# iGCSE Physics Section E

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## Density and pressure

Density = mass / volume
Volume = (width x depth x height) cm
³
Pressure = force / area
Pressure difference = height x density x gravity
Pressure increases with depth.

Brownian motion: The particles in matter are extremely small and, in liquids and gases, they are in a continuous state of rapid random motion.

The random motion of gas and liquid particles explains why pressure acts in all directions at any point.
The speed of molecules increases with temp, so as we heat gases in a rigid container, the pressure of the gas increases.
The temp of a gas inn  kelvin is proportional to the average kinetic energy of the gas molecules

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## Gas laws

Boyles law
For a fixed amount of gas at a constant temp
p x V = p x V
volume is inversely proportional to pressure
Absolute zero
For a fixed amount of gas at a costant volume
p / T = p / T
kelvin temperature scale: absolute zero = -273°
pressure is proportional to absolute temperature

To convert °C into kelvin, ADD 273°

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