1. GCSE: Extension Work A2 Level :Resonance
Cooking with Microwaves

2. Cooking with Microwaves
The microwave oven is well known as a fast, quick and efficient way of cooking food.
Its invention has changed the way we cook in the home.

3. Care when using the term ‘microwave’!
Watch how you express the term ‘microwave’ in your answers!
Distinguish clearly between
‘microwaves’ the waves – the photons of electromagnetic energy and
microwave ovens – the devices that cook using that energy

4. Cooking with Microwaves
The microwaves are produced by a magnetron - a high-powered vacuum tube that generates coherent (all ‘in step’ and of one wavelength) microwaves.
When you have the oven on a low setting you can hear the magnetron being switched on and off - delivering bursts of microwaves instead of a full stream of them.
The fan, that you can hear when the oven is on, is there to cool the magnetron - without it running the oven would overheat.

5. Cooking with Microwaves
The microwave oven cooks food by using high frequency electromagnetic waves (part of the electromagnetic spectrum - so their speed is 3 x 108 m/s in a vacuum) called microwaves at 2.45 GHz.
These are absorbed particularly well by water, and fat and sugar to a lesser extent.

6. Cooking with Microwaves
They get absorbed by the surface of the food and penetrate about 1cm into food before it is all absorbed.
The food is then hot on the surface and the heat is then conducted deeper into the cooler part of the food.

7. Cooking with Microwaves
The wavelength of mircowaves are between 1 millimetre and 30 centimetres.
Try calculating the wavelength of microwave oven microwaves! You should KNOW the wave equation
c = speed of electromagnetic radiation = 3 x 108 m/s
f = frequency of the waves = 2.45 GHz = 2.45 x 109 Hz

8. Cooking with Microwaves
c = fl
where
c = speed of electromagnetic radiation = 3 x 108 m/s
f = frequency of the waves = 2.45 GHz = 2.45 x 109 Hz
= wavelength of the microwaves
= c/f
= 3 x 108 / 2.45 x 109
= 0.12 m or 12 cm ANS

9. Cooking with Microwaves
Most food stuffs contain water.
Microwaves are most readily absorbed by water molecules (although fats and sugars also absorb too!).
These molecules then vibrate more quickly and therefore rise in temperature.

10. Cooking with Microwaves
Some science books say that the frequency of microwaves used in a microwave oven is the 'natural frequency' of water and resonance is the mechanism that causes the high amplitude vibration.
That is not the case – the chosen value of the frequency is not the resonant frequency of the water molecule.

11. Cooking with Microwaves
The vibration is down to the push and pull of the electric and magnetic forces from the wave that cause the vibration.
Water molecules (and sugars) are polar (one part is negative compared to the other).

12. Cooking with Microwaves
The electromagnetic waves have an electric field that 'pulls and pushes' charged particles such as the water molecule.
This causes the water molecules (that are attached together by hydrogen bonds into chains) to twist and turn.
Their vibrational and rotational energy increases - so their temperature rises.

13. Cooking with Microwaves
The frequency used in microwave ovens (2.45 GHz) is a sensible but not unique choice.
Waves of that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the foods.

14. Cooking with Microwaves
Leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications - the frequency was chosen in part because it would not interfere with existing communication systems.

15. Cooking with Microwaves
Using a frequency that water molecules responded to strongly (as in a resonance) would be a serious mistake--the microwaves would all be absorbed by water molecules at the surface of the food and none would be able to reach molecules deeper in the food.

16. Cooking with Microwaves
Conduction is a slow process.
So the food would not cook very well in the centre if the resonant frequency was used.

17. Cooking with Microwaves
The 2.45 GHz frequency was chosen because
it is absorbs well enough in liquid water so that the waves maintain good strength (even deep inside a typical piece of food).
Higher frequencies would penetrate less well and cook less evenly.

18. Cooking with Microwaves
The 2.45 GHz frequency was chosen because
Lower frequencies would penetrate better, but would be absorbed so weakly that they wouldn't cook well.
Also leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications - if lots of different frequencies were chosen it would cause a problem in communications..