Is it true that you shouldn’t put a speaker near a microwave oven?

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Is it true that you shouldn’t put a speaker near a microwave oven?

A microwave oven that’s built properly and not damaged emits so little electromagnetic radiation that the speaker should never notice. The speaker might have some magnetic field leakage outside its cabinet, and that might have some effect on a microwave oven. However, most microwaves have steel cases and the steel will shield the inner workings of the microwave oven from any magnetic fields leaking from the speaker. The two devices should be independent.

How does a phonograph work?

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How does a phonograph work? — MS

A phonograph record represents the air pressure fluctuations associated with sound as surface fluctuations in long, spiral groove. This groove is V-shaped, with two walls cut at right angles to one another—hence the “V”. Silence, the absence of pressure fluctuations in the air, is represented by a smooth portion of the V groove, while moments of sound are represented by a V-groove with ripples on its two walls. The depths and spacings of the ripples determine the volume and pitch of the sounds and the two walls represent the two stereo channels on which sound is recorded and reproduced.

To sense the ripples in the V-groove, a phonograph places a hard stylus in the groove and spins the record. As the stylus rides along the walls of the moving groove, it vibrates back and forth with each ripple in a wall. Two transducers attached to this stylus sense its motions and produce electric currents that are related to those motions. The two most common transduction techniques are electromagnetic (a coil of wire and a magnet move relative to one another as the stylus moves and this causes current to flow through the coil) and piezoelectric (an asymmetric crystal is squeezed or unsqueezed as the stylus moves and this causes charge to be transferred between its surfaces). The transducer current is amplified and used to reproduce the recorded sound.

Before you speak into the tape recorder, is the tape non-magnetic because half o…

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Before you speak into the tape recorder, is the tape non-magnetic because half of the magnets face one way and half the other way?

Exactly. When you switch your tape recorder to the record mode, it has a special erase head that becomes active. This erase head deliberately scrambles the magnetic orientations of the tape’s magnetic particles. The erase head does this by flipping the magnetizations back and forth very rapidly as the particles pass by the head, so that they are left in unpredictable orientations. There are, however, some inexpensive recorders that use permanent magnets to erase the tapes. This process magnetizes all the magnetic particles in one direction, effectively erasing a tape. Because it leaves the tape highly magnetized, this second technique isn’t as good as the first one. It tends to leave some noise on the recorded tape.

I am a mentor to a 7th grader who is doing a report on Einstein. How do I explai…

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I am a mentor to a 7th grader who is doing a report on Einstein. How do I explain his theory in a way that will be relevant to her? — MG

The basis for Einstein’s theory of relativity is the idea that everyone sees light moving at the same speed. In fact, the speed of light is so special that it doesn’t really depend on light at all. Even if light didn’t exist, the speed of light would still be a universal standard—the fastest possible speed for anything in our universe.

Once we recognize that the speed of light is special and that everyone sees light traveling at that speed, our views of space and time have to change. One of the classic “thought experiments” necessitating that change is the flashbulb in the boxcar experiment. Suppose that you are in a railroad boxcar with a flashbulb in its exact center. The flashbulb goes off and its light spreads outward rapidly in all directions. Since the bulb is in the center of the boxcar, its light naturally hits the front and back walls of the boxcar at the same instant and everything seems simple.

But your boxcar is actually hurtling forward on a track at an enormous speed and your friend is sitting in a station as the train rushes by. She looks into the boxcar through its window and sees the flashbulb go off. She watches light from the flashbulb spread out in all directions but it doesn’t hit the front and back walls of the boxcar simultaneously. Because the boxcar is moving forward, the front wall of the boxcar is moving away from the approaching light while the back wall of the boxcar is moving toward that light. Remarkably, light from the flashbulb strikes the back wall of the boxcar first, as seen by your stationary friend.

Something is odd here: you see the light strike both walls simultaneously while your stationary friend sees light strike the back wall first. Who is right? The answer, strangely enough, is that you’re both right. However, because you are moving at different velocities, the two of you perceive time and space somewhat differently. Because of these differences, you and your friend will not always agree about the distances between points in space or the intervals between moments in time. Most importantly, the two of you will not always agree about the distance or time separating two specific events and, in certain cases, may not even agree about which event happened first!

The remainder of the special theory of relativity builds on this groundwork, always treating the speed of light as a fundamental constant of nature. Einstein’s famous formula, E=mc2, is an unavoidable consequence of this line of reasoning.

What is the difference between a magnet and an electromagnet? Why are some metal…

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What is the difference between a magnet and an electromagnet? Why are some metals automatically magnetic?

Some metals are composed of microscopic permanent magnets, all lumped together. Such metals include iron, nickel, and cobalt. This magnetism is often masked by the fact that the tiny magnets in these metals are randomly oriented and cancel one another on a large scale. But the magnetism is revealed whenever you put one of these magnetic metals in an external magnetic field. The tiny magnets inside these metals then line up with the external field and the metal develops large scale magnetism.

