OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Explain how the motion of the molecules in a gas is related both to its…

    Full specification wording

    Pressure: explain how the motion of the molecules in a gas is related both to its temperature and its pressure (to include: application to closed systems only)

    In a gas the particles move randomly at high speed.

    Checked against the OCR J249 specification (opens in a new tab) on .

    • Point 3 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    In a gas the particles move randomly at high speed. Raising the temperature increases their average kinetic energy, so they move faster. In a closed system the number of particles and the volume are fixed, so faster particles strike the container walls more often and with greater force, raising the pressure. Conversely, compressing a fixed mass of gas at constant temperature crowds the particles so collisions with the walls become more frequent and pressure rises. A worked case: warming a sealed rigid can raises the temperature, the particles speed up, collisions with the walls become harder and more frequent, and the pressure inside increases. Keep the reasoning to closed systems, where mass and often volume stay constant.

    What earns the marks

    • States that gas particles move randomly and collide with the walls of their container.
    • Explains that raising the temperature increases the average kinetic energy of the particles so they move faster.
    • Links faster particle motion in a fixed volume to more frequent and more forceful collisions with the walls, raising the pressure.
    • Explains that compressing a fixed mass of gas at constant temperature increases collision frequency with the walls and so raises the pressure.
    • Applies the reasoning to a closed system in which the number of particles is fixed.

    Common mistakes and how to put them right

    • Claims that heating a gas makes the particles expand.

      Put it right: State instead that the particles move faster while the particles themselves stay the same size.

    • Says pressure rises because particles push harder on each other.

      Put it right: Explain that pressure comes from collisions of particles with the container walls.

    • Ignores the closed-system condition and lets particles escape.

      Put it right: Keep the number of particles fixed when relating temperature and pressure.

    • Describes temperature as the total kinetic energy of the gas.

      Put it right: Describe it as related to the average kinetic energy of the particles.

    Exam tips

    • Name the link in order: temperature, then particle speed or kinetic energy, then collision rate and force on the walls, then pressure.
    • State explicitly that the container is closed so the number of particles stays constant.
    • Use the words 'average kinetic energy' and 'collisions with the walls' to make the explanation precise.

    Try the lesson

    The start of the Pressure: explain how the motion of the molecules in a gas is related both to its temperature and its pressure (to include: application to closed systems only) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A fixed amount of gas is sealed in a rigid container. The gas is warmed so its temperature rises. What happens to the average speed of the gas molecules?

    Listen

    In a gas the particles move randomly at high speed. Raising the temperature increases their average kinetic energy, so they move faster. In a closed system the number of particles and the volume are fixed, so faster particles strike the container walls more often and with greater force, raising the pressure. Conversely, compressing a fixed mass of gas at constant temperature crowds the particles so collisions with the walls become more frequent and pressure rises. A worked case: warming a sealed rigid can raises the temperature, the particles speed up, collisions with the walls become harder and more frequent, and the pressure inside increases. Keep the reasoning to closed systems, where mass and often volume stay constant. A strong answer earns marks for these points. States that gas particles move randomly and collide with the walls of their container. Explains that raising the temperature increases the average kinetic energy of the particles so they move faster. Links faster particle motion in a fixed volume to more frequent and more forceful collisions with the walls, raising the pressure. Explains that compressing a fixed mass of gas at constant temperature increases collision frequency with the walls and so raises the pressure. Exam tip: Name the link in order: temperature, then particle speed or kinetic energy, then collision rate and force on the walls, then pressure.

    See it

    A flowchart. A top node says gas particles move randomly at high speed. It leads to two branches. The first branch goes to particles collide with the container walls, then to collisions with the walls produce pressure. A separate node saying closed system: number of particles fixed also points to that collisions-with-walls node. The second branch goes to raising the temperature increases average kinetic energy, then particles move faster, then more frequent and more forceful collisions with the walls, then pressure rises. A further chain starts at compressing a fixed mass at constant temperature, goes to particles crowded together, then more frequent collisions with the walls, and also ends at pressure rises.
    It shows how particle motion links to temperature and how collisions with the walls produce pressure, including the effects of heating and of compressing a fixed mass in a closed system.

    Key words

    Say what each word means before you turn the card.

    Kinetic model of a gas

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

    Temperature and particle speed

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

    Pressure from collisions with walls

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

    Closed systems

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

    See every definition
    Kinetic model of a gas
    Gas particles move randomly at high speed, and raising the temperature increases their average kinetic energy so they move faster.
    Temperature and particle speed
    Raising the temperature increases the average kinetic energy of the particles, so they move faster.
    Pressure from collisions with walls
    Faster particles strike the container walls more often and with greater force, raising the pressure.
    Closed systems
    The number of particles and the volume are fixed, so the reasoning relating temperature and pressure is kept to these systems.

    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 flowchart. A top node says gas particles move randomly at high speed. It leads to two branches. The first branch goes to particles collide with the container walls, then to collisions with the walls produce pressure. A separate node saying closed system: number of particles fixed also points to that collisions-with-walls node. The second branch goes to raising the temperature increases average kinetic energy, then particles move faster, then more frequent and more forceful collisions with the walls, then pressure rises. A further chain starts at compressing a fixed mass at constant temperature, goes to particles crowded together, then more frequent collisions with the walls, and also ends at pressure rises.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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