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.
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?
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