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What does temperature really measure, and how does a thermometer work?

Explain temperature and thermal energy flow, and describe how a liquid-in-glass thermometer works

Explain what temperature measures, how thermal energy flows from hot to cold, the Celsius scale and its fixed points, and how a liquid-in-glass thermometer works at N(A)-Level.

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What this dot point is asking

SEAB wants you to explain what temperature measures, to describe how thermal energy flows from a hotter object to a cooler one until they reach the same temperature, to know the Celsius scale and its fixed points, and to describe how a liquid-in-glass thermometer works. The big idea is that temperature tells you how hot something is, and thermal energy always flows from hot to cold.

The answer

Temperature and thermal energy

Temperature is a measure of how hot or cold an object is. It is measured in degrees Celsius (∘C^\circ\text{C}) at this level.

Thermal energy (heat) is the energy that flows because of a temperature difference. It always flows from a hotter object to a cooler one, never the other way on its own. A hot spoon in cold water cools down while the water warms up.

Thermal equilibrium

When two objects are in contact, thermal energy flows from the hotter to the cooler until they reach the same temperature. At that point there is no longer any net flow, and we say they are in thermal equilibrium. A drink left in a room eventually reaches room temperature for this reason.

The Celsius scale and fixed points

To mark a temperature scale we need two fixed, repeatable temperatures, called fixed points:

  • The lower fixed point is the melting point of pure ice, defined as 0 ∘C0\ ^\circ\text{C}.
  • The upper fixed point is the boiling point of pure water at normal atmospheric pressure, defined as 100 ∘C100\ ^\circ\text{C}.

The space between is divided into 100 equal degrees.

How a liquid-in-glass thermometer works

A liquid-in-glass thermometer has a bulb of liquid (often alcohol or mercury) joined to a thin tube. When the temperature rises:

  1. The liquid in the bulb gains thermal energy and expands.
  2. Because the tube is very thin, the liquid is pushed a long way up the tube.
  3. The height of the liquid against the marked scale gives the temperature.

When the temperature falls, the liquid contracts and moves back down.

Making a thermometer more sensitive

A thermometer is more sensitive if the liquid moves further for each degree. You can achieve this with a thinner tube or a larger bulb, so a small change in temperature gives a clear, easy-to-read movement.

Examples in context

Example 1. A fever thermometer. A clinical thermometer has a narrow tube and a small constriction that traps the liquid so the reading holds after the thermometer is removed. The narrow tube makes it sensitive enough to show the small difference between a healthy temperature and a fever.

Example 2. Cooling a hot pan. Running a hot pan under cold water cools it quickly because thermal energy flows from the hot metal to the cooler water. The flow continues until the pan and the water reach the same temperature. The bigger the temperature difference, the faster the energy flows at first.

Try this

  • Cue. State the direction of thermal energy flow when an ice cube is put in a warm drink. [1 mark] From the warm drink to the colder ice cube (hot to cold).

  • Cue. State the two fixed points of the Celsius scale. [2 marks] Pure ice melting at 0 ∘C0\ ^\circ\text{C} and pure water boiling at 100 ∘C100\ ^\circ\text{C} at normal atmospheric pressure.

  • Cue. Explain why a liquid-in-glass thermometer with a thinner tube is more sensitive. [2 marks] For the same expansion of liquid, a thinner tube makes the liquid rise further per degree, so the temperature can be read more precisely.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA hot drink is left in a cool room. (a) State the direction in which thermal energy flows. (b) Explain what eventually happens to the temperatures. (c) State what is meant by thermal equilibrium.
Show worked answer →

(a) Thermal energy flows from the hot drink to the cooler room (from hot to cold).

(b) The drink cools and the air near it warms slightly, until the drink reaches the same temperature as the room.

(c) Thermal equilibrium is when two objects in contact reach the same temperature, so there is no longer any net flow of thermal energy between them.

What markers reward: hot-to-cold flow, the temperatures becoming equal, and a correct definition of thermal equilibrium.

Original4 marksA liquid-in-glass thermometer is used to measure temperature. (a) Explain how it works as the temperature rises. (b) State the two fixed points used to mark the Celsius scale. (c) Suggest one way to make a thermometer more sensitive.
Show worked answer →

(a) When the temperature rises, the liquid in the bulb expands and rises up the thin tube. The height of the liquid shows the temperature on the scale.

(b) The lower fixed point is the melting point of pure ice (0 ∘C0\ ^\circ\text{C}) and the upper fixed point is the boiling point of pure water (100 ∘C100\ ^\circ\text{C}) at normal atmospheric pressure.

(c) Use a narrower (thinner) tube, so the same expansion of liquid moves further up the tube for each degree.

What markers reward: expansion of the liquid up the tube, the two fixed points at 0 ∘C0\ ^\circ\text{C} and 100 ∘C100\ ^\circ\text{C}, and a sensible way to increase sensitivity.

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