Celsius and kelvin temperatures

What you will learn

Absolute temperature conversions and temperature differences.

Specification mapping: C4.2

Prerequisites: C4.1

Physical principles

Temperature describes thermal state and determines the direction of spontaneous net heat transfer between bodies in contact: from higher to lower temperature. The kelvin is the SI unit of thermodynamic temperature. A kelvin interval has the same size as a Celsius-degree interval, but the scales have different zeros.

Convert using T(K) = θ(°C) + 273.15. Reverse with θ = T − 273.15. If an examination explicitly uses the rounded offset 273, use that convention and state it. Kelvin is written K without a degree sign. Absolute zero is 0 K, corresponding to −273.15 °C.

A temperature change has the same numerical value in K and °C: a rise from 20 to 35 °C is 15 °C or 15 K. Do not add 273.15 to a difference. In mcΔT calculations, use the change rather than the final absolute temperature. Ratios of Celsius temperatures do not have the same physical meaning as ratios on the absolute scale.

Scientific diagram and interpretation

Equations and relationships

T(K) = θ(°C)+273.15; ΔT(K) = Δθ(°C)

Worked examples and applications

Common misconceptions

Add the offset to a temperature, not to a temperature change. Kelvin has no degree symbol.

Examination technique

Keep θ for Celsius and T for kelvin when that helps distinguish the scales.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.