How does a microwave heat food?

1the myth

A popular claim says a microwave oven cooks food from the inside out. It does not. The waves are absorbed in the outer layer of the food, much as heat from other cooking methods starts at the surface. The inside of a thick food mostly warms by heat conducting in from that outer layer. There are odd cases, such as a dry crust over a moist filling, but the rule is outer layer first.

Microwaves heat the outer few centimetres first; the middle warms by conduction.

2the source

A part called the magnetron turns electric power into microwaves. At the usual household frequency of 2.45 gigahertz, the waves are 12.2 centimetres long. A short metal channel, the waveguide, carries them from the magnetron into the cooking box, which is closed with a metal mesh door whose holes are far smaller than the wavelength, so the waves stay in while light gets out.

A magnetron makes 12.2 cm waves and a waveguide feeds them into the box.

3lopsided water

A water molecule is a polar molecule: its two hydrogen atoms sit on one side, so that side carries a slight positive charge, and the oxygen side carries a slight negative charge. Fat molecules are polar in this way too. A polar molecule tries to line up with any electric field it sits in.

Water and fat molecules have a plus end and a minus end, so an electric field can twist them.

4jostling is heat

The microwaves are a rapidly changing electric field, swinging back and forth 2.45 billion times a second. Each swing tries to turn the lopsided molecules to line up, so they keep rotating and bumping their neighbours. That motion spreads through the food as heat. Water does not need to resonate with the wave for this: it heats across a wide range of frequencies, and 2.45 gigahertz was picked because that band is set aside for this kind of use worldwide.

A swinging electric field keeps twisting polar molecules, and the jostling is heat.

5how deep

At 2.45 gigahertz, the waves are absorbed mostly in the outer few centimetres of food that is rich in water. In liquid water, about half the power is absorbed in the outer centimetre or so. Thick food therefore heats in two stages: the waves heat the outer layer, then conduction carries that heat inward, which is why a microwaved dish often needs time to stand.

Waves heat only the outer few centimetres; conduction does the rest.

6hot spots

Microwaves bouncing around a metal box do not fill it evenly: some places get more energy and others less. A turntable, or in some ovens a metal stirrer fan, moves the food through the field so no part sits in a cold spot for the whole time. A hot spot or a cold spot is the reason a plate of leftovers can be scalding in one place and icy in another.

The field is uneven, so the turntable carries the food through the hot and cold spots.

7the whole idea

The whole oven on one page: a magnetron makes microwaves at 2.45 gigahertz, 12.2 centimetres long; they enter the box and swing the electric field billions of times a second; water and fat molecules twist back and forth and the jostling becomes heat; the heat starts in the outer few centimetres and conducts inward; a turntable moves the food through the hot and cold spots.

Magnetron waves jostle water and fat; the jostling is heat, from the outside in.