Why don’t microwaves get stuck in the food we put in the microwave oven?

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Why don’t microwaves get stuck in the food we put in the microwave oven?

Microwaves are like light—both are electromagnetic waves and both move extremely quickly. While it is possible to trap a light wave briefly between two mirrors, that wave will eventually be absorbed or released. The same is true of a microwave. It’s almost impossible to trap a microwave for more than 1 second, even in very exotic enclosures, so you needn’t worry about them becoming trapped in food. The food simply absorbs them and turns their energy into thermal energy.

There is an experiment involving grapes and microwaves that we found on the inte…

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There is an experiment involving grapes and microwaves that we found on the internet. If a grape is cut in half—with a piece of skin attached between the two halves—and it is then microwaves, sparks are produced. What is happening? — GB, Antioch, CA

This experiment is described in Fun with Grapes – A Case Study. While I haven’t tried it yet myself, I believe I know why it works. Grape juice is somewhat able to conduct electricity and the two halves of the grape are connected by a weak conducting path: the skin bridge. When the microwave oven is turned on, the microwaves not only heat the water in the grapes, they also push a few mobile electric charges back and forth through the skin bridge from one side of the grape to the other. This current releases energy as it passes through the narrow bridge and it heats the bridge extremely hot. The bridge soon catches fire and the electric current driven by the microwaves begins to pass through the flame. When current passes through a gas, it tends to ionize that gas (remove electrons from the gas atoms) so that the gas itself begins to conduct electricity. When current flows through atmospheric pressure air, it forms a brilliant arc. In this case, the arc that you see is powered by the microwaves as they push electric charges back and forth from one side of the grape to the other. An excellent set of movies showing this and other microwave oven experiments appears at http://www.physics.ohio-state.edu/~maarten/microwave/microwave.html.

Suppose you have two electric currents, one consisting of electrons and the othe…

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Suppose you have two electric currents, one consisting of electrons and the other of protons, moving in the same direction at the same velocity. Will the magnetic fields that these currents produce have identical magnitudes and directions? The right hand rule describes the direction of the magnetic field in terms of the direction of current, so it appears that it should be independent of the current’s charge. — ABD, Petersburg, VA

Current is defined as flowing in the direction of positive charge motion. Because electrons are negatively charged, the current they are carrying is flowing in the direction opposite their motion! In your question, you describe two beams, one of electrons and one of protons, and note that both beams are heading in the same direction at the same speed. The proton beam’s current is heading in the same direction as the beam while the electron beam’s current is heading in the opposite direction from the beam. Assuming that the two beams have equal numbers of particles per second, they will produce magnetic fields of equal magnitudes. But the magnetic field produced by the electron beam will be directed opposite that of produced by the proton beam!

A beam of hydrogen atoms—each of which consists of one proton and one electron—is a perfect example of this situation. The electrons in that atomic beam produce a magnetic field in one direction while the protons in that atomic beam produce a magnetic field in the opposite direction. The two fields cancel one another perfectly, as they must because a beam of neutral hydrogen atoms can’t produce any magnetic field.

In plain English that a child can understand, how does a magnet work?

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In plain English that a child can understand, how does a magnet work? — EK, Dale City, VA

There are several way in which objects in our universe can push or pull on one another and one of these ways is through electric or magnetic forces. Two objects that have electric charges are observed to push or pull on one another and two objects that have magnetic poles are also observed to push or pull on one another. That’s simply the way our universe works. With electric forces, things are relatively easy—when you pull a sock and shirt out of the dryer, the sock may well stick to the shirt because friction has given the two different electric charges (one is positively charged and the other negatively charged). By playing around with electrically charged objects, you can convince yourself that (1) there are two different types of electric charge—normally called “positive” and “negative”—and (2) that like charges repel while opposite charges attract.

