How does a microwave oven heat food?
A microwave oven uses a vacuum tube called a magnetron to create intense microwaves inside the cooking chamber. These microwaves are electromagnetic waves with a frequency of 2.45 gigahertz or 2,450,000,000 cycles per second. They are similar to normal radio waves, except that they have a higher frequency. Because of these microwaves, the electric field at any point inside the cooking chamber fluctuates back and forth 2.45 billion times each second. That means that an electrically charged particle at any point in the cooking chamber will be pulled first one way and then the other, back and forth 2.45 billion times each second. While water molecules aren’t electrically charged overall, they do have electrically charged ends—one end is positively charge and the other is negatively charged. In the presence of the microwave radiation, these water molecules find themselves twisted back and forth very rapidly. As they twist, they rub against one another and friction heats them up. The water becomes hot and this hot water, in turn, cooks the food. Food that doesn’t contain water (like salt or oil) won’t get hot. Neither will food in which the water molecules can’t turn (like ice or frozen food). That’s why it’s hard to defrost frozen food in a microwave.