Gases and the Absolute Scale of TemperatureCambridge IGCSE Physics: Revision notes
Section 1
How Does Particle Motion Relate to Temperature?
In the kinetic particle model, temperature is a measure of the average kinetic energy of the particles in a substance.
- As temperature rises, particles move faster on average and have more kinetic energy.
- As temperature falls, particles move slower and have less kinetic energy.
- There is a lowest possible temperature, called absolute zero, at which particles have the least possible kinetic energy. Absolute zero is approximately -273 degrees Celsius.
Absolute zero is a theoretical limit — particles never have zero kinetic energy at any real, achievable temperature.
Section 2
The Kelvin Scale
The kelvin scale is an absolute temperature scale that starts at absolute zero (0 K), so it has no negative values.
To convert between kelvin and Celsius:
T (in K) = theta (in degrees C) + 273
| Celsius | Kelvin |
|---|---|
| -273 degrees C | 0 K |
| 0 degrees C | 273 K |
| 100 degrees C | 373 K |
A change of 1 degree Celsius is exactly the same size as a change of 1 kelvin — only the starting point of the scale is different.
Forgetting to add 273 (not 273.15, which is not required at this level) when converting Celsius to kelvin, or subtracting instead of adding.
Room temperature of 20 degrees C = 20 + 273 = 293 K.
Section 3
Why Does a Gas Exert Pressure?
A gas exerts pressure on the walls of its container because its particles are in constant, random motion and repeatedly collide with the walls.
- Each collision exerts a tiny force on the wall.
- The pressure of the gas is the total force from all these collisions acting per unit area of the wall.
- More frequent collisions, or collisions with greater force, increase the pressure.
Section 4
How Does Temperature Change Affect Gas Pressure at Constant Volume?
If a fixed mass of gas is heated at constant volume:
- The average kinetic energy of the particles increases, so they move faster.
- The particles collide with the container walls more frequently and with greater force.
- This increases the pressure of the gas.
Cooling the gas at constant volume has the opposite effect: slower particles, less frequent and gentler collisions, and lower pressure.
Section 5
How Does Volume Change Affect Gas Pressure at Constant Temperature?
If a fixed mass of gas is compressed into a smaller volume at constant temperature:
- The particles are now closer together in a smaller space.
- They collide with the walls more often in a given time (the same number of particles hit a smaller area more frequently).
- This increases the pressure.
This relationship is described by the equation:
pV = constant (for a fixed mass of gas at constant temperature)
This means that if the volume decreases, the pressure must increase by the same proportion, and vice versa. On a graph of pressure against volume, this relationship produces a curve where pressure and volume are inversely proportional.
If a gas at 200 kPa in a 4 m3 container is compressed to 2 m3 at the same temperature, the new pressure is (200 x 4) / 2 = 400 kPa.
Must Know
- Temperature reflects the average kinetic energy of particles; higher temperature means faster-moving, more energetic particles.
- Absolute zero (approximately -273 degrees C) is the lowest possible temperature, where particles have the least possible kinetic energy.
- Convert Celsius to kelvin using T (K) = theta (degrees C) + 273.
- Gas pressure arises from particles colliding with the walls of their container.
- Heating a fixed mass of gas at constant volume increases pressure (faster, more frequent, harder collisions).
- Compressing a fixed mass of gas at constant temperature increases pressure, following pV = constant.
That's the notes covered.
Carry on to the next subtopic.