How It Ticks

Around the house · Exhibit 6

How does a microwave work?

Updated

A microwave oven is a metal box with a radio transmitter bolted to one side. The transmitter fills the box with waves that keep twisting the water molecules in food back and forth, and the food warms up from that. The waves bounce between the walls and pile up into a fixed pattern, so some places in the box get a lot of energy and some get almost none. That is why one bite of leftovers burns your tongue and the next is still cold.

Below is the floor of an oven seen from above, at the height where food sits. Drag the rim of the plate to turn it, or press Turning and let the motor do it. The buttons change how many standing waves are going at once.

An example box, 30 by 31 cm and 20 cm tall, not any particular oven, seen from above at food height. The pattern is worked out from the size of the box, not drawn by hand: 11 standing waves fit it between 2,400 and 2,500 MHz, and a box 1 cm lower holds only 8. The Other waves button picks a different mix of them. Simplifications are listed at the end.

Here is the whole chain in five steps:

  1. A vacuum tube called a magnetron turns electricity from the wall into radio waves at 2,450 MHz.
  2. The waves bounce around the metal inside of the oven.
  3. Bouncing back and forth, they add up into a fixed pattern of strong and weak places.
  4. Water in the food keeps trying to turn with the swinging field, and that jostling is heat.
  5. Food sitting in a strong place gets hot. Food in a weak place stays cold, unless it moves.

Why does a microwave heat food unevenly?

Think of a jump rope shaken from both ends at the right pace. Some points on it swing wide and some barely move, and the pattern stays put. Waves bouncing between the walls of an oven do the same thing in three dimensions. Where the reflections arrive in step, they add up and the field swings hard: a hot spot. Halfway between two of those, they cancel, and food there stays cold.

In the example box above, only 11 standing waves fit between 2,400 and 2,500 MHz, and a real oven sets up several of them at once.

How does a microwave heat food?

According to the FDA, microwaves are produced “by an electron tube called a magnetron.” They are “reflected within the metal interior of the oven where they are absorbed by food.” And they “cause water molecules in food to vibrate, producing heat that cooks the food.”

2,450 MHz has nothing to do with food. It’s one of the frequencies U.S. rules list for heating equipment, plus or minus 50 MHz, and the electric field in the box swings back and forth 2.45 billion times a second. A water molecule is lopsided, slightly positive at one end and slightly negative at the other, so the field keeps trying to turn it. In liquid water every molecule is tangled up with its neighbors and can’t follow cleanly. It lags, bumps into the molecules around it, and the water warms up. Salty water heats more, because the dissolved salt is pushed back and forth as well and rubs its way through the water.

Do microwaves heat water at a special resonant frequency?

No, though you’ll read it often. Water has no sharp resonance near the oven’s frequency. The part of its response that turns the field into heat rises into one broad hump, and physicists at the University of Augsburg, who measured it all the way to the far infrared, put the top at about 20 GHz. Ovens sit well below the top on purpose. Martin Chaplin, a water scientist at London South Bank University whose reference site on water has run since 2001, writes that the lower frequency is used “so that the radiation is not wholly absorbed by the first layer.”

I worked the numbers out myself from a standard model of water fitted to measurements, by Liebe, Hufford and Manabe. At 25 °C the top of the hump sits at 19.3 GHz, and there the power would be used up in the first half millimeter of food: a burnt skin around a cold middle. At 2.45 GHz the heating part is only a quarter of its top value, so the power fades nearly 40 times more slowly and reaches about 1.9 cm into water before it drops to a third. Chaplin’s figures for water give 1.4 cm. The two sets of measurements differ a little, and both land on a couple of centimeters.

Does a microwave cook from the inside out?

