What are the main temperature scales used in Physics?

In Physics, the main temperature scales used are Celsius, Fahrenheit, and Kelvin. Celsius is commonly used in most parts of the world for everyday temperature measurements. Fahrenheit is primarily used in the United States. Kelvin is the SI unit of temperature and is widely used in scientific contexts, including Cambridge International A Levels, because it starts at absolute zero, the theoretical point where all molecular motion ceases.

How do you convert temperatures between Celsius and Kelvin?

To convert a temperature from Celsius to Kelvin, you add 273.15 to the Celsius temperature. Conversely, to convert from Kelvin to Celsius, you subtract 273.15 from the Kelvin temperature. This conversion is crucial in Physics, especially in experiments and calculations involving thermodynamics, as it aligns with the absolute temperature scale.

Why is the Kelvin scale preferred in scientific measurements?

The Kelvin scale is preferred in scientific measurements because it is an absolute temperature scale that starts at absolute zero, the point where particles have minimal thermal motion. This makes it ideal for scientific calculations and experiments, as it provides a true measure of thermal energy. In the Cambridge International A Levels Physics curriculum, understanding the Kelvin scale is essential for topics like thermodynamics and kinetic theory.

What is the significance of absolute zero in temperature scales?

Absolute zero is the lowest possible temperature, theoretically the point at which all molecular motion stops. It is 0 Kelvin, equivalent to -273.15 degrees Celsius. Absolute zero is significant because it provides a baseline for the Kelvin scale, which is used in scientific research and education, including the Cambridge International A Levels Physics syllabus, to ensure precise and consistent temperature measurements.

How does the Fahrenheit scale differ from the Celsius scale?

The Fahrenheit scale differs from the Celsius scale in its starting point and the size of its degree. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees, whereas on the Celsius scale, water freezes at 0 degrees and boils at 100 degrees. The Fahrenheit scale is primarily used in the United States, while the Celsius scale is used worldwide and in scientific contexts, including the Cambridge International A Levels Physics curriculum.

Why is "Using Celsius in formulas" a common mistake in Temperature scales?

Why it happens: Habit. How to avoid it: Physics formulas (e.g. pV = nRT, E_k = 3/2kT) require KELVIN. Always convert. Source: 9702 Examiner Reports 2022-2024

TemperatureCambridge International A Level Physics 9702

Temperature scales

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Temperature Scales Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Celsius and Kelvin. Absolute zero. Always use K in physics formulas.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

14.2.1 — Convert between Celsius and Kelvin.
14.2.2 — State significance of absolute zero.

Celsius and Kelvin

Add 273.15.

Conversion. $T(K) = T(°C) + 273.15$ . Usually round to $+273$ .

Same size unit. 1 K is same interval as 1 $°$ C; only zero point differs.

Absolute zero (0 K). $-273.15°$ C. Theoretical minimum temperature where particles have minimum energy.

Why Kelvin in formulas? Many laws (gas, kinetic theory) imply that quantities scale with temperature ABOVE ABSOLUTE ZERO, not above an arbitrary reference like the melting point of ice.

Examples.

$25°$ C = 298 K.
400 K = $127°$ C.
Room temperature ~293-300 K.

Cambridge tip. Mark scheme rewards Kelvin in physics formulas; Celsius for everyday quoting.

The Kelvin and Celsius scales have the same size unit but differ in zero: 0 K = −273.15 °C is absolute zero.

$T(K) = T(°C) + 273$ .
Same-size units.
Absolute zero = 0 K.

See the full worked example for temperature scales →

How it’s examined

Temperature scales on every Paper 4. Most-tested: conversion (4 marks), absolute zero significance (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/42 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

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Step-by-step worked examples — Temperature scales

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1Celsius-Kelvin conversion (4 marks)
Extended• Adapted from 9702/42 May/Jun 2024• conversion
Question
Convert: (a) $25°$ C to K, (b) 400 K to $°$ C. (4 marks)
Step-by-step solution
1. Step 1
  $T(K) = T(°C) + 273.15$ .
2. Step 2
  (a) $25 + 273 = 298$ K (to 3 sf).
3. Step 3
  (b) $400 - 273 = 127°$ C.
Answer
(a) 298 K. (b) $127°$ C.
2Absolute zero (5 marks)
Extended• absolute zero
Question
Describe the significance of absolute zero. (5 marks)
Step-by-step solution
1. Step 1
  Absolute zero (0 K = $-273.15°$ C). Theoretical minimum temperature.
2. Step 2
  Particles have minimum (zero-point) energy. No further heat can be extracted.
3. Step 3
  Volume of ideal gas → 0 as $T \to 0$ K (in the limit).
Answer
Minimum temperature; particles at lowest energy; ideal gas volume → 0.

Key Formulae — Temperature scales

The formulae you need to memorise for temperature scales on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Celsius-Kelvin
$T(K) = T(°C) + 273.15$
When to use
Convert between scales. Kelvin scale used in all physics formulas.

Key Definitions and Keywords — Temperature scales

Definitions to memorise and the exact keywords mark schemes credit for temperature scales answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Absolute zero
Examiner keyword
Lowest possible temperature; 0 K = $-273.15°$ C. Particles at minimum energy.
Thermodynamic (absolute) temperature
Examiner keyword
Temperature measured in Kelvin from absolute zero. Used in physics formulas.

Common Mistakes and Misconceptions — Temperature scales

The traps other students keep falling into on temperature scales questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Using Celsius in formulas
9702 Examiner Reports 2022-2024
Why it happens
Habit.
How to avoid it
Physics formulas (e.g. $pV = nRT$ , $E_k = \frac{3}{2}kT$ ) require KELVIN. Always convert.

Practice questions

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Past paper style quiz

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Temperature scales — frequently asked questions

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Temperature scales

Short Study Notes in the form of Slides

Short Study Notes — Temperature scales

Detailed Notes

What you’ll learn

How it’s examined

1Celsius-Kelvin conversion (4 marks)

2Absolute zero (5 marks)

Celsius-Kelvin

Absolute zero

Thermodynamic (absolute) temperature

✕Using Celsius in formulas

Practice questions

Past paper style quiz

Assess your understanding

Temperature scales — frequently asked questions