PressureAQA GCSE Physics: Revision notes
Section 1
How do you calculate pressure for solid surfaces?
Pressure is the force acting per unit area. For solid surfaces, use the equation:
P = F/A
Where:
- P = pressure (measured in pascals, Pa or N/m²)
- F = force (measured in newtons, N)
- A = area (measured in m²)
Key points:
- Pressure is inversely proportional to area – the same force spread over a smaller area produces greater pressure
- This is why sharp objects (small area) pierce surfaces more easily than blunt ones (large area)
- To convert cm² to m², divide by 10,000
Units reminder: 1 Pa = 1 N/m²
A force of 600 N acts on a surface area of 0.5 m². Calculate the pressure. Solution: P = F/A = 600/0.5 = 1200 Pa. Remember to rearrange if finding force or area: F = P × A or A = F/P.
Examiners expect you to show the formula, substitute numbers with units, and state your final answer with units. Always check whether you need to convert cm² to m² before calculating.
Section 2
Why does pressure in liquids increase with depth?
Pressure in a liquid at a given depth depends on the weight of liquid above that point. As depth increases, more liquid is above, so the pressure increases.
The equation for pressure in a liquid is:
P = hρg
Where:
- P = pressure (in Pa or N/m²)
- h = depth/height of liquid column (in m)
- ρ = density of the liquid (in kg/m³)
- g = gravitational field strength (9.8 m/s² or 10 m/s²)
Why pressure increases with depth:
- At greater depths, there is a larger mass of liquid above pushing down
- This greater mass exerts a greater weight (force)
- The same cross-sectional area supports this greater force, so pressure increases
- Pressure increases linearly with depth – doubling depth doubles pressure
Pressure at the surface of a liquid is atmospheric pressure (approximately 101,000 Pa). Total pressure at depth = atmospheric pressure + pressure due to liquid column.
Calculate the pressure due to a water column of depth 5 m. Density of water = 1000 kg/m³, g = 10 m/s². Solution: P = hρg = 5 × 1000 × 10 = 50,000 Pa. This is the gauge pressure (due to the liquid alone).
Students often forget that total pressure at depth includes both atmospheric pressure and the liquid pressure. If asked for total pressure, add atmospheric pressure (≈ 101,000 Pa) to your calculated value.
Think of liquid particles as stacked building blocks. At the bottom, all the blocks above are pressing down. Go deeper (higher up the building), and fewer blocks press down, so pressure is lower.
Section 3
What is upthrust and how does it relate to floating?
Upthrust (also called buoyancy or buoyant force) is the upward force exerted by a fluid (liquid or gas) on an object immersed in it.
Why does upthrust occur?
- Pressure in a fluid increases with depth
- The pressure acting on the bottom of a submerged object is greater than the pressure on the top
- This difference in pressure creates a net upward force – the upthrust
Explaining floating using pressure differences:
- An object floats when upthrust = weight (object is in equilibrium)
- An object sinks when upthrust < weight (weight pulls it down faster)
- An object rises when upthrust > weight (upthrust accelerates it upward)
For a floating object:
- The object displaces a volume of fluid equal in weight to the object's weight
- Part of the object is submerged; part may be above the surface
- The upthrust force acts at the centre of buoyancy (the centre of the displaced fluid)
Key principle: Upthrust = weight of fluid displaced (Archimedes' principle)
- Larger objects displace more fluid, so they experience greater upthrust
- Denser fluids exert greater pressure at depth, producing greater upthrust
To explain upthrust in an exam, always mention pressure increases with depth, the greater pressure on the bottom than the top, and therefore the net upward force. Avoid just saying 'water pushes up' – explain why.
Imagine standing in a crowd. People push on you from all sides, but more push from below (closer to the ground) than from above, creating a net upward push. That's how pressure differences create upthrust.
Section 4
How is pressure transmitted in hydraulic systems? (Higher Tier)
Hydraulic systems use incompressible fluids (usually oil) to transmit force from one location to another. They allow small input forces to produce large output forces.
