Spark image

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)
 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
 
© Keith Gibbs 2020