OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Explain the relationship between the temperature of a gas and its pressure…

    Full specification wording

    Pressure: explain the relationship between the temperature of a gas and its pressure at constant volume (qualitative only)

    At constant volume, the number of gas particles and the space they occupy stay fixed.

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

    • Point 4 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    At constant volume, the number of gas particles and the space they occupy stay fixed. Heating the gas transfers energy to the particles, so their mean kinetic energy rises and they move faster. Faster particles strike the container walls more often and with greater force per collision, so the total force on a fixed area of wall increases and the pressure rises. Cooling reverses this: particles move more slowly, collide less often and less forcefully, and the pressure falls. The relationship is qualitative here, so you explain the direction of change and the particle reason rather than calculating values. A sealed rigid metal can heated on a bench is a useful picture: its volume cannot change, yet the pressure inside climbs as the temperature climbs.

    What earns the marks

    • States that at constant volume the particles keep the same space to move in, so only their speed and collision behaviour change.
    • Explains that raising the temperature increases the mean kinetic energy of the particles, so they move faster.
    • States that faster particles hit the container walls more frequently and with greater force, raising the pressure.
    • Describes the reverse case, in which cooling slows the particles and lowers the pressure at fixed volume.

    Common mistakes and how to put them right

    • Claims that heating a fixed-volume gas makes the particles expand.

      Put it right by saying the particles speed up while the container volume stays the same.

    • Says pressure rises because there are more particles after heating.

      Put it right by noting the number of particles is unchanged at constant volume.

    • Treats the link as proportional and calculates values.

      Put it right by giving a qualitative explanation of direction and particle behaviour only.

    Exam tips

    • Anchor every answer in the particle model: name the change in particle speed, then the change in collision frequency and force on the walls.
    • Check that your explanation keeps volume constant; if you mention the gas expanding, you have left the stated condition.
    • Use the words temperature, kinetic energy, collisions and pressure so the chain of reasoning is visible to the examiner.

    Try the lesson

    The start of the Pressure: explain the relationship between the temperature of a gas and its pressure at constant volume (qualitative only) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A gas is trapped in a strong glass flask whose volume cannot change. The flask is gently heated. What happens to the pressure of the trapped gas?

    Listen

    At constant volume, the number of gas particles and the space they occupy stay fixed. Heating the gas transfers energy to the particles, so their mean kinetic energy rises and they move faster. Faster particles strike the container walls more often and with greater force per collision, so the total force on a fixed area of wall increases and the pressure rises. Cooling reverses this: particles move more slowly, collide less often and less forcefully, and the pressure falls. The relationship is qualitative here, so you explain the direction of change and the particle reason rather than calculating values. A sealed rigid metal can heated on a bench is a useful picture: its volume cannot change, yet the pressure inside climbs as the temperature climbs. A strong answer earns marks for these points. States that at constant volume the particles keep the same space to move in, so only their speed and collision behaviour change. Explains that raising the temperature increases the mean kinetic energy of the particles, so they move faster. States that faster particles hit the container walls more frequently and with greater force, raising the pressure. Describes the reverse case, in which cooling slows the particles and lowers the pressure at fixed volume. A common slip: Claims that heating a fixed-volume gas makes the particles expand. Instead, saying the particles speed up while the container volume stays the same. Exam tip: Anchor every answer in the particle model: name the change in particle speed, then the change in collision frequency and force on the walls.

    See it

    A flowchart starting from a gas at constant volume, which splits into two paths. The heating path goes: temperature rises, then mean kinetic energy of particles rises, then particles move faster, then particles strike walls more often and with greater force, then pressure rises. The cooling path goes: temperature falls, then particles move more slowly, then collisions less often and less forceful, then pressure falls.
    It shows how heating a gas at constant volume raises pressure through faster particle motion and more forceful collisions, and how cooling reverses this.

    Key words

    Say what each word means before you turn the card.

    Particle model of gases

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

    Kinetic energy and temperature

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

    Collisions with container walls

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

    Pressure at constant volume

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

    See every definition
    Particle model of gases
    The model in which gas particles move and strike the container walls, used to explain how temperature changes affect pressure at constant volume.
    Kinetic energy and temperature
    Heating the gas transfers energy to the particles, so their mean kinetic energy rises and they move faster; cooling reverses this.
    Collisions with container walls
    Faster particles strike the walls more often and with greater force per collision, so the total force on a fixed area of wall increases.
    Pressure at constant volume
    With the number of particles and the space they occupy fixed, rising temperature raises the pressure and cooling lowers it.

    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 starting from a gas at constant volume, which splits into two paths. The heating path goes: temperature rises, then mean kinetic energy of particles rises, then particles move faster, then particles strike walls more often and with greater force, then pressure rises. The cooling path goes: temperature falls, then particles move more slowly, then collisions less often and less forceful, then pressure falls.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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