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4.12 What is the lowest temperature possible? The Kelvin absolute temperature scale

We know the temperatures of the cores of stars can be millions of degrees, so there doesn't seem to be any upper limit to temperature!

BUT, is there limit at the lower end of the temperature scale?  The answer is YES!

The first experimental evidence for a lower temperature limit came from graphs of volume versus temperature and pressure versus temperature for a fixed mass of gas.

The plots for these were linear until the gas liquefied or the liquid solidified.

BUT, if you extrapolated the gaseous data back, you find every line ended up at a theoretical pressure of zero and temperature of -273^oC.

Explanation ....

As you cool a material, the particles have less and less kinetic energy of movement around (gases or liquids) or vibration in fixed positions (solid).

The kinetic energy of particles is a function of temperature.

You can also say, that what we measure as temperature, is a measure of the average kinetic energy particles have.

BUT, eventually virtually all movement due to kinetic energy ceases at a temperature of -273^oC, particles have ~zero kinetic energy (theoretically the particle vibration of the solid material has stopped).

Therefore that's it as regards particle KE, and the temperature as we know it, cannot fall any further - there is no more internal energy to remove!

So, the lowest possible temperature that is -273^oC. (-273 on the Celsius scale, unit ^oC).

 

Theoretically, at this temperature, the particles have no kinetic energy of movement or vibration, the coldest they can be - nothing left in the kinetic energy store of the particles.

In fact by then, every substance will have solidified, but at -273^oC there is zero vibration of the particles.

In 1848 a Scottish-Irish scientist called William Thompson (later became Lord Kelvin) proposed a new temperature scale starting at zero (called absolute zero), which became known as the 'Kelvin' scale of temperature - unit K.

Therefore the difference between the Celsius and Kelvin scales is 273.

To convert from one to the other, the following simple formulae apply.

K = 273 + ^oC     or     ^oC = K - 273

(absolute zero 0 K is the same temperature as - 273^oC)

Examples of temperature Celsius (^oC) and Kelvin (K) scale conversions:

Freezing point of water = 0^oC, therefore 0 + 273 = 273 K.

Boiling point of water = 100^oC, therefore 100 + 273 = 373 K.

Melting point of pure iron = 1811 K, therefore 1811 - 273 = 1538 ^oC

Note: Do NOT write degrees Kelvin, do NOT write ^oK.

and don't write just C for Celsius, you need the degree symbol ^o too.

INDEX of notes on Particle model theory state changes and latent heat

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Key points Thermal energy & particle models

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured and exam-board-inclusive set of summary revision notes on Absolute Zero and the Kelvin Temperature Scale, tailored for GCSE/IGCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

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What Is the Lowest Possible Temperature?

Absolute Zero

• Defined as 0 Kelvin (K) or −273°C.
• It is the lowest possible temperature in the universe.
• At absolute zero:
  • Particles have zero kinetic energy.
  • No thermal motion occurs - particles stop moving.
  • No energy can be removed from the system.

Why Is It the lowest temperature possible?

• Temperature is a measure of average kinetic energy of particles.
• At 0 K, particles have no kinetic energy left to lose.
• It’s a theoretical limit - cannot be reached in practice, only approached.

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The Kelvin Temperature Scale

Key Features about the Kelvin scale of temperature

• Starts at absolute zero (0 K).
• No negative values - simplifies calculations.
• Directly proportional to particle kinetic energy.
• Used in physics because it reflects true thermal energy.

Conversion Between Celsius and Kelvin

• To convert Celsius to Kelvin:
  T(K) = T(°C) + 273
• To convert Kelvin to Celsius:
  T(°C) = T(K) - 273

Celsius (°C)	Kelvin (K)
−273	0
0	273
100	373

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Required Knowledge by Exam Boards

All boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR) require:

• Understanding of absolute zero and its significance.
• Use of the Kelvin scale in gas laws and thermal energy calculations.
• Ability to convert between Celsius and Kelvin.
• Explanation of temperature as particle kinetic energy.

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Exam Tips about the Kelvin scale of temperature

• Always use Kelvin in gas law equations.
• Remember: 0 K = −273°C.
• A change of 1 K = change of 1°C.
• Use Kelvin to avoid negative temperatures in calculations.
• Link temperature to particle motion in explanations.

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Common Misconceptions about the Kelvin scale of temperature

• Temperature can go below 0 K → It cannot.
• Particles still move at absolute zero → They don’t.
• Kelvin and Celsius have different intervals → They’re equal in size.
• Kelvin uses degrees → It uses K, not °K.

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Keywords, phrases and learning objectives for particle models and thermal energy

Appreciate from graphical data the evidence for a lower limit to the scale of temperature and this led to the formulation of the Kelvin scale temperature.

Know the lowest possible temperature possible -273^oC (0K on the Kelvin scale).

Know how to convert between Celsius and Kelvin temperature scales in calculations.

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WHAT NEXT?

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INDEX for physics notes on particle model theory, state changes,  latent heat, heating/cooling curves

INDEX of all my THERMAL ENERGY notes

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