- Explain temperature in terms of particle motion and convert between the Celsius and Kelvin scales
- Calculate the heat needed to warm a substance
- Distinguish conduction, convection and radiation
A metal spoon in hot tea heats up quickly, but a wooden spoon hardly warms at all. In summer the water of the Caspian Sea warms up much more slowly than the air. To understand this, we need to know what temperature and heat are and how they differ.
What does temperature tell us?
All substances are made of tiny particles — atoms and molecules — that are constantly moving in random directions. This is called thermal motion. Temperature shows how fast the particles are moving: the hotter an object, the faster its molecules move.
In everyday life we measure temperature on the Celsius scale: at normal atmospheric pressure ice melts at 0 °C and water boils at 100 °C. Physics also uses the Kelvin scale. Its zero point, absolute zero (−273 °C), is the temperature at which thermal motion is at its weakest — nothing can be cooled below it.
- Ttemperature on the Kelvin scale, in K
- ttemperature on the Celsius scale, in °C
For example, 20 °C = 293 K; a temperature difference is the same on both scales.
Internal energy and quantity of heat
The total energy of motion and interaction of all the particles in an object is its internal energy. It can be changed in two ways: by doing work (rubbing your hands together warms them) or by heat transfer (a spoon in boiling water warms up). The energy gained or lost by heat transfer is called the quantity of heat (Q), and it is also measured in joules.
- Qquantity of heat, in J
- cspecific heat capacity of the substance, in J/(kg · °C)
- mmass, in kg
ΔT — change in temperature (final − initial), in °C
| Substance | c, J/(kg · °C) |
|---|---|
| Water | 4200 |
| Ice | 2100 |
| Aluminium | 920 |
| Iron | 460 |
| Copper | 400 |
How much heat is needed to warm 2 kg of water in a kettle from 20 °C to boiling point (100 °C)?
Show solutionHide solution
Q = m · c · ΔT = 2 · 4200 · 80 = 672 000 J = 672 kJ.
1 kg of water and 1 kg of iron each receive the same amount of heat: 42 000 J. By how many degrees does the temperature of each rise?
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Water: ΔT = 42 000 / (1 · 4200) = 10 °C.
Iron: ΔT = 42 000 / (1 · 460) ≈ 91 °C.
Iron has a much smaller specific heat capacity, so its temperature rises about 9 times more than the water's.
How is heat transferred?
Heat always flows from a hotter object to a colder one and stops when their temperatures become equal. There are three ways this happens:
- Conduction — energy passes from particle to particle while the material itself stays in place. Metals are good conductors; wood, air and wool are poor ones.
- Convection — warmed liquid or gas rises and cooler liquid or gas sinks. This is how a radiator heats a room.
- Radiation — energy is carried by rays and can even cross empty space. This is how the Sun's heat reaches the Earth.
Key points
- Temperature shows how fast particles move in thermal motion; T = t + 273.
- Internal energy changes through work or heat transfer.
- Heat needed for warming: Q = m · c · ΔT.
- Water has a large specific heat capacity: 4200 J/(kg · °C).
- Heat is transferred by conduction, convection and radiation.
Check yourself
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