What are the common units used for measuring temperature in the Cambridge IGCSE Coordinated Science curriculum?

In the Cambridge IGCSE Coordinated Science curriculum, temperature is commonly measured in degrees Celsius (°C). However, students may also encounter the Kelvin (K) scale, which is the SI unit for temperature. Understanding both units is essential for solving problems related to thermal physics.

How does a thermometer work in measuring temperature?

A thermometer measures temperature by using a substance that changes in a predictable way with temperature changes. For example, in a liquid-in-glass thermometer, the liquid (usually mercury or alcohol) expands or contracts as the temperature rises or falls. This change in volume moves the liquid up or down the scale, allowing us to read the temperature.

What is the difference between Celsius and Kelvin scales in temperature measurement?

The Celsius scale is based on the freezing and boiling points of water, with 0°C as the freezing point and 100°C as the boiling point. The Kelvin scale, used in scientific contexts, starts at absolute zero, the theoretical point where all molecular motion stops, which is 0 K. To convert from Celsius to Kelvin, you add 273.15 to the Celsius temperature.

Why is it important to calibrate a thermometer in thermal physics experiments?

Calibrating a thermometer is crucial to ensure accurate temperature measurements. Calibration involves setting the thermometer to known reference points, such as the freezing and boiling points of water, to correct any discrepancies. This ensures that the readings are reliable, which is essential for conducting precise experiments in thermal physics.

What are some common types of thermometers used in measuring temperature?

Common types of thermometers include liquid-in-glass thermometers, digital thermometers, and infrared thermometers. Liquid-in-glass thermometers use mercury or alcohol, digital thermometers use electronic sensors, and infrared thermometers measure temperature from a distance by detecting infrared radiation. Each type has its specific applications and advantages in different scenarios.

Why is "Describing fixed points without mentioning standard atmospheric pressure" a common mistake in Measurement of Temperature ?

Why it happens: Students know the temperatures but forget that boiling/freezing points depend on pressure. How to avoid it: Always state 'at standard (atmospheric) pressure' when describing fixed points. Boiling point of water changes with pressure.

Why is "Assuming all thermometric properties are linear with temperature without stating this assumption" a common mistake in Measurement of Temperature ?

Why it happens: Interpolation calculations assume linearity, but students do not flag this. How to avoid it: Write 'assuming a linear relationship' before applying the interpolation formula.

Why is "Using $T(\text{K}) = T(°\text{C}) + 273$ but then using the °C value in calculations" a common mistake in Measurement of Temperature ?

Why it happens: Students convert correctly but then accidentally use the original Celsius value. How to avoid it: After converting, cross out the Celsius value and only use the kelvin value in gas law calculations.

Thermal PhysicsCambridge IGCSE Coordinated Sciences 0654

Measurement of Temperature

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Measurement of Temperature — Cambridge IGCSE Coordinated Science 0654

Temperature is measured by thermometric properties — physical quantities that change measurably with temperature. Cambridge tests thermometer types, their advantages, and the Celsius scale vs Kelvin scale.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

P3.4a — Describe the physical properties used in thermometers.
P3.4b — Compare different types of thermometers and their suitability.
P3.4c — Recall the Celsius and Kelvin scales and convert between them.

Types of Thermometers

Different thermometers suit different ranges, response times, and situations.

Thermometric properties: A useful thermometric property must:

Change measurably with temperature
Change consistently and reproducibly
Be easy to read/measure

Liquid-in-glass thermometer:

Thermometric property: volume (length) of liquid in capillary tube
Mercury: range −39°C to 357°C; accurate; toxic; visible silvery colour
Alcohol (ethanol): range −115°C to 78°C; non-toxic; less accurate; coloured (red/blue)
Advantages: simple, cheap, self-contained, easy to read
Disadvantages: fragile; slow to reach equilibrium; cannot be used remotely

Thermocouple thermometer:

Two different metal wires joined at two junctions; e.m.f. generated proportional to temperature difference between junctions
Advantages: very wide temperature range (−200°C to +2000°C); fast response; can be very small (e.g., measure temperature of a tiny spot); can be read remotely
Disadvantages: requires reference temperature; needs voltmeter to read; less intuitive

