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2.1.3Thermal Physics

Gases AND THE Absolute Scale OF Temperature — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.1.3 Gases and the Absolute Scale of Temperature — IGCSE PHYSICS notes: Describe how the pressure and volume of a gas change with temperature and volume, convert °C to kelvin and use pV = constant (Extended).Constant volume, fixed mass — increasing temperature increases pressure…

  • Constant temperature, fixed mass
  • Constant pressure
  • Conversion
  • Absolute zero
  • Boyle's law
  • Kinetic theory

Part 1 of 6Gas changes explained by particles

Gas changes explained by particles
Constant volume, fixed mass
  • Constant volume, fixed mass increasing temperature increases pressure because particles move faster and hit the walls more often and harder.
Constant temperature, fixed mass
  • Constant temperature, fixed mass decreasing the volume increases pressure because particles hit the walls more often per unit area per second.
Constant pressure
  • Constant pressure increasing temperature increases volume.

Part 2 of 6Temperature conversion and absolute zero

Conversion
  • T (in K) = θ (in °C) + 273.
Absolute zero
0 K = −273 °C.
The Kelvin scale starts at absolute zero, so it has no negative values.
A change of 1 K equals a change of 1 °C.
Temperatures must be in kelvin in gas calculations.

Part 3 of 6Boyle's law (Extended)

Boyle's law
Boyle's law for a fixed mass of gas at constant temperature, pV = constant, so p1V1 = p2V2.
Pressure is inversely proportional to volume.
A graph of p against V is a curve.
A graph of p against 1/V is a straight line through the origin.

Part 4 of 6Solving gas problems

1
Note the fixed quantity stated: pressure, volume or temperature.
2
Convert every temperature to kelvin by adding 273.
3
Choose the correct gas relationship for the fixed quantity.
4
Substitute the two sets of values.
5
Solve and check the answer is physically sensible.

Part 5 of 6Investigating Boyle's law

1
Trap a fixed mass of air in a sealed syringe or tube connected to a pressure gauge.
2
Record the initial pressure and volume.
3
Increase the pressure in steps and wait each time for the temperature to return to room temperature.
4
Record the pressure and volume at each step.
5
Calculate p × V for each step (constant) or plot p against 1/V (straight line through the origin).

Part 6 of 6More from the specification

More from the specification
Kinetic contrast between gases and solids
  • Kinetic contrast between gases and solids widely separated gas particles move freely at high speeds, unlike tightly constrained vibrating atoms in dense solids.
Kinetic theory
  • Kinetic theory gas molecules move randomly at high speed and collide with each other and the container walls
Brownian motion
  • Brownian motion visible smoke or pollen particles jiggle randomly because they are hit unevenly by fast invisible air or water molecules
Gas pressure
  • Gas pressure caused by molecules colliding with the container walls, each collision exerting a small force
Temperature and energy
  • Temperature and energy a higher temperature means greater average kinetic energy of molecules, so more frequent and forceful collisions
Absolute zero
  • Absolute zero the lowest possible temperature, 0 K or -273 degrees Celsius, where molecular motion is at a minimum;
    • Kelvin = Celsius + 273
Kinetic theory
  • Kinetic theory gas molecules move randomly at high speed and collide with each other and the container walls
Brownian motion
  • Brownian motion visible smoke or pollen particles jiggle randomly because they are hit unevenly by fast invisible air or water molecules
Gas pressure
  • Gas pressure caused by molecules colliding with the container walls, each collision exerting a small force
Temperature and energy
  • Temperature and energy a higher temperature means greater average kinetic energy of molecules, so more frequent and forceful collisions
Absolute zero
  • Absolute zero the lowest possible temperature, 0 K or -273 degrees Celsius, where molecular motion is at a minimum;
    • Kelvin = Celsius + 273
Solid
  • State with a fixed shape and a fixed volume.
Brownian motion
  • Random jiggling of small visible particles hit by fluid molecules.
Particle model
  • Representing an object as a single point.
Random motion
  • Movement in no specific path with sudden changes at collisions.
Kinetic energy
  • K = ½mv2, energy of motion.
Average speed
  • Total distance divided by total time.
Molecule
  • Tiny particle of the fluid, too small to be seen.
Liquid
  • State with a fixed volume that takes the shape of its container.
Random
  • Without a pattern in speed or direction.
Collision
  • Short interaction with large internal forces.
Force
  • A push or pull measured in newtons.

Key terms

TermMeaning
Kinetic theoryParticles in constant random motion.
Absolute zero0 K or −273 °C.
Boyle's lawpV is constant for a fixed mass of gas at constant temperature.
KelvinSI unit of temperature on the absolute scale.
Fixed massSame number of particles throughout.

Avoid these mistakes

1
Using Celsius temperatures in gas equations — convert to kelvin.
2
Applying Boyle's law when the temperature changes.
3
Believing the Kelvin scale has negative values.
4
Forgetting that the mass of gas must be fixed.

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