Principle of pressure transmission:
- Pressure applied to a fluid is transmitted equally in all directions (Pascal's principle)
- Pressure = Force / Area, so the same pressure can produce different forces depending on the area
The hydraulic equation:
F₁/A₁ = F₂/A₂
Where:
- F₁ = input force (in N)
- A₁ = input piston area (in m²)
- F₂ = output force (in N)
- A₂ = output piston area (in m²)
Since pressure is the same throughout the system:
- If A₂ > A₁, then F₂ > F₁ (force is amplified)
- The ratio of output to input force = A₂/A₁ (the mechanical advantage)
Practical applications:
- Car brakes: small foot force on a small piston produces large braking force
- Hydraulic lifts and jacks: small effort force lifts large loads
- Excavator arms: precise control with significant force multiplication
Energy consideration: Although force is amplified, the input work (F₁ × d₁) equals the output work (F₂ × d₂). A larger distance is moved at the input side for the same work output.
A hydraulic system has an input piston of area 0.01 m² and an output piston of area 0.5 m². A force of 100 N is applied to the input piston. Find the output force. Solution: F₁/A₁ = F₂/A₂, so 100/0.01 = F₂/0.5, therefore F₂ = (100 × 0.5)/0.01 = 5000 N. Mechanical advantage = 50.
Examiners test whether you can rearrange the hydraulic equation. Know all three forms: F₂ = (F₁ × A₂)/A₁, A₂ = (F₂ × A₁)/F₁, and A₁ = (F₁ × A₂)/F₂. Show your working clearly.
Section 5
What is atmospheric pressure and how does it change with altitude?
Atmospheric pressure is the pressure exerted by the weight of the air above a given point. At sea level, it is approximately 101,000 Pa (or 101 kPa).
Why does atmospheric pressure exist?
- Air has mass and therefore weight
- The column of air above the Earth exerts a downward force
- This force spread over the surface area creates atmospheric pressure
- Atmospheric pressure acts in all directions (downward, sideways, and even upward)
How atmospheric pressure changes with altitude:
| Altitude | Atmospheric Pressure | Explanation |
|---|---|---|
| Sea level | ~101,000 Pa | Maximum pressure; full column of air above |
| Higher altitudes | Decreases | Less air above; smaller mass exerts less weight |
| Mountains | ~50,000 Pa | Approximately half the pressure at sea level |
| Space | ~0 Pa | No air; no atmospheric pressure |
Key points:
- Atmospheric pressure decreases non-linearly with altitude (decreases faster initially, then more gradually)
- As altitude increases, the density of air decreases, so pressure decreases
- The relationship is approximately exponential (halves roughly every 5.5 km)
- Atmospheric pressure is the reason weather systems form and change
Practical effects of atmospheric pressure changes:
- Difficulty breathing at high altitude (less oxygen available)
- Boiling point of water decreases at high altitude (lower atmospheric pressure)
- Aircraft cabins are pressurised to maintain passenger comfort
When discussing atmospheric pressure changes, remember it's caused by less air above at higher altitudes, not by gravity changing. Always relate pressure to the weight of the air column.
Students often state that atmospheric pressure 'gets weaker' with altitude but forget to explain why: the mass of air above decreases, so the weight decreases, and therefore pressure decreases.
Must Know
- Pressure on solids: P = F/A (measured in Pa or N/m²); pressure increases when force increases or area decreases
- Pressure in liquids: P = hρg; pressure increases with depth because more liquid above means greater weight pushing down
- Upthrust: Results from pressure difference (greater pressure at depth than at surface); an object floats when upthrust equals weight
- Hydraulic systems: Pressure is transmitted equally throughout incompressible fluid; F₁/A₁ = F₂/A₂ allows force amplification when A₂ > A₁
- Atmospheric pressure: Caused by weight of air above; approximately 101,000 Pa at sea level; decreases with altitude because less air is above
- Key principle: Pressure = Force/Area; larger areas experience greater total force at the same pressure; smaller areas experience larger pressure from the same force
That's the notes covered.
Carry on to the next subtopic.