Convert between Celsius and kelvin temperatures
Use the selected 273 offset in both directions and write the correct units.
Section 2.1.3 · Gases and the absolute scale of temperature
Core and Extended; Physics 0625, 2026–2028, section 2.1.3. Explain pressure changes for a fixed gas mass at constant volume or constant temperature, and convert between kelvin and degrees Celsius using the selected +273 approximation. Numerical pV calculations and graphs belong to the labelled Supplement lessons. Written practice is self-checked.
Before you begin
- Identify absolute zero as the lowest-temperature reference.
- Add and subtract signed numbers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is −10 + 273?
What you will learn
- Recall T in K = θ in °C + 273 for the selected syllabus.
- Convert positive and negative Celsius readings and kelvin readings consistently.
Add the offset for a Celsius reading
The selected syllabus uses T (K) = θ (°C) + 273. For 20°C, T = 293 K; for −10°C, T = 263 K. Kelvin temperatures use the symbol K without a degree sign. Negative Celsius temperatures can still correspond to positive kelvin temperatures, because the two scales have different zero points.
The school-level approximation places −273°C at 0 K and 0°C at 273 K. The exact thermodynamic offset is 273.15, but use the selected 273 when answering these syllabus-model questions. Do not alternate offsets halfway through a calculation or silently replace a supplied approximation with another precision.
Subtract the offset to return to Celsius
Rearrange the equation as θ (°C) = T (K) − 273. A reading of 310 K becomes 37°C, and 250 K becomes −23°C. Adding 273 to a kelvin reading would move the conversion in the wrong direction. Keep the input scale explicit before deciding which operation to use.
A proposed equilibrium temperature below 0 K is outside this absolute-temperature model. This does not make every negative Celsius reading impossible: −23°C corresponds to positive 250 K. The distinction is between the temperature's physical lower bound and the arbitrary zero of the Celsius scale.
Pause and explain
Convert 250 K to degrees Celsius using the selected approximation.
Worked example
Convert −10°C to kelvin and 310 K to degrees Celsius using the selected approximation.
Show the worked solution
- For Celsius to kelvin, add 273: −10 + 273 = 263.
- For kelvin to Celsius, subtract 273: 310 − 273 = 37.
- Use K without a degree sign and °C for the Celsius result.
Answer 263 K; 37°C.
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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 −10°C to kelvin and 310 K to degrees Celsius.
Give me a hint
Identify the input scale before adding or subtracting 273.
Compare my reasoning
- Celsius input needs +273: −10 + 273 = 263 K.
- Kelvin input needs −273: 310 − 273 = 37°C.
- Write each result with its appropriate scale symbol.
263 K; 37°C.
Look for these in your work
- Choose the direction correctly.
- Preserve the negative input sign.
Convert 25°C, −40°C and 373 K using the selected approximation.
Give me a hint
Use the same offset for all three conversions.
Compare my reasoning
- 25 + 273 = 298 K.
- −40 + 273 = 233 K.
- 373 − 273 = 100°C.
298 K; 233 K; 100°C.
Look for these in your work
- Convert both scales.
- Use K without a degree sign.
A conversion gives −280°C as −7 K. Check the arithmetic and whether the proposed equilibrium temperature fits the absolute-temperature model.
Give me a hint
Correct arithmetic can still reveal a physically inadmissible input.
Compare my reasoning
- Using the selected formula, −280 + 273 = −7.
- That kelvin result is below the model's 0 K lower bound.
- The proposed equilibrium temperature is inadmissible; a negative Celsius reading alone would not be sufficient to reject a different temperature.
The arithmetic gives −7 K, revealing an inadmissible equilibrium temperature in this model.
Look for these in your work
- Separate arithmetic from admissibility.
- Do not reject all negative Celsius readings.
Common mistakes
- Add for °C to K; subtract for K to °C.
- Kelvin uses K, not °K.
- Negative °C does not mean negative K.
What can you explain now?
What is 0°C in the selected kelvin conversion?
Compare with the explanation
273 K.
Add the selected offset: 0 + 273 = 273.
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Make it stick
Convert readings both ways, state the selected offset and distinguish negative Celsius from the absolute lower bound.
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