4.3.1Properties of Matter
Pressure
5 min read 5 parts Lesson 7 of 11
Section 1 of 5Pressure as Force on an Area
Pressure depends on two quantities at once, so changing either the force or the area changes the result.
Definition
- Pressure is the force acting normally (at right angles) on a surface divided by the area of that surface,
- so it measures how concentrated a force is.
Equation
- p = F ÷ A, where p is pressure in pascals (Pa), F is force in newtons (N) and A is area in square metres (m²).
The pascal
- One pascal is one newton acting on one square metre, so 1 Pa = 1 N/m²;
- everyday pressures are often quoted in kilopascals (kPa).
Large area
- Spreading the same force over a bigger area lowers the pressure, which is why snowshoes and wide tractor tyres stop you sinking.
Small area
- Reducing the area raises the pressure, which is why knives, nails and drawing pins are sharpened to a fine edge or point.
| Change | Effect on pressure |
|---|---|
| Force doubled, area unchanged | Pressure doubles |
| Area doubled, force unchanged | Pressure halves |
| Force and area both doubled | Pressure stays the same |
Section 2 of 5Pressure in a Gas
Pressure in a Gas
Random motion
- Gas particles move rapidly in random directions and frequently collide with each other and with the walls of their container.
Collisions with walls
- Each collision exerts a tiny force on the wall, and the huge number of collisions per second produces a steady overall force.
Direction
- Gas pressure acts equally in all directions and always at right angles to any surface it touches.
Size of the pressure
- Pressure is the total force from all these collisions divided by the area of wall they strike.
Vacuum
- With no particles there are no collisions, so the pressure inside a perfect vacuum is zero.
Section 3 of 5Ways to Change Gas Pressure
1
Heating the gas gives the particles more kinetic energy, so they move faster and strike the walls harder and more often, raising the pressure.
2
Squeezing the gas into a smaller volume packs the particles closer so they reach the walls more frequently, which raises the pressure.
3
Adding more gas to a fixed container increases the number of particles, so more collisions occur each second and the pressure rises.
4
Cooling the gas slows the particles, so collisions become weaker and less frequent and the pressure falls.
5
Allowing the gas to expand into a larger volume spaces the collisions out over a bigger wall area, so the pressure falls.
Section 4 of 5Worked Example
Question
- A girl of mass 50 kg stands on two feet.
- The total area of the soles in contact with the floor is 0.040 m².
- Calculate the pressure she exerts on the floor.
- (Gravitational field strength = 9.8 N/kg)
- Weight is the force on the floor: W = mg = 50 × 9.8 = 490 N.What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
- Use p = F ÷ A with F = 490 N and A = 0.040 m².
- p = 490 ÷ 0.040 = 12 250 Pa.
- Round to two significant figures to match the data given: 12 000 Pa.
Answer
- p ≈ 1.2 × 10⁴ Pa (12 kPa).
- If she stood on one foot the area would halve and the pressure would double.
Section 5 of 5Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Pressure Force acting at right angles per unit area of a surface, measured in pascals.
- Pascal (Pa)
- The unit of pressure;
- one pascal equals one newton per square metre.
- The unit of pressure;
- Normal force A force acting at 90° to the surface it presses on.
- Gas pressure The effect of billions of particle collisions with the container walls each second.
- Vacuum A space containing no particles, so it exerts no gas pressure.
Common Mistakes
- Using area in cm² or mm² without converting to m² before applying p = F/A gives an answer in the wrong units.
- Saying a gas pressure is caused by particles pushing on each other, when it is caused by collisions with the container walls.
- Thinking a larger force always means a larger pressure, forgetting that a larger area can cancel the effect.
- Using mass in kilograms as the force instead of converting to weight in newtons.