Thermodynamic temperature and Celsius conversion
Key idea: H2 Physics lessons on temperature, ideal gases, internal energy and thermodynamic systems.
Continue where you stopped
The core idea
Build the idea
Learn the idea
Big question: Why is thermodynamic temperature more than a relabelled Celsius scale?
Kelvin and Celsius intervals are equal, but their zeros differ: T/K = θ/°C + 273.15. Thermodynamic temperature begins at absolute zero and has no degree symbol. Temperature indicates the direction of spontaneous thermal transfer and, for an ideal gas, links to mean molecular kinetic energy.
Use an absolute zero and equal-sized intervals
Thermodynamic temperature is an absolute, substance-independent scale. Its zero, 0 K, is absolute zero; Celsius uses the same interval size but places 0 °C at 273.15 K. Thus T/K = θ/°C + 273.15 for readings.
A temperature change of 8 K equals a change of 8 °C because the offset cancels. Ratios such as 'twice as hot' are physically meaningful only with kelvin: 40 °C is not twice 20 °C.
Check your understanding: A sample warms from 15 °C to 35 °C. What is its temperature change in kelvin?
20 K. Do not add 273.15 to a temperature interval.
Connect temperature to equilibrium
Temperature determines the direction of net energy transfer by heating: from the system at higher temperature to the one at lower temperature. At thermal equilibrium the temperatures are equal and there is no net transfer, although microscopic exchanges continue.
For an ideal gas, thermodynamic temperature is proportional to mean translational kinetic energy per molecule. Absolute zero is a limiting zero of that model; it does not mean every microscopic contribution to energy in all matter vanishes.
Check your understanding: Two objects have the same internal energy. Must they be in thermal equilibrium?
No. Equilibrium requires equal temperature, whereas total internal energy also depends on amount and microscopic potential energy.
Key ideas to keep
- Write K, not °K.
- A temperature difference has the same numerical value in kelvin and Celsius degrees.
- Zero Celsius is not zero molecular motion.
See the reasoning
Worked example
Convert a reading without confusing it with an interval
Question: A resistance thermometer suggests a temperature of −40.0 °C. Convert it to kelvin and compare a 10 °C interval with a 10 K interval.
Step 1: Identify what is being converted
Why: A temperature reading needs the zero-point offset; a temperature change does not.
Working: The stated value is a reading of −40.0 °C.
Step 2: Apply the reading conversion
Why: The kelvin zero lies 273.15 units below the Celsius zero.
Working: T = −40.0 + 273.15 = 233.15 K.
Step 3: Compare interval sizes
Why: Kelvin and Celsius increments have the same size.
Working: A rise of 10 °C is exactly a rise of 10 K; no offset is added to a difference.
Answer: T = −40.0 + 273.15 = 233.15 K. Celsius and kelvin have equal-sized intervals, so a temperature change of 10 °C is exactly 10 K even though their zero points differ.
Check: 233.15 K is above absolute zero, and the kelvin unit has no degree sign.
Use a hint if needed
Practise with support
Try this
A gas changes from 250 K to 310 K. Give both Celsius temperatures and the temperature change.
Hint: Subtract 273.15 for readings; offsets cancel in a difference.
Check your answer
The readings are −23.15 °C and 36.85 °C. The change is 60 K or 60 °C; do not add 273.15 to an interval.
Now work without the hint
Practise independently
Your turn
Explain the two defining features of the thermodynamic scale and convert 68.0 °C to kelvin.
Check your answer
It is absolute, with zero at absolute zero, and substance-independent. T = 68.0 + 273.15 = 341.15 K.
Avoid these traps
Common mistakes
Common mistake
Zero degrees Celsius is absolute zero.
What is wrong with this reasoning?
Show better thinking
Absolute zero is 0 K or −273.15 °C; 0 °C is 273.15 K.
Write for the examiner
Exam guidance
Convert temperatures to kelvin before using gas or molecular-energy equations.
Exam-style practice [5 marks]
Define thermodynamic temperature, locate absolute zero and convert −18.0 °C to kelvin.
Plan before you answer
- Define the scale before calculating.
- Use 0 K for absolute zero.
- Apply the 273.15 offset once.
Mark your answer and compare the model
Marking points
Tick each point only if your answer states it clearly.
Model answer
It is an absolute, substance-independent temperature scale; absolute zero is 0 K. T = 255.15 K.
Come back in three days
Check what stayed with you
Recall question
Convert 315.0 K to Celsius and explain why kelvin has no degree sign.
Check the answer
θ = 315.0 − 273.15 = 41.85 °C. Kelvin is the SI thermodynamic-temperature unit on an absolute scale, written K rather than °K.
Syllabus and review details
This lesson covers the listed H2 Physics 9478 outcomes. Temperature and ideal-gas ideas lead into the first law of thermodynamics. Use ΔU = Q + W, where W is work done on the system; for expansion against constant external pressure, work done by the gas is pΔV and work done on the gas is −pΔV.
- GCE A-Level H2 PhysicsTopic 12(a) / Topic 12(b) · 2027Checked against the syllabus · partial topic coverageOfficial 9478 syllabus
Course and syllabus information
- Course
- GCE A-Level H2 Physics
- Edition
- GCE A-Level H2 Physics 2027