7 min readConcept sheet1-mark MCQ and Part II (2 marks) for definitions, units and temperature conversions

Temperature, heat and thermal equilibrium

Temperature tells how hot a body is; heat is energy that flows from a hotter body to a colder one until both reach the same temperature.

TemperatureDegree of hotness or coldness of a body; SI unit kelvin (K)

HeatEnergy that flows because of a temperature difference; SI unit joule (J)

Celsius to kelvinT (K) = t (°C) + 273

CalorieHeat needed to raise the temperature of 1 g of water by 1 °C; 1 cal ≈ 4.186 J

Temperature vs heat

TemperatureThe degree of hotness or coldness of a body. It is a measure of the average kinetic energy of the body's molecules. It decides the direction in which heat flows. SI unit: kelvin (K). Other scales are the degree Celsius (°C) and the degree Fahrenheit (°F). It is measured with a thermometer.

HeatA form of energy that flows from a hotter body to a colder body because of their temperature difference. SI unit: joule (J). An older unit is the calorie: the heat needed to raise the temperature of 1 g of water by 1 °C.

A bucket of warm water can give out more heat than a cup of hot tea. This is true even though the tea is at a higher temperature.

Temperature scales

Point	Celsius	Kelvin	Fahrenheit
Absolute zero (the lowest possible temperature)	−273 °C	0 K	About −460 °F
Ice melts (freezing point of water)	0 °C	273 K	32 °F
Normal body temperature	37 °C	310 K	98.6 °F
Water boils (at normal pressure)	100 °C	373 K	212 °F

Thermal equilibrium

Put a hot body and a cold body in contact. Heat flows from the hot one to the cold one. The hot body cools and the cold body warms up. If no heat escapes, the heat lost by the hot body equals the heat gained by the cold body. The mass of a body does not change when it is heated or cooled. The flow stops when both reach the same temperature. The two bodies are then in thermal equilibrium: there is no net flow of heat between them. A thermometer works on this idea. It is kept in contact with a body until both are at the same temperature. Then it shows that temperature.

Activity: reaching thermal equilibrium

1
Pour hot water into a steel tumbler. Stand the tumbler in a bowl of cold water.
2
Put a thermometer in each and note both readings every minute.
3
Observe: the hot water's reading falls and the cold water's reading rises.
4★
After some time, both thermometers show the same reading. Heat has flowed until the two are in thermal equilibrium.

★ key step

Converting temperatures

Problem

Convert (i) 27 °C into kelvin (ii) 373 K into degree Celsius (iii) 37 °C into degree Fahrenheit.

Working

(i) T = t + 273 = 27 + 273 = 300 K.
(ii) t = T − 273 = 373 − 273 = 100 °C.
(iii) F = (9/5) × C + 32 = (9/5) × 37 + 32 = 66.6 + 32 = 98.6 °F.

Answer

300 K, 100 °C and 98.6 °F. (The exact Celsius to kelvin step is 273.15; the board uses 273.)

In real life

A clinical thermometer is kept under the tongue for about a minute so that it reaches thermal equilibrium with your body. Food energy on a biscuit packet is printed in kilocalories (kcal), a unit of heat energy.

Exam angle

1 marks: “What is the SI unit of temperature?” Kelvin (K).

2 marks: “Distinguish between heat and temperature.” Heat is energy that flows because of a temperature difference (unit joule). Temperature is the degree of hotness that decides the direction of heat flow (unit kelvin).

2 marks: “What is thermal equilibrium?” The state in which two bodies in contact are at the same temperature, so there is no net flow of heat between them.

Remember it

Heat flows downhill in temperature: from hot to cold, and it stops when the temperatures are equal.