Theme B — The Particulate Nature of Matter

B.1.3 Temperature Scales — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 53 1 quiz

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Lesson summary

B.1.3 Temperature Scales — IB PHYSICS SL notes: Convert between the Celsius and kelvin scales and explain the meaning of absolute zero and the fixed points.Temperature — a measure of how hot or cold an object is, related to the average random kinetic energy of the particles.Direc…

Direction of transfer
Not thermal energy
Liquid-in-glass thermometer
Other thermometric properties
Never negative
Conversion

Part 1 of 7What temperature measures

Temperature

Temperature a measure of how hot or cold an object is, related to the average random kinetic energy of the particles.

Direction of transfer

Direction of transfer thermal energy flows spontaneously from the object at higher temperature to the one at lower temperature.

Not thermal energy

Not thermal energy a bath of warm water holds far more energy than a hot spark but is at a lower temperature.

Part 2 of 7Celsius scale

1Fixed points

0 °C for the melting point of ice and 100 °C for the boiling point of water at standard pressure.

2Interval

Interval it is divided into 100 equal degrees.

3Liquid-in-glass thermometer

Liquid-in-glass thermometer it assumes a linear relation between the column length and the temperature, calibrated at the fixed points.

4Other thermometric properties

Other thermometric properties the resistance of a thermistor, the e.m.f. of a thermocouple and the pressure of a gas at constant volume.

Part 3 of 7Kelvin scale and absolute zero

Kelvin

Kelvin the SI unit of temperature, quoted with the symbol K and no degree sign.

Absolute zero

0 K or −273 °C, the lowest possible temperature.
It is defined as the temperature at which the particles have zero kinetic energy (minimum energy).
No more energy can be removed from a system at 0 K.

Never negative

Never negative a temperature in kelvin cannot be negative.

Space

Space the temperature of empty space is about 2.7 K.

Part 4 of 7Converting between scales

Conversion

T/K = θ/°C + 273 and θ/°C = T/K − 273.
The exact offset is 273.15, but the data booklet uses 273.

Equal divisions

Equal divisions a change of 1 K equals a change of 1 °C.

Values

Values water freezes at 273 K and boils at 373 K, 20 °C is 293 K and room temperature of 300 K is 27 °C.

Remembering

Remembering the data booklet lists the conversion under unit conversions.

Part 5 of 7Choosing and using a scale

Use kelvin

Use kelvin for equations where temperature is a multiplier or in a ratio.
pV = nRT, E_k = (3/2)k_BT and L = σAT⁴ are examples.
What the symbols mean: p = pressure (Pa) · V = volume (m³) · n = amount of gas (mol) · R = 8.31 J mol⁻¹ K⁻¹ · T = temperature (K = °C + 273)

Either scale

Either scale when only a temperature change ΔT matters, as in Q = mcΔT.

Equal changes only

20 K is the same change as 20 °C, but a temperature of 20 K is not 20 °C.

Why absolute

Why absolute a value in °C would give wrong ratios in gas laws and radiation laws.

Definition

Definition the kelvin is defined by fixing the Boltzmann constant, k_B = 1.38 × 10⁻²³ J K⁻¹.

Part 6 of 7Thermometers

Thermometer	Advantages	Limitations
Liquid-in-glass	Cheap and direct	Limited range, slow, cannot be read remotely
Thermistor	Rapid response, high sensitivity, can be logged	Narrow range, non-linear
Thermocouple	Very wide range, fast because of small mass	Needs a reference junction
Gas thermometer	Accurate and defines a scale	Bulky

Equilibrium

Equilibrium the thermometer must reach thermal equilibrium with the object.

Small heat capacity

Small heat capacity so that it does not change the temperature it measures.

Choosing

Choosing compare range, sensitivity, response time and whether it disturbs the system.

Calibration

Calibration repeat at fixed points.

Part 7 of 7More from the specification

Calibrating thermodynamic temperature scales

Calibrating thermodynamic temperature scales referencing fixed ice points and boiling points on Earth establishes empirical Celsius and thermodynamic Kelvin scales.

Absolute Temperature and Molecular Kinetic Energy

Absolute temperature in kelvin (K) is a direct measure of the average random translational kinetic energy of molecules within a substance, defined by Ek = (3/2) kB T.

Kelvin and Celsius Temperature Scales

Temperature scales are related by T(K) = theta(degrees C) + 273.15,
where absolute zero (0 K) represents the lowest theoretical temperature at which particle thermal kinetic energy is minimal.

Specific Heat Capacity versus Latent Heat

Specific heat capacity c measures the energy needed to raise the temperature of 1 kg of a substance by 1 K (Q = mc delta_T),
whereas specific latent heat L governs phase changes at constant temperature (Q = mL).
What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg⁻¹)

Conduction, Convection, and Radiation

Thermal energy is conducted through solids via lattice vibrations and free electrons, convected through fluids by bulk circulation,
and radiated across empty space as electromagnetic waves.

Potential energy

Energy stored by the configuration of a system.

Wavelength

Distance between a point on one wave and the same point on the next wave

Solid

State with a fixed shape and a fixed volume.

Fusion

Joining of light nuclei into a heavier one.

Intensity

Power per unit area perpendicular to the wave direction.

Apparent brightness

The power received per unit area at the observer

Phase change

Change of matter from one state to another

Thermal conductivity

Material constant relating the rate of conduction to the temperature gradient and area

Surface temperature

Effective temperature of the star's radiating surface.

Copper

Metal widely used for electrical wires.

Convection

Transfer of thermal energy by the movement of a heated fluid

Conduction

Transfer of thermal energy by lattice vibrations and free electrons

Molecule

Tiny particle of the fluid, too small to be seen.

Luminosity

The total power of radiation emitted by a star.

Density

Mass per unit volume.

Key terms

Term	Meaning
Temperature	Measure of how hot or cold an object is; related to average random kinetic energy
Absolute zero	Lowest possible temperature, 0 K or −273 °C
Kelvin	SI unit of temperature
Fixed point	Reference temperature such as the melting point of ice
Thermometric property	Physical property that varies with temperature and is used in a thermometer
Boltzmann constant	k_B = 1.38 × 10⁻²³ J K⁻¹

Avoid these mistakes

Writing the conversion with the wrong sign — add 273 to go from °C to K.

Using °C in gas-law equations — use kelvin.

Writing a degree sign with kelvin — it is K, not °K.

Saying 20 K is 20 °C — only the changes are equal.

Confusing temperature with thermal energy — temperature is about average kinetic energy.

Last updated: 15 October 2026