Thermistor:

Semiconductor whose resistance decreases as temperature increases (NTC — negative temperature coefficient)
Used in: digital thermometers, patient monitors, engines, smart devices
Advantages: very fast response; easily interfaced with digital circuits

Clinical thermometer:

Mercury-in-glass with constriction (kink) that stops mercury falling back — records maximum temperature
Range: 35–42°C (human body temperature range)

Choosing a thermometer:

Requirement	Best thermometer
Very high temperature	Thermocouple
Very low temperature	Alcohol-in-glass
Fast response	Thermocouple or thermistor
Remote sensing	Thermocouple or thermistor
Simple/cheap	Liquid-in-glass

Liquid-in-glass: simple, cheap; uses thermal expansion of liquid.
Thermocouple: wide range (−200 to +2000°C); fast; remote use.
Thermistor: resistance decreases with temperature; used in digital devices.

Temperature Scales

The Celsius scale is defined by ice and steam points; Kelvin scale is the absolute scale.

Celsius scale:

0°C: melting point of pure ice at standard atmospheric pressure (lower fixed point)
100°C: boiling point of pure water at standard atmospheric pressure (upper fixed point)
Defined so there are exactly 100 divisions (degrees) between fixed points

Kelvin (absolute) scale:

T(K) = T(°C) + 273

0 K = absolute zero = −273°C (lowest possible temperature)
No negative temperatures on Kelvin scale
1 degree Celsius = 1 kelvin (same size unit, different starting point)
Used in gas law calculations (always)

Key temperature conversions:

Celsius	Kelvin
−273°C	0 K
0°C	273 K
20°C	293 K
100°C	373 K
1000°C	1273 K

0°C = 273 K. T(K) = T(°C) + 273.
Absolute zero = 0 K = −273°C; minimum possible temperature.
Both scales: 1 degree = 1 kelvin (same size interval).

How it’s examined

Paper 4: 'Convert 37°C to Kelvin' (1 mark — 310 K). 'State ONE advantage of a thermocouple over a liquid-in-glass thermometer' (1 mark — wider range / faster response / can be used remotely). 'State the thermometric property used in a liquid-in-glass thermometer' (1 mark — volume/length of liquid). MCQ: choosing the best thermometer for a given application.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (P3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Measurement of Temperature

Step-by-step solutions to past-paper-style questions on measurement of temperature , written exactly the way a tutor would explain them at the board.

1Calibrating a thermometer
Core• calibration, fixed points, Celsius
Question
Describe how to calibrate a liquid-in-glass thermometer using two fixed points.
Step-by-step solution
1. Step 1
  Immerse the thermometer bulb in a mixture of pure melting ice and water at standard atmospheric pressure. Mark the liquid level — this is $0\,°\text{C}$ (the lower fixed point).
2. Step 2
  Immerse the bulb in steam above boiling pure water at standard atmospheric pressure. Mark the liquid level — this is $100\,°\text{C}$ (the upper fixed point).
3. Step 3
  Divide the distance between the two marks into 100 equal divisions. Each division represents $1\,°\text{C}$ .
Answer
Mark 0 °C (ice point) and 100 °C (steam point) at standard pressure, then divide the interval into 100 equal divisions.
2Thermocouple thermometer — advantages
Extended• thermocouple, thermometric property
Question
Suggest two advantages of a thermocouple thermometer over a liquid-in-glass thermometer for measuring the temperature of a small, rapidly changing heat source.
Step-by-step solution
1. Step 1
  A thermocouple has a very small thermal mass (junction is tiny metal wires), so it responds rapidly to temperature changes — suitable for rapidly fluctuating temperatures.
2. Step 2
  A thermocouple can measure very high temperatures (well above $300\,°\text{C}$ ) at which a liquid-in-glass thermometer would break or the liquid would boil.
Answer
Faster response time (small thermal mass) and wider temperature range (can measure very high temperatures).
3Reading an unmarked thermometer using interpolation
Challenge• calibration, interpolation, thermometric property
Question
A resistance thermometer has resistance $200\,\Omega$ at $0\,°\text{C}$ and $280\,\Omega$ at $100\,°\text{C}$ . When placed in a liquid, its resistance is $248\,\Omega$ . Assuming a linear response, calculate the temperature of the liquid.
Step-by-step solution
1. Step 1
  Change in resistance over full range: $280 - 200 = 80\,\Omega$ corresponds to $100\,°\text{C}$ .
2. Step 2
  Change at the unknown temperature: $248 - 200 = 48\,\Omega$ .
3. Step 3
  Temperature by proportion.
  $\theta = \dfrac{48}{80} \times 100 = 60\,°\text{C}$
Answer
$\theta = 60\,°\text{C}$
Examiner tip
This interpolation method works for any thermometric property that varies linearly with temperature.

