Cooking with electromagnetic Waves

A gas or electric stove generates thermal energy (heat) that warms and cooks food. A microwave oven generates electromagnetic energy (microwaves) produced by a device called a magnetron (see Figure 20-1).

Figure 20-1:

Your typical microwave oven.

Figure 20-1:

Your typical microwave oven.

How hot is boiling water?

Water is a molecule (H2O) composed of three atoms: two hydrogen and one oxygen. When water is exposed to energy (heat), some of the water molecules vaporize (or separate into their gaseous components). These vapors collect in tiny pockets at the bottom of the vessel (pot) in which the water's contained. Continued heating energizes the vapors, and they begin to push up against the water.

To break through the water's surface, the vapors must acquire enough energy to equal the force (pressure) of the atmosphere (air) pushing down on the water. The temperature at which this happens is called the boiling point.

At sea level (elevation: 0 feet), the atmosphere is heavier (has more oxygen) than at higher elevations. That's why you breathe more easily in Miami, Florida, (elevation: 10 feet) than atop Mount McKinley in Alaska (elevation: 20,320 feet).

The heavier air at sea level exerts more pressure against the surface of the water in your pot, so making the water boil takes more energy (higher heat).

At sea level, the boiling point of water is 212 degrees Fahrenheit (100 degrees on the Celsius—

C — scale). As a general rule, the boiling point of water drops one degree Fahrenheit for every 500-foot increase in altitude above sea level. In other words, at an altitude 500 feet above sea level, the boiling point for water is 211 degrees Fahrenheit (99.4 degrees Celsius); at 1,000 feet, it's 210 degrees Fahrenheit (98.9 degrees Celsius).

The following chart shows the approximate boiling points for water in specific American cities at specific altitudes.

Altitude Place

Boiling Boiling Point 0F Point 0C

Sea level Atlantic City, NJ

500 feet Austin, TX

5,000 Denver, CO

6,000 Cheyenne, WY 200

7,000 Santa Fe, NM 198

211 202

The World Almanac and Book of Facts 1994 (Mahwah, NJ: World Almanac, 1993)

Note: How fast the water is boiling does not affect the temperature; a slow boil (few bubbles) is as hot as a fast one (lots of bubbles).

Microwaves transmit energy that excites water molecules in food. The water molecules leap about like hyperactive 3-year-olds, producing friction, which then produces the heat that cooks the food. The dish holding food in a microwave oven generally stays cool because it has so few water molecules.

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