2.5 microwaves

• 1.2.5 Microwave safety January 4, 2011 What radiations are on either side of microwave radiation in the electromagnetic radiation

2. 3. 4. Aims

• Explain how microwave radiation is able to heat by inducing vibrations

• Explain the idea of intensity (H: falls with distance)

• Examine the balance of risks and benefits associated with the use of microwave ovens

5. 6. Microwave ovens

• Microwave radiation transfers energy to absorbing materials

• Once the radiation is absorbed it loses all its energy and ceases to exist

• Molecules of water, fat, and sugar are all good absorbers of microwave radiation which makes them vibrate. Food containing these molecules get hot

• A potato is made mostly of water, carbohydrate and just a little fat

• Why does crockery get hot?

7. 8. 9. How deep? 10. 11. How deep?

• Absorption does not take place until the radiation enters the material

• Water in a potato is good at absorbing microwave radiation

• But it is not so good that the energy is all absorbed near the surface of the potato

• Some energy is transferred quite deeply into the potato

12. How cooked?

• Because most of the energy goes into heating the food, using microwaves is an energy efficient way of cooking

• Microwave ovens are typically rated at 600-800 W

• The heating effect of non-ionising radiation on an absorbing material always depends on itsintensity(the energy arrives every second) and itsduration(the exposure time)

13. 14. How cooked?

• You control the amount of cooking by adjusting

• • The power settings

• • The cooking time

15. 16. Safety features

• People contain water, fats and sugars so we are very good absorbers of microwave radiation

• Exposure to this radiation would literally cook you

• The oven door has a metal grid to reflect the radiation back and a hidden switch to stop the oven when the door is open

17. 18. Questions

• Why doesnt microwave radiation cause ionisation?

• In a conventional oven (not microwave), how does energy reach the centre of a potato to cook it?

• A simple microwave oven is not very good at making baked potatoes. To get crispy skin you need a microwave with a grill function emitting infra-red radiation. Why is infrared radiation good for producing crispy skin but not good for heating through.

19. Question

• How much larger is the power of a microwave oven than a 60 W lamp?

• Why is it important that the walls and door of a microwave oven reflect the microwave radiation?