With magnetic forces, there is an annoying complication: magnetic poles (the magnetic equivalent of “charge”) always come in equal but opposite pairs. As with electric charges, there are two types of magnetic poles—normally called “north” and “south”—and like poles repel while opposite poles attract. However, you won’t be able to find a pure north pole anywhere; it always comes attached to a south pole (and vice versa). So any magnet you find will have at least one north pole and at least one south pole (while they typically have only one of each, they can also have many of each). The forces that these poles exert on one another are fundamental to our universe—I can’t explain them in terms of more basic phenomena because they are already basic except at a very abstract level. (In fact, electric and magnetic forces are intimately related to one another and it is actually electric charges that are creating the magnetic poles that you observe in a magnet.) If you play around with several magnets for a while, you should be able to convince yourself about the existence of two different poles and that like poles repel while opposite poles attract. You should also notice that the magnets push one another directly toward or away from them (the forces between poles are parallel to the line separating them) and that the forces become stronger as the poles become nearer (the force is inversely proportional to the square of the distance separating the poles).

As for how a permanent magnet works, it’s made from a material that contains ordered electrons. Electrons are intrinsically magnetic and, in a few special materials, that magnetism as organized so that the overall materials are themselves magnetic. Each electron has its own north and south pole, but together they give the material a giant north and south pole.

How do circuits work, what are they made of, and who came up with the concept?

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How do circuits work, what are they made of, and who came up with the concept? — RK, New Albany, IN

Circuits themselves are as old as electricity. A circuit is literally a complete loop through which electric current can flow. For example, a flashlight contains a circuit whenever it’s turned on—the current flows from the battery’s positive terminal, through the switch (which is on), through the filament of the light bulb (which glows), and back to the battery’s negative terminal. The battery then gives the current some more energy and sends it around this “circuit” again and again.

But electronic “circuits” are much more modern. Here the word circuit is equivalent to “device,” “board,” or “chip.” Such electronic devices date to somewhere around the beginning of the twentieth century. As radio developed, with tube amplifiers and other electronic components, so did these circuits. Modern electronic systems place many of the components involved in an electronic device on a single sheet of plastic or fiberglass and many of the components on that board may exist on the surface of one or more tiny silicon wafers. These single wafer circuits, called integrated circuits, were invented in 1959 by Texas Instruments and became commercial products at Fairchild Semiconductors in 1965.

Is time constant?

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Is time constant? — RH, Boise, Idaho

That’s a complicated and interesting question. To begin with, consider how we measure time: we generally use repetitive mechanical systems to tick off short intervals of time and then count as those intervals pass by. Thus we measure time in terms of the swinging of a clock’s pendulum or the vibration of a quartz crystal or the motion of an atom’s electrons around its nucleus. If time were to speed up or slow down, it would affect the mechanical motions in our bodies just as much as it would affect the mechanical motions of our clocks, so we wouldn’t notice any change in the ticking of our clocks. If time were somehow to begin passing half as fast as normal and you were to look at your watch, your watch would still appear to tick off seconds at the same rate. So the first answer to your question is that we can’t tell if time is constant, so long as any changes in time occur uniformly and instantly throughout the entire universe.

The reason for including the bit about “uniformly and instantly throughout the entire universe” is that we can tell if time changes at one location but not another. For example, if time were to slow down near you but not near me, I would be able to look at your watch and see that it’s running slow just as you would be able to look at my watch and see that it’s running fast. Alternatively, we could synchronize our watches, wait a while, and then compare our watches again. Since your time is running more slowly than mine, our watches would no longer be synchronized. While this situation sounds unlikely, it does occur. The rate at which time passes depends on where you are and on how fast you are moving, a result described by the Special and General Theories of Relativity. Our universe mingles space and time in a complicated way and also permits gravity to influence the passage of time. In short, the faster you are moving or the nearer you are to a large gravitating object, the more slowly time passes for you.

