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12.2.2Particle Model

Absolute Zero — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 13

📚 Edexcel GCSE Combined Science 1SC0 - Physics Hub / Topical Revision
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Part 1 of 7Cooling a gas

As a gas is cooled, its molecules slow down.

Cooling a gas
Slower molecules hit the walls less often and less hard, so the pressure of a fixed volume of gas falls.
If we plot pressure against temperature in °C for a fixed mass of gas at constant volume we get a straight line which does not pass through the origin (the pressure is still large at 0 °C).

Part 2 of 7Extrapolating to zero pressure

Experiments with different gases and different starting amounts all give straight lines. When the lines are extended (extrapolated) backwards, they all reach zero pressure at the same temperature, about −273 °C. Real gases liquefy before they get there, so the line is extended by calculation, not measurement.

Zero pressure would mean the molecules were no longer hitting the walls, that is, they would have almost no kinetic energy. So −273 °C is the lowest temperature possible. We call it absolute zero.

Part 3 of 7What absolute zero means

What absolute zero means
At absolute zero particles have the minimum possible kinetic energy.
  • They do not stop completely, but at IGCSE we treat them as having essentially none.
    • (quantum effects)
It is impossible to cool anything to exactly absolute zero or below it.
The value is −273 °C (more precisely −273.15 °C), or 0 K.

Notice that absolute zero was found using the behaviour of gases, but it applies to every substance: it is the lower limit for temperature itself.

Part 4 of 7The Kelvin scale

The kelvin (K) is the SI unit of temperature. The Kelvin scale starts at absolute zero, so it has no negative values. A temperature step of 1 K is exactly the same size as a step of 1 °C. To convert:

T (K) = θ (°C) + 273

θ (°C) = T (K) − 273

The Kelvin scale
Water freezes at 273 K (0 °C).
Water boils at 373 K (100 °C).
Room temperature, 20 °C, is 293 K.

Note that the kelvin is written K with no degree sign.

Part 5 of 7Why use the Kelvin scale?

The Celsius scale is based on the properties of water, so its zero is arbitrary.

Why use the Kelvin scale?
The Kelvin scale is based on a real physical zero, so temperatures in kelvin are proportional to the average kinetic energy of the particles.
Doubling the kelvin temperature doubles the average kinetic energy; doubling the Celsius temperature means nothing physically.
For this reason all gas-law calculations (5.3.4 to 5.3.6) must use kelvin.

Part 6 of 7Evidence from experiments

A simple way to find absolute zero is to use a flask of air at constant volume connected to a pressure gauge.

1
Immerse it in a bath at several temperatures, for example in melting ice at 0 °C, in warm water at 40 °C and in nearly boiling water at 90 °C.
2
Plot pressure against temperature in °C and draw a line of best fit.
3
Extend this line back until it reaches zero pressure on the horizontal axis.
4
The intercept is close to −273 °C whatever gas or mass of gas is used.
5
The result is only approximate because of the small volume of gas in the connecting tube and the time it takes for the temperatures to equalise,
  • but it is convincing evidence that there is a minimum possible temperature.

Low temperature in practice

Scientists can cool materials to within a tiny fraction of a kelvin above absolute zero using liquid helium (boiling at about 4 K) and further techniques.

Evidence from experiments
At these temperatures some materials show remarkable behaviour, such as superconductivity, where the electrical resistance falls to zero.
Absolute zero itself can never be reached, only approached more and more closely,
  • which is a statement of the third law of thermodynamics encountered in later physics courses.

Part 7 of 7Worked examples

  1. Convert (a) 25 °C to kelvin and (b) 150 K to °C.

  2. A student says 'the temperature rose from 10 °C to 20 °C, so the particles have twice the energy'. Is this correct? Use the Kelvin scale to decide.

Key terms

TermMeaning
Absolute zeroLowest possible temperature: 0 K or about −273 °C.
Kelvin (K)SI unit of temperature, starting from absolute zero.
ExtrapolateExtend a graph line beyond the measured data.
Kinetic energyEnergy an object has because of its motion.

Avoid these mistakes

1
Adding or subtracting 273 the wrong way round when converting.
2
Using °C in a gas-law calculation. Always convert to K first.
3
Writing '°K'. The unit is simply K.
4
Thinking particles are completely motionless at absolute zero; they have the minimum possible energy.

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