Microwave ovens
Microwave ovens use microwaves with a wavelength of
about 12 cm and a frequency of 2.45 GHz to cook food. The big advantage of microwave
ovens over 'normal' cookers is that the food cooks from the inside.
Water and cooking with a microwave
oven
It is the water in food that allows us to cook it using microwaves. To
understand how the microwaves heat the food imagine a crowd watching a tennis match. As
the ball moves form one end of the court to the other the crowd turns backwards and
forwards facing first one way and then the other. If they are packed close together every time
they turn they rub against each other and so generate heat. This is rather similar to what
happens in the food in a microwave oven. The water molecules in the food absorb energy
from the microwaves and oscillate, turning first one way and then the other. They do this
rather faster than the tennis crowd – changing direction about 5 thousand million times a
second! This absorption of energy heats up the water and so cooks to the food.
The right frequency
It is important to get the frequency right – if
it is too high the microwaves would pass right through the food without heating it, if it is too
low they would be so strongly absorbed by the surface of the food that it would not cook
properly. If you have heard of resonance it is important to understand that this does NOT
take place in microwave cooking. If it did the molecules of water would absorb so much
energy that once again the microwaves could only penetrate a short distance into the
food.
Although the microwaves reflect from the wall of the oven the 'table' inside the
microwave oven rotates to make sure that the food is cooked evenly. If this was not done you
would get 'hot spots' within the oven and much of the food would not heat up.
Some
people have even used a microwave oven without a rotating table to measure the speed of
electromagnetic radiation! If you are interested you will need to read the section on Waves in the
Creative Teaching Ideas section on the CD that accompanies this site.
Defrost
cycle
Microwave ovens have a special cycle to defrost food. This is because while
water absorbs the microwaves strongly ice does not. Therefore as the ice melts the water
formed gets very hot quickly and so you can have ice and very hot water in the same portion
of food. Therefore in the defrost cycle the microwave power is switched on and off so that
there is time for the heat to spread out from the melted
water.
Safety
You want to cook the food inside the oven and not
yourself and so the door of a microwave oven has two safety features:
(i) a cut off that
switches off the microwave power as soon as the door is opened
(ii) a metal grill in the
glass of the door that has small enough holes to prevent the 12 cm wavelength microwaves
from escaping
(For a more technical explanation see:
16-19 Microwave ovens)