I suspect that the amount of water on the earth is constant and therefore cannot…

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I suspect that the amount of water on the earth is constant and therefore cannot be used up. Is this true? If so, would it not follow that we don’t need to worry about water conservation so much as water pollution? — AP, Kansas City, MO

While the number of water molecules on earth doesn’t change very much, it isn’t exactly constant. Water molecules are consumed in some chemical reactions (particularly photosynthesis in plants) and produced in other reactions (particularly the burning of petroleum). However, most of the water on our planet is mixed with salt and is therefore unsuitable for drinking. The amount of fresh water on earth is not constant and it can be used up. That’s why both the conservation of fresh water and the control of water pollution are important.

How does the trajectory of a ball change when you give it a spin?

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How does the trajectory of a ball change when you give it a spin? — BHL, Stavanger, Norway

As spinning ball tends to curve in flight. That’s because the ball deflects the airflow around it in one direction and accelerates in the opposite direction. There are two ways in which the spinning ball deflects the air. First, the spinning ball pulls the air it encounters around with it in one direction and produces an imbalance in the airspeeds on its two sides. The air flowing around the side of the ball that is turning back toward the thrower travels faster than the air flowing around the other side of the ball. Since the faster moving air has converted more of its total energy into kinetic energy, the energy of motion, it has less of its energy in the form of pressure. Thus the air pressure on the side of the ball turning toward the thrower is lower than the air pressure on the other side of the ball. The ball accelerates and curves toward the side turning toward the thrower. This effect is called the Magnus effect.

Second, a ball moving at any reasonable speed leaves behind it a turbulent wake and experiences a type of air resistance we call “pressure drag.” When the ball is spinning, this wake forms asymmetrically behind the ball and the pressure drag is not even balanced. The ball pushes the air in the wake to one side and the air pushes back. As a result, the ball accelerates sideways—to the same side as occurs with the Magnus force. In both cases, the ball curves toward the side turning toward the thrower. This second effect is called the wake deflection effect.

The direction in which a thrown ball curves depends on its direction of spin. If the left side of the ball turns back toward you after you have thrown it, the ball will curve toward your left. If the right side turns back toward you, it will curve toward your right. If the bottom turns back toward you, the ball will arc downward faster than it would with gravity alone (for example, topspin in tennis). If the top turns back toward you, the ball will arc upward or will at least not arc downward as much as it would with gravity alone (for example, backspin in golf and hanging fastballs in baseball).

What types of sound can humans hear? What types of materials are soundproof? How…

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What types of sound can humans hear? What types of materials are soundproof? How is the volume of a sound changed? Is the speed of sound the same in all types of media, such as water or air? — JM, Fairfax, VA

In air, sounds are disturbances that consist of compressions and rarefactions—the air molecules are packed either more tightly or less tightly than normal. These regions of too high or too low pressure and density move through the air at about 330 meters per second—the speed of sound and when they pass our ears, we may hear them as sound. As a particular sound passes our ears, the air pressure rises and falls and then rises again, over and over. The number of full cycles—a pressure rise then a pressure fall—that pass our ears each second determines the pitch of the sound we hear. The lowest pitch that our ears are sensitive to is about 20 cycles per second and the highest pitch that we can detect is about 20,000 cycles per second. While other pitches are possible, we simply can’t hear them with our ears.

A sound’s volume is determined by the extent to which the air pressure fluctuates as the sound passes. A loud sound involves a stronger pressure fluctuation than a soft sound. Soundproof materials are ones that decrease the volume of the sound passing through them by weakening the pressure fluctuations. There are two ways to decrease the volume of sound passing through a material: by absorbing the sound or by reflecting it. Soft materials such as carpet or foam rubber absorb sound by allowing the sound’s pressure fluctuations to waste their energies bending the materials. The sound’s energy is converted into thermal energy. Hard, dense materials reflect sounds by making the sounds change speed. Sound travels quickly through most solids and liquids—typically about 5 to 10 kilometers per second. Whenever a wave changes speed in passing from one medium to another, part of that wave is reflected. Thus as sound speeds up in entering a hard surface from the air and as that sound slows down when reentering the air, much of the sound reflects.

What are gas permeable contact lenses made from and what do they use to pigment …

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What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK

A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear’s eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or “vulcanized” to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I’m not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.