Density and Pressure
Density and pressure for Cambridge IGCSE Physics 0625. Measuring density, floating and sinking, pressure in solids and in liquids, and the manometer.
What you need to know
- Density is mass per unit volume. To measure it for an irregular solid, find the mass on a balance and the volume by displacement in a measuring cylinder.
- An object floats if its density is less than the density of the liquid.
- Pressure in a solid is force divided by the area it acts on. A smaller area gives a bigger pressure for the same force.
- Pressure in a liquid increases with depth and with the density of the liquid, and it acts equally in all directions at a given depth.
- Pressure at a point in a liquid does not depend on the shape or the width of the container.
Equations
- density = mass / volume
- pressure = force / area
- pressure difference in a liquid = density x g x height
- density of water is 1000 kg per cubic metre
Density and Pressure questions students ask
What is density?
Mass per unit volume. Divide the mass in kilograms by the volume in cubic metres to get kilograms per cubic metre.
How do you measure the density of an irregular solid?
Find the mass on a balance. Find the volume by lowering it into a measuring cylinder of water and recording the rise in level. Then divide mass by volume.
Why do objects float?
An object floats if its average density is less than the density of the liquid. If it is denser it sinks.
How do you convert grams per cubic centimetre to kilograms per cubic metre?
Multiply by 1000. Water is 1 gram per cubic centimetre, which is 1000 kilograms per cubic metre.
How do you calculate pressure?
Pressure equals force divided by the area the force acts on, measured in pascals, which are newtons per square metre.
Why do sharp knives cut better than blunt ones?
The edge has a much smaller area, so the same force produces a much greater pressure.
Why does pressure increase with depth in a liquid?
There is a greater weight of liquid above the point. The pressure difference equals density times g times the depth.
Does the shape of the container affect the pressure at a given depth?
No. Pressure in a liquid depends only on the depth, the density of the liquid and g. The width and shape of the container make no difference.
Does pressure in a liquid act in all directions?
Yes. At any given depth it acts equally in every direction, not just downwards.
Mistakes that cost marks
- Mixing up cubic centimetres and cubic metres. 1 g per cubic centimetre equals 1000 kg per cubic metre.
- Using the total depth of the container instead of the depth of the point in question.
- Forgetting that pressure acts in all directions in a fluid, not just downwards.
- Reading the volume from the wrong part of the meniscus in a displacement experiment.
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