However, most metals don’t have any internal magnetic order at all and there is nothing to line up with an external field. Metals such as copper and aluminum have no magnetic order in them—they don’t have any tiny magnets present. The only way to make aluminum or copper magnetic is to run a current through it.

How does electric current create magnetic poles in metal? When the current goes …

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How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it positive and negative?

An electric current is itself magnetic—it creates a structure in the space around it that exerts forces on any magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire—the current’s magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the wire up into a coil, the magnetic field takes on a more familiar shape. The current-carrying coil effectively develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your right hand, then your thumb points to the north pole that develops at one end of the coil.

How do the sizes of two magnets determine how much paper can be held between the…

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How do the sizes of two magnets determine how much paper can be held between them? — D

While the full answer to this question is complicated, the most important issues are the strengths and locations of the magnetic poles in each magnet. Since each magnet has north poles and south poles of equal strengths, there are always attractive and repulsive forces at work between a pair of magnets—their opposite poles always attract and their like poles always repel. You can make two magnets attract one another by turning them so that their opposite poles are closer together than their like poles (e.g. by turning a north pole toward a south pole).

To maximize the attraction between the magnets, opposite magnetic poles should be as near together as possible while like magnetic poles are as far apart as possible. With long bar magnets, you align the magnets head to toe so that you have the north pole of one magnet opposite the south pole of the other magnet and vice versa. But long magnets also tend to have weaker poles than short stubby magnets because it takes energy to separate a magnet’s north pole from its south pole. With short stubby magnets, the best you can do is to bring the north pole of one magnet close to the south pole of the other magnet while leaving their other poles pointing away from one another. Horseshoe magnets combine some of the best of both magnets—they can have the strong poles of short stubby magnets with more distance separating those poles.

Returning to the paper question, size is less important than pole strength and separation. The stronger the magnets and the farther apart their poles, the more paper you can hold between them.

I live under the flight path that leads into Sydney’s International/Domestic Air…

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I live under the flight path that leads into Sydney’s International/Domestic Airport. As planes fly over, a sound follows them (3-4 seconds) like air folding in on itself. A slurping sound similar to sucking air in through your cheeks. This phenomenon does not happen all the time, but seems to happen when overcast. Any clues as to what is happening? — TA, Sydney, Australia

The sound you hear may be related to the vortices that swirl behind a plane’s wingtips as it moves through the air. These vortices form as a consequence of the wing’s lift-generating processes. Because the air pressure above a wing is lower than the air pressure below the wing, air is sucked around the wingtip and creates a swirling vortex. The two vortices, one at each wingtip, trail behind the plane for miles and gradually descend. You may be hearing them reach the ground after the airplane has passed low over your home. If someone reading this has another explanation, please let me know.

How do the automatic soda dispensers at fast food joints know when the cup is fu…

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How do the automatic soda dispensers at fast food joints know when the cup is full? — MB, San Diego, CA

Those dispensers measure the volume of liquid they dispense and shut off when they’ve delivered enough liquid to fill the cup. They don’t monitor where that liquid is going, so if you put the wrong sized cup below them or press the button twice, you’re in trouble.

I’ve heard that there are only four basic forces in nature: gravitational, elect…

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I’ve heard that there are only four basic forces in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear. Is this true, and if so, what are the basic differences? — SH, Purdue, Indiana

The number of “basic forces” has changed over the years, increasing as new forces are discovered and decreasing as seemingly separate forces are joined together under a more sophisticated umbrella. A good example of this evolution of understanding is electromagnetism—electric and magnetic forces were once thought separate but gradually became unified, particularly as our understanding of time and space improved. More recently, weak interactions have joined electromagnetic interactions to become electroweak interactions. In all likelihood, strong and gravitational interactions will eventually join electroweak to give us one grand system of interactions between objects in our universe.

But regardless of counting scheme, I can still answer your question about how the four basic forces differ. Gravitational forces are attractive interactions between concentrations of mass/energy. Everything with mass/energy attracts everything else with mass/energy. Because this gravitational attraction is exceedingly weak, we only notice it when there are huge objects around to enhance its effects.

Electromagnetic forces are strong interactions between objects carrying electric charge or magnetic pole. While most of these interactions can be characterized as attractive or repulsive, that’s something of an oversimplification whenever motion is involved.

Weak interactions are too complicated to call “forces” because they almost always do more than simply pull two objects together or push them apart. Weak interactions often change the very natures of the particles that experience them. But the weak interactions are rare because they involve the exchange of exotic particles that are difficult to form and live for exceedingly short times. Weak interactions are responsible for much of natural radioactivity.

Strong forces are also very complicated, primarily because the particles that convey the strong force themselves experience the strong force. Strong forces are what hold quarks together to form familiar particles like protons and neutrons.