2.5 Microwave safety 26 August 2015 What radiations are on either side of microwave radiation in the electromagnetic radiation.

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Presentation transcript:

2.5 Microwave safety 26 August 2015 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?

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 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?