Explain temperature as a measure of average particle energy and describe how a liquid-in-glass thermometer is calibrated

What this dot point is asking

SEAB wants you to understand temperature as a measure of how hot something is, related to the average energy of its particles, to know that thermal energy flows from hot to cold until thermal equilibrium, and to describe how a liquid-in-glass thermometer works and is calibrated using fixed points. The big idea is that temperature and total thermal energy are different things.

The answer

What temperature measures

Temperature is a measure of how hot or cold an object is. On the particle model, the higher the temperature, the greater the average kinetic energy of the particles. Temperature is measured in degrees Celsius ($^\circ\text{C}$) at O-Level.

Temperature versus thermal energy

These are not the same. Temperature is about the average energy per particle. Thermal energy (internal energy) is the total energy of all the particles. A bathtub of warm water at $40\,^\circ\text{C}$ holds far more thermal energy than a cup of boiling water at $100\,^\circ\text{C}$, because it contains so many more particles, even though its temperature is lower.

Heat flow and thermal equilibrium

When two objects at different temperatures touch, thermal energy flows from the hotter to the colder one. This continues until both reach the same temperature, a state called thermal equilibrium, after which there is no net flow.

The liquid-in-glass thermometer

A liquid-in-glass thermometer has a bulb of liquid (such as mercury or coloured alcohol) joined to a thin tube. When the bulb is warmed, the liquid expands and rises up the narrow tube; the height of the liquid shows the temperature.

Calibration with fixed points

To mark the scale, two fixed points are used:

• Lower fixed point: pure melting ice, marked $0\,^\circ\text{C}$.
• Upper fixed point: steam above boiling water at standard atmospheric pressure, marked $100\,^\circ\text{C}$.

The distance between these marks is divided into $100$ equal degrees to complete the Celsius scale.

Examples in context

Example 1. A spark versus a bath. A tiny spark from a firework can be at thousands of degrees yet carry so little thermal energy that it does no harm if it lands on your skin. A warm bath at only $40\,^\circ\text{C}$ holds vastly more thermal energy, because it contains so many more particles. This is the clearest illustration that high temperature is not the same as a lot of thermal energy.

Example 2. A hot drink cooling. A hot drink left on a table cools because thermal energy flows from the hotter drink to the cooler surroundings. The flow continues until the drink reaches room temperature, at which point it is in thermal equilibrium with the room and stops cooling.

Try this

Q1. State the temperatures assigned to melting ice and to steam on the Celsius scale. [2 marks]

• Cue. Melting ice is $0\,^\circ\text{C}$; steam (at standard atmospheric pressure) is $100\,^\circ\text{C}$.

Q2. State the direction of thermal energy flow between a hot and a cold object in contact. [1 mark]

• Cue. From the hotter object to the colder object.

Q3. Explain the difference between the temperature and the thermal energy of an object. [2 marks]

• Cue. Temperature is the average energy per particle; thermal energy is the total energy of all the particles, depending on how much substance there is.