Use thermodynamic and Celsius temperature scales
Convert between kelvin and degrees Celsius, explain absolute zero and distinguish the thermodynamic scale from sensor properties.
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 14. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use temperature differences, proportional reasoning, energy, mass and SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement correctly converts Celsius temperature θ to kelvin T?
What you will learn
- Understand that the thermodynamic scale does not depend on a particular substance
- Convert between kelvin and degrees Celsius
- Identify 0 K as absolute zero and the lowest possible temperature
The thermodynamic scale is independent of the thermometer material
A thermometer may use a changing property such as resistance or liquid volume, but the thermodynamic temperature scale does not depend on the property of any one substance. Different sensor materials can be calibrated against the same temperature scale.
Kelvin is the SI unit of thermodynamic temperature. The kelvin scale begins at absolute zero, the lowest possible temperature; 0 K is not the same as 0 °C.
Convert using the offset between the scales
The syllabus relation is T/K = θ/°C + 273.15. For a numerical conversion, T in kelvin equals θ in degrees Celsius plus 273.15; conversely θ in degrees Celsius equals T in kelvin minus 273.15.
A temperature interval has the same numerical size in kelvins and degrees Celsius: a rise of 5 K is a rise of 5 °C. The offset applies to absolute temperatures, not to temperature differences.
Pause and explain
What is 0 K called?
Worked example
Convert 27.0 °C to kelvin and 300.0 K to degrees Celsius.
Show the worked solution
- For Celsius to kelvin, add 273.15.
- For kelvin to Celsius, subtract 273.15.
- Round both answers to one decimal place.
Answer 27.0 °C = 300.2 K; 300.0 K = 26.9 °C.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert −18.0 °C to kelvin.
Give me a hint
Add 273.15 to the Celsius temperature, keeping the sign of −18.0.
Compare my reasoning
- T/K = θ/°C + 273.15.
- T = −18.0 + 273.15 K.
- Round to one decimal place.
T = 255.2 K.
Look for these in your work
- Correct offset used
- Negative Celsius value handled
- Kelvin unit stated
Convert 350 K to degrees Celsius.
Give me a hint
Subtract 273.15 from the kelvin value.
Compare my reasoning
- θ/°C = T/K − 273.15.
- θ = 350 − 273.15 °C.
- Round to one decimal place.
θ = 76.9 °C.
Look for these in your work
- Conversion direction correct
- Offset subtracted
- Celsius unit stated
A metal-resistance thermometer and a liquid thermometer are both calibrated against the thermodynamic scale. Explain why their different sensing materials do not create different temperature scales.
Give me a hint
Distinguish each thermometer's temperature-dependent property from the scale used to assign temperature.
Compare my reasoning
- Resistance and liquid volume are sensor properties.
- Each property can be calibrated against thermodynamic temperature.
- The thermodynamic scale is independent of the particular substance used in the thermometer.
They use different signals but can be calibrated to the same substance-independent thermodynamic temperature scale.
Look for these in your work
- Sensor signals distinguished
- Calibration described
- Scale independence stated
Common mistakes
- Using 273.15 as a multiplier
- Adding 273.15 when converting kelvin to Celsius
- Applying the offset to a temperature change
What can you explain now?
Which statement correctly converts Celsius temperature θ to kelvin T?
Compare with the explanation
T/K = θ/°C + 273.15
The Celsius value is shifted by 273.15 to give absolute temperature.
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Make it stick
How do you convert an absolute temperature between Celsius and kelvin, and what is the lowest possible kelvin value?
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