Heat and temperature¶
Heat and matter · level 4, lesson 2 of 6
A question to start¶
The sparks from a sparkler are over 1,000 °C, yet they land on your hand without burning it. A bath at 40 °C is much cooler. Which holds more heat?
The idea¶
Temperature and heat are not the same thing.
- Temperature tells you how fast the particles are jiggling on average. It is measured in °C or K.
- Thermal energy is the total energy of all that jiggling, so it depends on how much stuff there is as well. It is measured in joules (J).
- Heat is thermal energy on the move, from hotter to colder.
A spark is very hot but tiny: it holds very little thermal energy, and it cools the moment it touches you. A bath is only warm but holds a huge amount.
Worked examples¶
1. Cup and bucket. A cup and a bucket of water, both at 80 °C, have the same temperature. The bucket holds far more thermal energy and would melt far more ice.
2. Mixing. Pour the cup into the bucket: the temperature stays 80 °C, but the thermal energy adds up.
3. Mars air. At the same temperature, a cubic metre of Mars air holds about a ninetieth of the thermal energy of Earth air, because there is so little of it. That is why it cannot keep the ground warm at night.
Try it¶
- What does temperature tell you about the particles?
- Which has more thermal energy, a lake at 10 °C or a cup of tea at 70 °C?
- Why does a spark from a sparkler not burn you?
- Two pans of water are both at 50 °C, one holding 1 kg and one 2 kg. Which takes longer to cool to 20 °C on the same cooker turned off? Why?
Answers
- How fast they are moving on average.
- The lake: it is far bigger.
- It holds so little thermal energy that it cools almost at once.
- The 2 kg pan, if both lose heat at about the same rate: it has twice as much thermal energy to lose.
Common mistake¶
"Hotter always means more heat." Temperature is about each particle; thermal energy is about all of them. A small hot thing can hold less than a big warm one.