National 5PhysicsSQARevision NotesProperties Of MatterGas Laws & The Kinetic ModelPressure

Pressure (SQA National 5 Physics): Revision Note

Exam code: X857 75

Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Updated on

Pressure

  • Pressure is defined as:

Force exerted per unit area

  • The area should always be the cross-sectional area of the object

  • So that the force is at a right angle to the surface it is acting upon

  • The average pressure exerted on a surface can be calculated using the following relationship:

    p space equals fraction numerator space F over denominator A end fraction

  • Where:

    • p = pressure measured in pascals (Pa)

      • 1 pascal is equivalent to 1 newton per meter squared

      • 1 Pa = 1 N m-2

    • F = force measured in newtons (N)

    • A = area measured in metres squared (m2)

  • The pressure relationship can be rearranged using a triangle

Pressure, force, area triangle
Pressure, force, area relationship triangle

Effects of changing variables

  • For a constant force:

    • Reducing the area increases the pressure

    • Increasing the area reduces the pressure

    • Pressure is inversely proportional to area

p space proportional to fraction numerator space 1 over denominator A end fraction

  • For a constant area:

    • Increasing the force increases the pressure

    • Decreasing the force decreases the pressure

    • Pressure is directly proportional to force

p space proportional to space F

Diagram comparing pressure distribution from a heeled shoe with high pressure and a flat shoe with low pressure, highlighting area and pressure effects.
When the same force is applied over a smaller surface area, the pressure exerted is greater

Examples of pressure

  • Pressure can be exerted by a solid on another solid

    • For example, the pressure exerted by a person standing on the ground

A finger pressing a drawing pin into a surface, with a label indicating "drawing pin (thumb tack)" next to it.
The force exerted by the small surface area of the pin point on the cork board results in a greater pressure on the cork board. Whereas, the large surface area of the push-plate exerts a smaller pressure on the finger.

  • Pressure can be exerted by a gas on a surface

    • For example, when gas particles collide with the surfaces of a container

Diagram of gas particles in a container, showing blue spheres with green arrows indicating random motion and collisions, representing kinetic theory.
Gas particles exert pressure on the walls of a container due to the force of the particles colliding with the surfaces

Air pressure

  • Air pressure arises due to the collisions of air particles with surfaces

  • The weight of the air above an object exerts a force at ground or sea level

Diagram depicting air pressure changes with altitude, showing less dense molecules at higher altitudes and more dense molecules near the surface.
Air pressure arises from the weight of the air above a surface exerting a force on the surface

Worked Example

The diagram below shows the parts of the lifting machine used to move the concert platform up and down.

Pressure Worked Example GCSE, downloadable IGCSE & GCSE Physics revision notes

The pump creates a pressure in the liquid of 5.28 × 105 Pa to move the platform upwards.

Calculate the force that the liquid applies to the piston.

Answer:

Step 1: List the known quantities

  • Cross-sectional area = 2.73 × 10-2 m2

  • Pressure = 5.28 × 105 Pa

Step 2: Write down the relevant relationship

p space equals fraction numerator space F over denominator A end fraction

Step 3: Rearrange for the force, F

F space equals space p A

Step 4: Substitute the values into the equation

F = (5.28 × 105) × (2.73 × 10-2) = 1.4414× 104

Step 5: Round to the appropriate number of significant figures and quote the correct unit

  • The least precise input value is 3 s.f.

  • Therefore, the answer can only be given to the same precision

F = 1.44× 104 N (3 s.f.)

Examiner Tips and Tricks

Exam questions will often use an unfamiliar context to make a question more difficult. These are called 'application questions'. It just means that you have to apply your understanding to a new situation. You already know the physics, you just have to think about how what you already know fits into the new situation.

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics Content Creator

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Caroline Carroll

    Reviewer: Caroline Carroll

    Expertise: Physics & Chemistry Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.