AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    Pressure in a fluid 1: A fluid can be either a liquid or a gas.

    A fluid is anything that flows, which covers both liquids and gases and excludes solids.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 1 of 9
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A fluid is anything that flows, which covers both liquids and gases and excludes solids. In a solid the particles are held in fixed positions and can only vibrate, so a solid keeps its shape. In a liquid the particles touch but slide past one another, so a liquid takes the shape of its container while keeping its volume. In a gas the particles are far apart and move randomly at speed, so a gas fills whatever space it is given. Because their particles can move, both liquids and gases press outwards on every surface they touch, and both can be pumped along a pipe. The differences matter too: a gas can be compressed because of the space between its particles, and its density is far lower, which is why pressure changes with depth much faster in water than in air.

    What earns the marks

    • A fluid identified as a substance that can flow.
    • A particle arrangement that allows the particles to move past one another.
    • Stating that a fluid exerts pressure on every surface it touches.

    Common mistakes and how to put them right

    • Restricting the word fluid to liquids only.

    • Saying a gas has no mass or exerts no pressure.

    • Describing the particles in a liquid as far apart, which is the arrangement in a gas.

    • Claiming a liquid can be compressed as easily as a gas.

    Exam tips

    • If a question says fluid, check whether the substance in the figure is a gas; the same equations still apply.
    • Use the word flow in any definition, because that is what is being tested.

    Try the lesson

    The start of the Pressure in a fluid 1: A fluid can be either a liquid or a gas. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which of the following substances is a fluid?

    Listen

    A fluid is anything that flows, which covers both liquids and gases and excludes solids. In a solid the particles are held in fixed positions and can only vibrate, so a solid keeps its shape. In a liquid the particles touch but slide past one another, so a liquid takes the shape of its container while keeping its volume. In a gas the particles are far apart and move randomly at speed, so a gas fills whatever space it is given. Because their particles can move, both liquids and gases press outwards on every surface they touch, and both can be pumped along a pipe. The differences matter too: a gas can be compressed because of the space between its particles, and its density is far lower, which is why pressure changes with depth much faster in water than in air. A strong answer earns marks for these points. a fluid identified as a substance that can flow. a particle arrangement that allows the particles to move past one another. stating that a fluid exerts pressure on every surface it touches. Exam tip: If a question says fluid, check whether the substance in the figure is a gas; the same equations still apply.

    See it

    A top-down flowchart. A box labelled Substance leads to a decision box asking Can it flow?. The Yes branch leads to a box labelled Fluid, and the No branch leads to a box labelled Solid. The Fluid box splits into two boxes, Liquid and Gas. Liquid leads to a box reading Particles touch but slide past one another, and Gas leads to a box reading Particles far apart, move randomly at speed. Both of these particle boxes lead to a final box reading Exerts pressure on every surface it touches.
    It shows how substances are classified as fluids or solids by whether they flow, with liquids and gases as the two fluids and their particle arrangements.

    Key words

    Say what each word means before you turn the card.

    fluids

    Try answering aloud or in your head before turning the card.

    particle arrangement

    Try answering aloud or in your head before turning the card.

    liquids and gases

    Try answering aloud or in your head before turning the card.

    compressibility

    Try answering aloud or in your head before turning the card.

    See every definition
    fluids
    Substances that can flow, covering both liquids and gases and excluding solids.
    particle arrangement
    In a liquid the particles touch but slide past one another, while in a gas they are far apart and move randomly at speed.
    liquids and gases
    Both can be pumped along a pipe and both press outwards on every surface they touch.
    compressibility
    A gas can be compressed because of the space between its particles, whereas a liquid can barely be compressed.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A box labelled Substance leads to a decision box asking Can it flow?. The Yes branch leads to a box labelled Fluid, and the No branch leads to a box labelled Solid. The Fluid box splits into two boxes, Liquid and Gas. Liquid leads to a box reading Particles touch but slide past one another, and Gas leads to a box reading Particles far apart, move randomly at speed. Both of these particle boxes lead to a final box reading Exerts pressure on every surface it touches.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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