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P2.1.3P2. Thermal Physics

Gases and the absolute scale of temperature — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 14 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P2.1.3 Gases and the absolute scale of temperature — COMBINED SCIENCE notes: Convert between Celsius and kelvin, and describe and explain how the pressure, volume and temperature of a gas are related.Kelvin scale — temperature measured in kelvin (K) is called absolute temperature…

  • Absolute zero
  • Size of a step
  • Celsius to kelvin
  • Kelvin to Celsius
  • Fixed points
  • Compressing a gas

Part 1 of 7Absolute temperature

1Kelvin scale
  • Kelvin scale temperature measured in kelvin (K) is called absolute temperature.
2Absolute zero
  • 0 K = −273 °C, and no temperature can be lower, so a kelvin temperature is never negative.
3Size of a step
  • Size of a step a change of 1 K is the same as a change of 1 °C.

Part 2 of 7Converting between kelvin and Celsius

Celsius to kelvin
  • T (K) = θ (°C) + 273.
Kelvin to Celsius
  • θ (°C) = T (K) − 273.
Fixed points
  • 0 °C = 273 K and 20 °C = 293 K.

Part 3 of 7Pressure and volume at constant temperature

Pressure and volume at constant temperature
Compressing a gas
  • Compressing a gas decreasing the volume increases the pressure.
Expanding a gas
  • Expanding a gas increasing the volume decreases the pressure.
Particle explanation
  • Particle explanation in a smaller volume the particles hit the walls more often, so the total force on each wall and the pressure increase.

Part 4 of 7Boyle's law

Statement
  • Statement for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.
Equation
  • Equation pV = constant, so p1V1 = p2V2.
Halving the volume
  • Halving the volume doubling the pressure on a fixed mass of gas halves its volume.
Units
  • Units any consistent units can be used for p and V, provided the initial and final units match.

Part 5 of 7Pressure and temperature at constant volume

Pressure and temperature at constant volume
Effect
  • Effect raising the temperature of a fixed mass of gas at constant volume increases its pressure.
Particle explanation
  • Particle explanation the particles move faster, so they hit the walls more often and with greater force.
Graph
  • Graph with temperature in kelvin, pressure is directly proportional to temperature and the line passes through the origin.

Part 6 of 7Volume and temperature at constant pressure

Volume and temperature at constant pressure
Effect
  • Effect raising the temperature of a gas at constant pressure increases its volume.
Particle explanation
  • Particle explanation faster particles push harder on the walls, so the gas expands until the pressure falls back to the constant value.

Part 7 of 7More from the specification

1Intermolecular forces in crystalline solids
  • Intermolecular forces in crystalline solids strong cohesive forces hold constituent particles in fixed regular lattice positions throughout dense crystalline solids.
2Kinetic model
  • Kinetic model particles of a gas move randomly at high speed in all directions, colliding with the container walls.
3Brownian motion
  • Brownian motion smoke or pollen particles jitter because of random collisions with fast, invisible molecules.
4Gas pressure
  • Gas pressure it comes from the force of molecular collisions with the walls, rising with temperature as molecules move faster.
5Absolute temperature
  • Absolute temperature the kelvin scale starts at absolute zero where particles have minimum energy, with K equal to degrees Celsius plus 273.
6States of matter
  • States of matter a solid has a fixed shape, a liquid takes the shape of its container, and a gas fills all the space available.
7Kinetic model
  • Kinetic model particles of a gas move randomly at high speed in all directions, colliding with the container walls.
8Brownian motion
  • Brownian motion smoke or pollen particles jitter because of random collisions with fast, invisible molecules.
9Gas pressure
  • Gas pressure it comes from the force of molecular collisions with the walls, rising with temperature as molecules move faster.
10Absolute temperature
  • Absolute temperature the kelvin scale starts at absolute zero where particles have minimum energy, with K equal to degrees Celsius plus 273.
11States of matter
  • States of matter a solid has a fixed shape, a liquid takes the shape of its container, and a gas fills all the space available.
12Solid
  • A state of matter with a fixed shape and fixed volume.
13Brownian motion
  • The random movement of particles.
14Kinetic energy
  • Energy stored in a moving object.
15Average speed
  • Total distance travelled divided by total time taken.
16Molecule
  • Two or more atoms joined together chemically.
17Liquid
  • A state of matter with a fixed volume that takes the shape of its container.
18Collision
  • Particles hitting each other.

Key terms

TermMeaning
KelvinThe SI unit of absolute temperature.
Absolute zero0 K or −273 °C, the lowest possible temperature.
Boyle's lawAt constant temperature the pressure of a fixed mass of gas is inversely proportional to its volume.
Inversely proportionalWhen one quantity doubles, the other halves.
CompressionReducing the volume of a gas.

Avoid these mistakes

1
Adding 273 when converting kelvin to Celsius — subtract 273 to go from kelvin to Celsius.
2
Using Celsius in a proportionality — pressure is proportional to temperature only in kelvin.
3
Saying particles stop at absolute zero — they have the least possible kinetic energy.
4
Forgetting that Boyle's law needs constant temperature and a fixed mass of gas.

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