No. In the FDA’s words, “microwave ovens do not cook food from the 'inside out.'” The waves arrive from outside and lose energy as they go, so the outer couple of centimeters take most of it. With the same water model, the power drops to a third at 0.8 cm in water at 0 °C, 1.6 cm at 20 °C and 2.7 cm at 40 °C. A bowl of soup heats all the way through because it isn’t much thicker than that. In a thick piece of meat, the FDA says, the inside “is cooked mainly by the conduction of heat from the hot outer layers,” just as in a regular oven. The USDA’s advice for large cuts follows from the same physics: cook them on medium power, 50%, for longer, which “allows heat to reach the center without overcooking outer areas.”

What does 50% power mean on a microwave?

In most microwaves, the ones without an inverter, 50% power doesn’t mean a weaker beam: the magnetron runs at full strength half the time and is switched off the rest. According to Maytag’s guide, power levels “represent a percentage of ‘on’ time for the magnetron tube. The magnetron tube cycles on and off to reach each level.” Averaged over a minute, a 1,000-watt oven on 50% delivers about 500 watts, so the same dish takes roughly twice as long, and between the bursts the heat in the outer layers has time to spread inward. Panasonic, which sells inverter ovens and says they hold a lower power steadily, argues that the on-off kind “can result in overcooked edges and cool spots in the middle.”

Why does frozen food thaw in patches?

Ice molecules are locked into a crystal and can’t turn anywhere near fast enough, so frozen food lets most of the waves pass and barely warms. Once one spot thaws, the water there soaks up the power and races ahead. Chaplin: “the thawed material may get very hot while unthawed material stays frozen.” So the edges of a frozen dinner boil while the middle is still ice.

Why is a microwave faster than an oven?

Because the heat starts inside the food. In a regular oven, heat reaches the food from the hot air and the hot walls, but only at the surface, and from there it creeps inward by conduction, one slow millimeter after another. A microwave puts its energy into the outer couple of centimeters at once, so anything thinner than that has little left for conduction to do. In a thick roast the middle still waits for it, just as in a regular oven, which is why the USDA’s 50% advice above exists. A refrigerator does a different job: it pumps heat out of the food and into the kitchen.

Why does the plate turn?

The hot spots stay where they are, so the food has to move through them. In 1966 Sharp sold the R-600, the first microwave oven in Japan with a turntable. Turn the plate in the toy above and watch the spots smear into rings: every bit of food at the same distance from the middle passes through the same hot and cold places, so it ends up with the same heat.

The turntable can’t do everything. It evens out the heat around each circle, never between the middle and the rim, and the very middle of the plate doesn’t move at all. In the starting set of waves, after a full turn the middle centimeter of the plate gets about 15% of the plate’s average heat and the very center almost none, however long you keep turning. That’s what stirring halfway is for: the spoon moves food across the circles, which the plate can’t. Some ovens have no turntable at all. I haven’t found a primary source for how each maker spreads the heat in those, so I’ll leave them out rather than guess.

Does a microwave kill bacteria?

A microwave kills bacteria only where the food gets hot enough for long enough, and cold spots like the ones in the toy above are where it might not. The heat kills them: in the FDA’s words, the wave energy “is changed to heat as it is absorbed by food.” The USDA’s food safety agency warns that microwave ovens “can cook unevenly and leave 'cold spots,' where harmful bacteria can survive.” Its advice is to finish the turntable’s job by hand: “Stir or rotate food midway through the microwaving time to eliminate cold spots where harmful bacteria can survive.” Then let the food stand, which completes the cooking, and check it with a food thermometer; reheated leftovers should reach 165 °F (74 °C).

How do you measure the speed of light with a microwave?

