Transcript
2.5 Microwave safetyApril 12, 2023
What radiations are on either side of microwave
radiation in the electromagnetic radiation
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
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?
How deep?
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
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 its intensity (the energy arrives every second) and its duration (the exposure time)
How cooked?
• You control the amount of cooking by adjusting– The power settings– The cooking time
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
Questions
• Why doesn’t 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.
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?
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