Liquids and gases transfer heat by convection. z When the particles of a liquid or gas are heated, they begin to rise. When they get to a certain point, they fall back down.
z If this process repeats, a convection current of moving particles is created.
Convection is the reason hot air and hot water rise. Radiation
Radiation does not need a medium to transfer heat. Heat is transferred in the form of infrared waves.
z Hot bodies give out infrared radiation – the hotter the body, the greater the amount of infrared it radiates. This is how the Sun heats the air.
Learn more about the Earth and the Sun in Earth and Space 23.1
z Dark surfaces absorb and radiate infrared heat faster than bright surfaces. For this reason, houses in hot climates can be painted white to keep the inside cool.
Dark versus bright
A student set up the following apparatus to test the hypothesis: ‘Dark surfaces absorb radiation better than bright surfaces.’
1. How did the student ensure that this was a fair test?
2. Predict what will happen and explain the reasons for your predictions.
3. Set up this apparatus in class and carry out the investigation. Write down the method followed and record your observations.
Fig. 19.5.4 The infrared radiation from a hot body can be seen using a thermal imaging camera. What does this picture tell you about the heat radiated by spiders and humans?
Water
Dissolved dye particles rise due
to convection
Convection current
Particles fall again
19.5
Bunsen burner set a distance away from coins
4. Explain why your predictions did or did not match your observations.
5. Comment on the original hypothesis in light of your results.