Key Formulae — Measurement of Temperature

The formulae you need to memorise for measurement of temperature on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.

Temperature by interpolation
$\theta = \dfrac{X_{\theta} - X_{0}}{X_{100} - X_{0}} \times 100\,°\text{C}$
$X_{\theta}$
thermometric property at unknown temperature
$X_{0}$
thermometric property at 0 °C
$X_{100}$
thermometric property at 100 °C
When to use
Reading a thermometer calibrated with two fixed points assuming a linear response.
Kelvin-Celsius conversion
$T(\text{K}) = T(°\text{C}) + 273$
$T(\text{K})$
temperature in kelvin
$T(°\text{C})$
temperature in degrees Celsius
When to use
Converting between Celsius and kelvin scales (required for all gas law calculations).

Key Definitions and Keywords — Measurement of Temperature

Definitions to memorise and the exact keywords mark schemes credit for measurement of temperature answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Thermometric property
Examiner keyword
A physical property that changes measurably and (ideally) linearly with temperature and can be used to measure it. Examples: length of a liquid column, electrical resistance, EMF of a thermocouple.
Fixed point
Examiner keyword
A standard, reproducible temperature used to calibrate thermometers. The lower fixed point is $0\,°\text{C}$ (pure ice point); the upper is $100\,°\text{C}$ (pure steam point), both at standard atmospheric pressure.
Thermocouple thermometer
A thermometer that uses the EMF generated at the junction of two different metals as its thermometric property. Advantages: fast response, wide range, can be made very small.
Celsius scale
A temperature scale with $0\,°\text{C}$ at the ice point and $100\,°\text{C}$ at the steam point of water at standard atmospheric pressure.

Common Mistakes and Misconceptions — Measurement of Temperature

The traps other students keep falling into on measurement of temperature questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Describing fixed points without mentioning standard atmospheric pressure
Why it happens
Students know the temperatures but forget that boiling/freezing points depend on pressure.
How to avoid it
Always state 'at standard (atmospheric) pressure' when describing fixed points. Boiling point of water changes with pressure.
✕Assuming all thermometric properties are linear with temperature without stating this assumption
Why it happens
Interpolation calculations assume linearity, but students do not flag this.
How to avoid it
Write 'assuming a linear relationship' before applying the interpolation formula.
✕Using $T(\text{K}) = T(°\text{C}) + 273$ but then using the °C value in calculations
Why it happens
Students convert correctly but then accidentally use the original Celsius value.
How to avoid it
After converting, cross out the Celsius value and only use the kelvin value in gas law calculations.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Measurement of Temperature — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Measurement of Temperature

Short Study Notes in the form of Slides

Short Study Notes — Measurement of Temperature

Detailed Notes

What you’ll learn

How it’s examined

1Calibrating a thermometer

2Thermocouple thermometer — advantages

3Reading an unmarked thermometer using interpolation

Temperature by interpolation

Kelvin-Celsius conversion

Thermometric property

Fixed point

Thermocouple thermometer

Celsius scale

✕Describing fixed points without mentioning standard atmospheric pressure

✕Assuming all thermometric properties are linear with temperature without stating this assumption

✕Using T(K)=T(°C)+273T(\text{K}) = T(°\text{C}) + 273T(K)=T(°C)+273 but then using the °C value in calculations

Practice questions

Past paper style quiz

Assess your understanding

Measurement of Temperature — frequently asked questions

✕Using $T(\text{K}) = T(°\text{C}) + 273$ but then using the °C value in calculations