You need a microwave oven, a plate, a bar of chocolate (or marshmallows, as in the original) and a ruler:

  1. Stop the plate from turning. Take the turntable out if your oven’s manual allows it, or stand the plate on an upturned bowl so it doesn’t ride on the turntable.
  2. Put the chocolate on the plate and heat it on low, watching it, until a few spots go soft. Stop there: overheated chocolate scorches and smokes.
  3. Measure the gap between two neighboring melted spots. It’s roughly half a wavelength.
  4. Multiply the gap by two, then by the frequency on the oven’s label. That’s the speed of light.
Robert Stauffer, a high school teacher, published the experiment with marshmallows in 1997 and wrote that his class often came within 5%. Oven labels say 2,450 MHz, and the band ovens may use runs from 2,400 to 2,500 MHz. Here the spots sit exactly half a wavelength apart. In a real oven the waves cross the box at an angle, which can stretch the gap but never shrink it, so an error tends to come out fast.

Why can you see through the door but microwaves can’t get out?

The door window has a metal sheet full of small holes. A wave meets a hole much smaller than itself almost as if the sheet were solid. In 1944 Hans Bethe worked out how much gets through: the share of the power landing on a hole that makes it to the other side falls with the fourth power of the hole’s size. Halve the hole and each bit of wave landing on it is only a sixteenth as likely to get through. The smaller hole also catches a quarter of the wave. Together, a sixty-fourth of the power. A light wave is about 0.0005 mm long, so every hole is huge to it. The oven’s wave is 122 mm long, and the holes are about 1 mm.

One hole in perfect metal, hit straight on. Bethe’s formula holds for holes much smaller than the wave, so the toy uses it up to 12 mm and leaves a question mark between 12 mm and about 72 mm, where the wave starts to fit through.

A Matsushita patent for a door screen, filed in 1976 and granted in 1977, asks for holes no bigger than 1.2 mm. By Bethe’s formula, a hole that size lets through about one part in 4.6 million of the power that lands on it, before the thickness of the sheet adds anything.

Why does metal spark in a microwave?

Metal reflects the waves, just as the walls of the box do. The FDA’s advice is that “generally, metal pans or aluminum foil should also not be used in a microwave oven, as the microwaves are reflected off these materials causing the food to cook unevenly and possibly damaging the oven.” Note the “generally”: if your oven came with a metal rack, its manual is the authority on that rack.

The sparks come from edges and points. The waves push electric charge around on any metal, and on a smooth wall it spreads out. On a fork tine or a crumpled corner of foil it crowds into the tip, and the field there gets strong enough to break down the air. In a General Mills patent on foil packaging for microwave meals, “Arcing occurs when the local electric field at the surface of a metal exceeds the dielectric breakdown strength of the material or media surrounding it,” and the sharpness of the edge “plays a critical role.”

Is it safe to stand in front of a microwave?

Yes, as long as the door is intact. The FDA warns that “A damaged microwave oven may present a risk of microwave energy leaks,” and it recommends not using an oven “if the door doesn’t close firmly or is bent, warped, or otherwise damaged.” A U.S. federal standard, 21 CFR 1030.10, caps leakage at 1 milliwatt per square centimeter for a new oven and 5 over its lifetime, measured 5 cm or more from the surface. The same standard demands at least two safety interlocks, which the FDA describes as systems “that stop the production of microwaves the moment the latch is released or the door is opened.” So the oven can’t run with the door open, and when it’s off, the magnetron makes no waves at all. The FDA also answers the other common worry: the energy “is changed to heat as it is absorbed by food, and does not make food 'radioactive' or 'contaminated.'”

The one thing in most kitchens that does hold something radioactive is on the ceiling: an ionization smoke detector carries a speck of americium. How much it adds works out to less than an ordinary afternoon of natural background.

Who invented the microwave oven?

Percy Spencer, an engineer at Raytheon, which built magnetrons for radar during the war. His patent, “Method of treating foodstuffs”, was filed on October 8, 1945, and granted in 1950. The well-known story that a chocolate bar melted in his pocket while he stood near some microwave equipment comes from later retellings, such as the IEEE’s history wiki, not from the patent. That same wiki dates it to “around 1946,” after the patent was already filed, so I’d treat it as a good story rather than a record.

What do the toys simplify?

Sources