Electromagnetic Radiation
Quantum and Nuclear | Light, Sound and Waves

Radiating: energy stores and pathways

Teaching Guidance for 14-16 Supporting Physics Teaching

Thinking about radiation from the Sun in terms of energy in stores and power in pathways

Think of a car left in a Mediterranean beach car park, where there is no shade. You get back to the car at the end of the day and open the doors. The metal of the car is too hot to touch: You could fry an egg on the bonnet! The air inside is stifling hot.

This familiar event is not too difficult to account for. The Sun has been shining on the car.

Now try to build in more links – that's what physics is all about. Can you give a good account in terms of energy in stores and power in pathways?

Richer descriptions

Thinking about stores

In trying to provide a deeper explanation, you can go back to the energy ideas of the SPT: Energy topic. Using these you can think of the Sun as a nuclear store and the car as a thermal store, with the two connected through a heating-by-radiation pathway.

As the long hot summer day progresses, the Sun's nuclear store is gradually depleted (not by very much!) and the thermal store of the car fills as energy is shifted from one to the other. The quantity of energy in the Sun's nuclear store is reduced: some of that energy ends up in the thermal stores of the car (of course, most of the energy from the Sun's store ends up in other stores).

In energy terms, that's the full story.

Thinking about pathways

You may wish to go a step further by thinking more deeply about the radiation pathway. In this case of the Sun heating the car, no physical material travels from the Sun to car. The heating-by-radiation pathway works through electromagnetic radiation. Electromagnetic oscillations, changing electric and magnetic fields, which are created in the Sun travel to Earth and set matter (such as the stuff that the car is made from) oscillating as they are absorbed by it. In this way energy is shifted by the heating-by-radiation pathway.

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