Pressure in gases Revision Notes for AQA GCSE Physics

Pressure in gases

What happens when you change gas volume

When you have a gas in a container, pressure and volume have a special relationship. When one goes up, the other goes down. This is called being inversely proportional.

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Being inversely proportional means that as one quantity increases, the other decreases by the same proportion. If you double the pressure, the volume becomes half the original size (assuming temperature and mass remain constant).

The key rule is: if mass and temperature stay the same, pressure and volume are inversely proportional to each other.

Changing volume at constant temperature

Imagine a gas in a syringe. If you push the plunger down:

The volume gets smaller
The gas particles get squashed together
They hit the walls more often
The pressure increases

If you pull the plunger out:

The volume gets bigger
The gas particles spread out more
They hit the walls less often
The pressure decreases

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Key fact: The pressure produces a net force at right angles to the walls of the container.

The gas equation

You can work out pressure and volume changes using this equation:

pressure × volume = constant

Written as: $pV = \text{constant}$

This means: $p_1V_1 = p_2V_2$

Where:

$p_1$ = initial pressure
$V_1$ = initial volume
$p_2$ = final pressure
$V_2$ = final volume

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Units you need to know:

Pressure: pascals (Pa)
Volume: cubic metres (m³)
1 kPa = 1000 Pa

Working out pressure and volume changes

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Worked Example: Calculating Volume Change

A gas at 100 kPa has a volume of 28 cm³. The pressure increases to 250 kPa. What's the new volume?

Step 1: Write down what you know

$p_1 = 100$ kPa, $V_1 = 28$ cm³
$p_2 = 250$ kPa, $V_2 = ?$

Step 2: Use the equation $p_1V_1 = p_2V_2$

Step 3: Rearrange to find $V_2$

$V_2 = \frac{p_1 \times V_1}{p_2} = \frac{100 \times 28}{250} = 11.2 \text{ cm}^3$

Answer: The new volume is 11.2 cm³

Remember!

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Key Points to Remember:

Pressure and volume are inversely proportional - when one increases, the other decreases
Temperature and mass must stay constant for this relationship to work
Use $pV = \text{constant}$ or $p_1V_1 = p_2V_2$ for calculations
Units: pressure in pascals (Pa), volume in cubic metres (m³)
When gas is compressed, pressure goes up and volume goes down

Pressure in gases (AQA GCSE Physics): Revision Notes