How do microwaves heat up food?

There is no better feeling than putting a leftover pizza in the microwave for just under a minute. It heats it up for you to perfection, but how? What is going on in the microwave that allows this to happen? How does it happen so quick?

A microwave emits waves called microwaves. It contains 3 main components:

1. A magnetron: this is a vacuum tube which generates the microwaves, which provides energy that heats food.
2. A waveguide: these are guides located as holes in the walls of the microwave when you open the door, which direct the waves produced by the magnetron to the chamber.
3. A chamber: this does the obvious and holds the food whilst maintaining a decent level of safety – containing the microwave radiation

Let’s define what heat is at a molecular level.

Heat is the transfer of energy due to a difference in temperature. At a molecular scale when molecules begin to heat up they vibrate more vigorously. This increased intensity of vibration is observed as a rise in temperature.

NOTE: heat can only travel from hot to cold.

When an oven or a stove are used to cook, the food is usually put on a heat insulating pan. This absorbs the heat, which then passes it on to the outside layer of the food. This cooks the outside and as more heat energy is supplied, heat from the cooked parts are passed on to the neighbouring colder uncooked parts; heating the food from the outside to the inside.

What is the key difference with using a Microwave? Food is cooked simultaneously throughout. But how?

Our food is filled with water - even dry foods contain water, and water molecules are polar. This means they are charged with one end being positive and the other being negative. To give these molecules more energy, we expose the food with electromagnetic waves which are produced by the magnetron; which are the microwaves.

Electromagnetic waves are two waves in 1; one is magnetic and the other is electric, which travel perpendicular to each other at all times – as shown below.

In a microwave, the magnetic and electric fields of the microwaves switch rapidly (imagine blue wave becoming red wave and vice versa on animation above). In fact they switch at a mind blowing 2.5 billion times a second.

The water in the food then tries to align with the electric field which is rapidly changing. This causes the water molecule to vibrate rapidly and due to molecular friction, heat is produced, thus heating up our food.

In conclusion, microwaves heat food a lot quicker than ovens as the food is heated inside out simultaneously. If the food had no water or in better context if it had no polar molecules, the microwave would pretty much be useless.

Hope this has helped and until next time, take care.

~ Mystifact