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Pressure increases when volume decreases because the particles hit the walls more frequently. Free GCSE AQA Physics notes with key definitions and 15 FAQs.
Volume and Pressure is part of Particle model and pressure in GCSE AQA Higher Physics.
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Pressure increases when volume decreases because the particles hit the walls more frequently. Free GCSE AQA Physics notes with key definitions and 15 FAQs.
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Use this lesson to stay inside Particle model and pressure while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from Volume and Pressure before testing yourself from memory.
Picture squeezing a balloon until it bursts. You're forcing the same amount of air into a smaller space, and eventually the balloon can't handle the pressure. This everyday experience reveals a fundamental relationship in physics.
When you compress a gas, you're not changing how many particles are inside. You're changing how much space they have to move around in. This creates a dramatic effect on pressure that we can predict and calculate.
Understanding this relationship helps explain everything from how syringes work to why crisp packets expand on aeroplanes. The particle model gives us the tools to see what's happening at the molecular level.
Imagine a box filled with bouncing balls. The balls represent gas particles, constantly moving in random directions at high speed. Every time a ball hits the wall, it exerts a tiny force. Pressure is the total effect of billions of these collisions happening every second.
The faster the particles move, the harder they hit the walls. The more particles you have, the more collisions occur. But there's another crucial factor: the size of the container. If you squeeze the same particles into a smaller space, something interesting happens.
In a smaller container, particles don't have as far to travel between collisions. They hit the walls more frequently. More collisions per second means higher pressure. This is the key insight that explains why compressing gases increases pressure.
Robert Boyle discovered that pressure and volume are inversely proportional. When one doubles, the other halves. When one triples, the other becomes one-third. This isn't just a mathematical curiosity - it's a direct consequence of particle behaviour.
Think about our bouncing ball model again. If you halve the container size, each particle hits the walls twice as often. Since pressure depends on collision frequency, the pressure doubles. The mathematical relationship P₁V₁ = P₂V₂ captures this perfectly.
This relationship only works when temperature stays constant. If temperature changes, particle speed changes too, which affects collision force. For now, we're focusing on situations where only volume changes.
Boyle's Law calculations appear frequently in exams. The key is identifying what you know and what you need to find. Always check your units match - pressure in pascals, volume in cubic metres.
Worked example: Boyle's Law Calculation
A gas occupies 2.0 m³ at a pressure of 100,000 Pa. What is the pressure when the volume is compressed to 0.5 m³? [3 marks]
Answer: 400,000 Pa (or 4.0 × 10⁵ Pa)
Notice how the pressure increased by a factor of four when the volume decreased by a factor of four. This makes sense - compressing to one-quarter volume means four times as many collisions per second.
**Trap**: Many students forget to rearrange the formula correctly. Remember that P₂ = P₁V₁/V₂, not P₁V₂/V₁. The initial values (P₁ and V₁) go together in the numerator.
Boyle's Law explains many everyday phenomena. When you pull the plunger of a syringe, you increase the volume inside. This decreases pressure, creating suction that draws liquid in. Push the plunger down, and you compress the liquid out.
Scuba divers experience Boyle's Law directly. As they descend, water pressure increases, compressing the air in their lungs and equipment. A balloon taken 10 metres underwater shrinks to half its volume because pressure doubles.
Aircraft cabins are pressurised because atmospheric pressure decreases with altitude. At 10,000 metres, air pressure is only one-quarter of sea level pressure. Without pressurisation, sealed containers would expand dramatically.
Car tyres demonstrate the impact of temperature on gas pressure. In cold weather, the kinetic energy of the gas particles decreases, leading to a reduction in pressure inside the tyre.
Boyle's Law questions test your ability to manipulate the P₁V₁ = P₂V₂ equation. Examiners often give you three values and ask for the fourth. The key is systematic rearrangement and careful unit checking.
Watch out for unit traps. Pressure might be given in kPa (kilopascals) but you need Pa (pascals) for your answer. Volume could be in cm³, dm³, or m³. Always convert to consistent units before calculating.
When explaining pressure changes using the particle model, examiners want specific language. Mention 'collision frequency', 'force per unit area', and 'particle motion'. Avoid vague terms like 'molecules get squashed' or 'pressure builds up'.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Pulling the plunger of a syringe increases the volume inside, which decreases the pressure and creates suction that draws liquid in. This is Boyle's Law: for a fixed mass of gas at constant temperature, pressure and volume are inversely proportional.
Pressure increases when volume decreases because the particles do not have as far to travel between collisions, so they hit the walls more frequently. Halving the container size makes each particle hit the walls twice as often, so the pressure doubles.
Boyle's Law states that for a fixed mass of gas at constant temperature, pressure multiplied by volume remains constant (P₁V₁ = P₂V₂). Pressure and volume are inversely proportional: when one doubles, the other halves.
Write P₁V₁ = P₂V₂, rearrange to P₂ = P₁V₁/V₂, then substitute. A gas at 100,000 Pa with a volume of 2.0 m³ compressed to 0.5 m³ has P₂ = 200,000 ÷ 0.5 = 400,000 Pa, four times the pressure for one-quarter of the volume.
A balloon shrinks underwater because water pressure increases with depth, compressing the air inside. A balloon taken 10 metres underwater shrinks to half its volume because the pressure doubles, as Boyle's Law predicts.
The Boyle's law equation is P₁V₁ = P₂V₂: for a fixed mass of gas at constant temperature, pressure multiplied by volume stays constant. Pressure is in pascals and volume in cubic metres.
Boyle's law only works when temperature stays constant. If the temperature changes, particle speed changes too, which affects how hard particles hit the walls, so pressure no longer depends only on volume.
If the volume of a gas halves at constant temperature, the pressure doubles. Each particle hits the walls twice as often, and since pressure depends on collision frequency, the pressure doubles.
At 10,000 metres, air pressure is only one-quarter of sea level pressure, because atmospheric pressure decreases with altitude. This is why aircraft cabins are pressurised and why crisp packets expand on aeroplanes. Without pressurisation, sealed containers would expand dramatically.
Boyle's law was discovered by Robert Boyle, who found that the pressure and volume of a gas are inversely proportional at constant temperature: when one doubles, the other halves.
Car tyres lose pressure in cold weather because the kinetic energy of the gas particles decreases, so they hit the tyre walls less hard. This leads to a reduction in pressure inside the tyre.
Boyle's law P₁V₁ = P₂V₂ is rearranged to find P₂ by dividing both sides by V₂: P₂ = P₁V₁/V₂. A common trap is writing P₁V₂/V₁; the initial values P₁ and V₁ go together.
Compressing 2.0 m³ of gas at 100,000 Pa to 0.5 m³ raises the pressure to 400,000 Pa. Using P₂ = P₁V₁/V₂ = 100,000 × 2.0 ÷ 0.5 = 400,000 Pa: a quarter of the volume gives four times the pressure.
Scuba divers experience Boyle's law because water pressure increases as they descend, compressing the air in their lungs and equipment. A balloon taken 10 metres underwater shrinks to half its volume because the pressure doubles.
Answers about gas pressure changes should use specific terms such as collision frequency, force per unit area and particle motion. Vague terms like 'molecules get squashed' or 'pressure builds up' should be avoided.