Can we add a section to a microwave oven that gets the food or drinks cold? – MH

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Can we add a section to a microwave oven that gets the food or drinks cold? – MH

Not without adding a full-blown refrigerator. While it’s relatively easy to add thermal energy to food or drink, it’s much harder to remove that thermal energy. Since energy is conserved, the thermal energy that you remove from the food must be transferred elsewhere. Since heat (moving thermal energy) normally flows from a hotter object to a colder object, you must make something colder than the food before the heat will leave the food. While it’s possible to cool an object to a temperature lower than its surroundings, this cooling process requires a heat pump, a device that actively pumps heat from a cold object to a hot object (against its natural direction of flow). A refrigerator is such a heat pump.

How do you boil ice water? (I think it has something to do with a vacuum.) – MW

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How do you boil ice water? (I think it has something to do with a vacuum.) – MW

You’re right, it does have to do with a vacuum. While water molecules can evaporate from the surface of liquid water at almost any temperature, boiling can only occur when the evaporation rate is high enough to support the appearance of evaporation bubbles inside the body of the liquid water. Normally, atmospheric pressure pushes inward on cold water so hard that any evaporation bubble that appears inside the water is immediately crushed out of existence. But in water that’s at 100° C, evaporation is so rapid that the evaporation bubbles in the liquid water can survive and grow, despite the crushing inward forces of atmospheric pressure. The hot water boils.

Water boils not because it’s hot but because any evaporation bubble that forms inside it is able to survive and grow despite the surrounding atmospheric pressure. At normal atmospheric pressure, the water does have to be hot for this to happen. But if you remove the atmospheric pressure, the water can boil at much lower temperatures. In fact, at sufficiently low pressures, even ice water will boil! It’s funny to see ice cubes floating in a container of boiling water, but it happens when you remove the air from around the ice water.

How does a light bulb work?

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How does a light bulb work? — DH, Casselberry, FL (and also KH)

In a common incandescent light bulb, an electric current flows through a double-spiral coil of very thin tungsten wire. As the electric charges in the current flow through this tungsten filament, they collide periodically with the tungsten atoms and transfer energy to those tungsten atoms. The current gives up its energy to the tungsten filament and the filament’s temperature rises to about 2500° C. While all objects emit thermal radiation, very hot objects emit some of the thermal radiation as visible light. A 2500° C object emits about 12% of its heat as visible light and this is the light that you see coming from the bulb. Most of the remaining heat emerges from the bulb as invisible infrared light or “heat” light. The glass enclosure shields the filament from oxygen because tungsten burns in air.

Can a compound have triple bonds? If so, please give an example.

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Can a compound have triple bonds? If so, please give an example. — BA, IL

Yes, some compounds contain triple bonds. Acetylene is the simplest such molecule, with two carbon atoms connected by a triple bond. Each carbon atom has one hydrogen atom attached to it, so the entire molecule is a four-atom chain: hydrogen-carbon-carbon-hydrogen. The triple bond between carbon atoms is extremely strong—the atoms are sharing 6 electrons between them.

How do light sticks work? – AE

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How do light sticks work? – AE

When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized (“burned”) is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.

Does the volume in the cooking chamber of a microwave oven affect the rate at wh…

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Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? – RP

The size of a microwave oven’s cooking chamber should have little or no effect on how quickly it cooks food. The oven’s magnetron tube delivers a certain amount of microwave power to the cooking chamber and virtually all of that power will eventually be absorbed in the food. It may take a few moments longer for a large cooking chamber to fill with microwaves when you first start the oven, but soon the food inside it will be exposed to the same intensity of microwaves as food cooking inside a smaller microwave oven with a similar magnetron power.

On the other hand, the magnetron’s power does affect cooking speed so that an oven with a more powerful magnetron will cook food faster than one with a less powerful magnetron. The speed of cooking in a microwave oven also depends on how much food it contains because the food shares the microwave power. In general, doubling the amount of food in the microwave doubles the cooking time.

What is the “sound barrier”? – EL

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What is the “sound barrier”? – EL

The “sound barrier” is more a psychological barrier than a real impediment. In the early days of high-speed flight, there was concern that a plane flying at or beyond the speed of sound in air would encounter unanticipated phenomena that would rip it apart. However, when Chuck Yeager finally did exceed the speed of sound for the first time in 1947, he found the transition from subsonic to supersonic uneventful. The only way that he could tell he was traveling faster than the speed of sound was with the help of his instruments.

What is a shockwave and a sonic boom? – EL

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What is a shockwave and a sonic boom? – EL

A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can’t hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a “sonic boom”. Note that the sonic boom occurs when the shockwave passes your ears, not when the plane “breaks the sound barrier”. When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.

If airplane cabins are pressurized to provide adequate oxygen for the passengers…

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If airplane cabins are pressurized to provide adequate oxygen for the passengers to breathe, what provides this compressed air? – EL

The air that you breathe inside an airplane is actually pumped into the cabin through the jet engines. The first component of a jet engine is a compressor that takes the low-density air outside and boosts its pressure and density. While most of this air then continues through the engine to the combustion chamber, part of it is diverted to the cabin. But before it can be released into the cabin, the air must be chilled by an air conditioner. That’s because compressing air adds energy to it and raises its temperature. The compressed air leaving the jet engine’s compressor is hot, even though no combustion has taken place yet. So the air is first cooled and then sent into the cabin.