5.7
What you need to know
- Calculate pressure from force and area
- Explain why liquid pressure increases with depth
- Use p = rho g h with all three factors
Active recall
Quick check
Answer each question before opening the answer.
What three factors determine liquid pressure?
Density, gravitational field strength and depth
What is the unit of pressure?
The pascal, Pa
Why does pressure increase with depth?
There is a greater weight of liquid above the point
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