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2.5 Microwave safety January 4, 2011 What radiations are on either side of microwave radiation in the electromagnetic radi...
 
 
Aims <ul><li>Explain how microwave radiation is able to heat by inducing vibrations </li></ul><ul><li>Explain the idea of ...
 
Microwave ovens <ul><li>Microwave radiation transfers energy to absorbing materials </li></ul><ul><li>Once the radiation i...
 
 
How deep?
 
How deep? <ul><li>Absorption does not take place until the radiation enters the material </li></ul><ul><li>Water in a pota...
How cooked? <ul><li>Because most of the energy goes into heating the food, using microwaves is an energy efficient way of ...
 
How cooked? <ul><li>You control the amount of cooking by adjusting </li></ul><ul><ul><li>The power settings </li></ul></ul...
 
Safety features <ul><li>People contain water, fats and sugars so we are very good absorbers of microwave radiation </li></...
 
Questions <ul><li>Why doesn’t microwave radiation cause ionisation? </li></ul><ul><li>In a conventional oven (not microwav...
Question <ul><li>How much larger is the power of a microwave oven than a 60 W lamp? </li></ul><ul><li>Why is it important ...
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2.5 microwaves

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2.5 microwaves

  1. 1. 2.5 Microwave safety January 4, 2011 What radiations are on either side of microwave radiation in the electromagnetic radiation
  2. 4. Aims <ul><li>Explain how microwave radiation is able to heat by inducing vibrations </li></ul><ul><li>Explain the idea of intensity (H: falls with distance) </li></ul><ul><li>Examine the balance of risks and benefits associated with the use of microwave ovens </li></ul>
  3. 6. Microwave ovens <ul><li>Microwave radiation transfers energy to absorbing materials </li></ul><ul><li>Once the radiation is absorbed it loses all its energy and ceases to exist </li></ul><ul><li>Molecules of water, fat, and sugar are all good absorbers of microwave radiation which makes them vibrate. Food containing these molecules get hot </li></ul><ul><li>A potato is made mostly of water, carbohydrate and just a little fat </li></ul><ul><li>Why does crockery get hot? </li></ul>
  4. 9. How deep?
  5. 11. How deep? <ul><li>Absorption does not take place until the radiation enters the material </li></ul><ul><li>Water in a potato is good at absorbing microwave radiation </li></ul><ul><li>But it is not so good that the energy is all absorbed near the surface of the potato </li></ul><ul><li>Some energy is transferred quite deeply into the potato </li></ul>
  6. 12. How cooked? <ul><li>Because most of the energy goes into heating the food, using microwaves is an energy efficient way of cooking </li></ul><ul><li>Microwave ovens are typically rated at 600-800 W </li></ul><ul><li>The heating effect of non-ionising radiation on an absorbing material always depends on its intensity (the energy arrives every second) and its duration (the exposure time) </li></ul>
  7. 14. How cooked? <ul><li>You control the amount of cooking by adjusting </li></ul><ul><ul><li>The power settings </li></ul></ul><ul><ul><li>The cooking time </li></ul></ul>
  8. 16. Safety features <ul><li>People contain water, fats and sugars so we are very good absorbers of microwave radiation </li></ul><ul><li>Exposure to this radiation would literally cook you </li></ul><ul><li>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 </li></ul>
  9. 18. Questions <ul><li>Why doesn’t microwave radiation cause ionisation? </li></ul><ul><li>In a conventional oven (not microwave), how does energy reach the centre of a potato to cook it? </li></ul><ul><li>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. </li></ul>
  10. 19. Question <ul><li>How much larger is the power of a microwave oven than a 60 W lamp? </li></ul><ul><li>Why is it important that the walls and door of a microwave oven reflect the microwave radiation? </li></ul>

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