OCR A Level Physics

Revision Notes

5.2.3 Gas Laws

Test Yourself

Investigating Gas Laws

Boyle’s Law

  • If the temperature T of an ideal gas is constant, then Boyle’s Law is given by:

Boyle's Law_2

  • This means the pressure is inversely proportional to the volume of a gas

Gas Volumes at Low Temperatures & High Pressures

Pressure increases when a gas is compressed

  • The relationship between the pressure and volume for a fixed mass of gas at constant temperature can also be written as:

P1V1 = P2V2

  • Where:
    • P1 = initial pressure (Pa)
    • P2 = final pressure (Pa)
    • V1 = initial volume (m3)
    • V2 = final volume (m3)

Boyles Law, downloadable AS & A Level Physics revision notes

Boyle's Law graph representing pressure inversely proportional to volume

  • If the temperature increases, the graph is further from the origin and vice versa

Pressure Law

  • If the volume V of an ideal gas is constant, the Pressure law is given by:

PT

  • This means the pressure is proportional to the temperature

  • The relationship between the pressure and thermodynamic temperature for a fixed mass of gas at constant volume can also be written as:

Pressure Law

  • Where:
    • P1 = initial pressure (Pa)
    • P2 = final pressure (Pa)
    • T1 = initial temperature (K)
    • T2 = final temperature (K)

Pressure Law, downloadable AS & A Level Physics revision notes

Pressure Law graph representing temperature (in °C) directly proportional to the volume

Worked example

The pressure inside a bicycle tyre is 5.10 × 105 Pa when the temperature is 279 K. After the bicycle has been ridden, the temperature of the air in the tyre is 299 K.

Calculate the new pressure in the tyre, assuming the volume is unchanged.

Pressure Law Worked Example _2

Exam Tip

Remember when using any ideal gas law, including the ideal gas equation, the temperature must always be in kelvin (K)

Estimating Absolute Zero

  • The value of absolute zero can be estimated by using the gas law Charles' Law
  • If the pressure P of an ideal gas is constant, then Charles’s law is given by:

V ∝ T

  • This means the volume is proportional to the temperature of a gas

  • The relationship between the volume and thermodynamic temperature for a fixed mass of gas at constant pressure can also be written as:

Charles's Law

  • Where:
    • V1 = initial volume (m3)
    • V2 = final volume (m3)
    • T1 = initial temperature (K)
    • T2 = final temperature (K)

Charles Law, downloadable AS & A Level Physics revision notes

Charles's Law graph representing temperature (in °C) directly proportional to the volume

  • The Charles's Law graph for temperature in Kelvin against volume is identical except that it is a straight line through the origin
    • Extrapolating backwards leads to a temperature of absolute zero at which the gas will have a volume of zero m3

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