Why do microwaves heat water

It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Today I learnt that microwaves heat food by blasting electromagnetic waves through the water molecules found in the food.

And how in that case is a microwave able to melt plastics etc. Microwave heating is largely caused by the changing electric and magnetic fields i. As the direction of the electric field changes over time, the polar molecules often, of water attempt to follow the field by changing their orientation inside the material to line up along the field lines in an energetically favorable configuration namely, with the positive side pointing in the same direction as the field lines.

As these molecules change direction rapidly millions of times per second at least , they gain energy - which increases the temperature of the material.

This process is called dielectric heating. However, water is not the only polar molecule in the world. You can test for yourself that most plastics don't heat in a microwave while most glass and ceramic objects do. So, a microwave oven melting your plastic bowl has more to do with it over-heating your food than over-heating that food's container. EDIT: After doing some research to address some questions brought up in the comments to this post, I've found some very interesting information about why glass and ceramics heat up in the microwave which I will share here.

First of all, according to this article from the Royal Society of Chemistry so-called "earthenware" ceramics are fired at categorically lower temperatures than "stoneware.

The conclusion is that earthenware ceramics heat up in the microwave because they have the polar water molecules in them which undergo dielectric heating. On the other hand, stoneware and apparently porcelain will not heat in the microwave due to their respective lack of water molecules. Either way, I still wouldn't recommend microwaving your grandmother's porcelain china to find out. Second, glass' molecular structure is apparently locally tetrahedral but without long-range order i.

These impurities are only loosely bound and are able to move around within the amorphous structure of the glass. These ions of sodium or other elements have a net charge they are ions after all which means that the oscillating electric field produced by the microwave oven causes the ions to jostle back and forth, gaining energy.

The idea is very similar to the rotations of polar molecules which have an electric dipole but no net charge , but the mechanism is different namely, translational energy rather than rotational energy. So in summary, ceramics apparently heat up because they still contain some water, while glass heats up mostly because of the presence of semi-free, charged ions.

Microwave heating actually has nothing to do with the moisture content of items. It has everything to do with the amount of electric dipoles polar molecules in the item of concern. Water molecules with many other organic molecules happen to be electric dipoles. That is, one side of the molecule has a positive charge and the other side has a negative charge. When the oven uses a microwave to make an electric field, all electric dipoles move to align with that field.

If the direction of that field quickly flips, you give these dipoles kinetic energy. As you increase a group of molecules' kinetic energy, you increase the temperature of that group.

Any material containing significant amounts of electric dipoles will heat in a microwave. Also, resonance of water molecules have nothing to do with heating food in microwave ovens. The oscillations of the waves in microwave ovens is too slow for resonance to play a role. For further reading, check out wikipedia's articles on Microwave Ovens and Dielectric Heating , which should answer your questions more throughly.

A broadband electromagnetic wave contains many frequencies. You need a monochromatic wave a nearly single-frequency wave in order to tune to a specific frequency. Laser beams are monochromatic. Radio waves from simple antennas are monochromatic. Microwaves in an microwave oven are not monochromatic.

The microwaves in a microwave oven are created by a device called a magnetron, which is a resonant cavity that causes current to naturally oscillate at high frequency, and thereby emit electromagnetic waves. The oscillation of the current in the magnetron is not caused by a delicately-controlled external circuit. Rather, the oscillation arises naturally from the way that electrons emitted by the cathode hit the anode in a random fashion and then slosh around as directed by the magnetron's shape.

This randomness causes the magnetron to emit many frequencies. Furthermore, the random nature of the oscillation generation also causes the frequencies to be unstable and quickly jump around. Microwaves use electromagnetic waves to rapidly move and cause friction between water molecules to generate heat.

One study on how various microwaving temperatures affect the properties of water confirmed that microwaves can heat water to boiling temperature 1. That said, the electromagnetic waves in microwaves heat water molecules in random spots. This means that if the water is not heated long enough, pockets of boiling water may develop below a layer of cooler water.

You should also use microwave-safe cups when boiling water in a microwave. The health effects of microwaves remain controversial. You can boil water in a microwave.

However, microwaves may heat water unevenly, so make sure to stir it before use. Studies on the health effects of microwaves remain inconclusive. Spilling boiling water can be dangerous.

To protect your skin from burns , be sure to use hot pads when removing water from your microwave. You should only boil water in the microwave in approved containers. Steam vapor can also cause burns.

Therefore, be sure to protect your skin and do not put your hands directly above boiling water until it has cooled slightly. When boiling water in the microwave, be sure to take proper precautions.