How does a magnet work and is there a way that I can determine which end of the …

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How does a magnet work and is there a way that I can determine which end of the magnet is north and which end is south?

The magnetic fields that are responsible for the interesting behaviors of magnets can be created either by (1) moving electric charge or (2) changing electric fields. We can ignore the second process because it has very little to do with permanent magnets. Instead, let’s focus our attention on the first process: moving electric charge producing magnetic fields. Whenever electric charges flow through a wire, a phenomenon that we call an electric current, they create magnetism. Many appliances use electricity and electric currents to create magnetism, notably televisions, motors, and audio speakers. But a permanent magnet doesn’t use an obvious electric current to create its magnetic field. Instead, it uses the spinning character of the electrons inside the material from which that magnet is made. Electrons are electrically charged and they have an intrinsic spinning character. A simplistic view of an electron is as a spinning, electrically charged ball. Since its charge is in motion, an electron acts as a magnet and has both a north pole and a south pole. In most materials, the magnetic electrons are turned in opposite directions, canceling out one another’s magnetism so that the overall material is non-magnetic. But in a few special materials, including most steels, the cancellation is imperfect and some magnetism remains. In a permanent magnet, this remaining magnetism is particularly apparent. The material is, in effect, a big collection of magnetic electrons that all work together to create a large magnet.

To determine which end of a permanent magnet is its north pole and which is its south, take a compass and hold it a reasonable distance from one end of the magnet. If the north end (often the red end) of the compass needle points toward this end of the magnet, you know that this end of the magnet is a south pole! That’s because opposite poles attract and the “north” end of the compass needle, a north pole, is attracted to south poles. Interestingly enough, the magnetic pole near the earth’s geographic north pole is actually a south magnetic pole. That’s why the north pole of the compass needle points toward the earth’s north geographic pole. When you use a compass to detect which pole of the magnet is north, be careful not to bring the compass needle too close to the permanent magnet. A strong permanent magnet can remagnetize the compass needle and reverse its poles. To make sure that this hasn’t occurred, check to see whether the compass still points toward the north pole after you bring it near strong permanent magnets.

How does the wattage of a candle compare to the wattage of a light bulb?

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How does the wattage of a candle compare to the wattage of a light bulb?

A 60 watt light bulb emits about 6 watts of visible light while wasting the remaining 54 watts of electric power as other forms of thermal energy. A candle probably also consumes about 60 watts of chemical energy (the paraffin wax) but emits much less than 3 watts of visible light. The light bulb is clearly not very efficient at converting electric power into visible light but the candle is even less efficient. That’s because the candle flame operates at a lower temperature (about 1700° C) than the filament of the light bulb (about 2500° C) and the spectrum of light emitted by a hot object depends strongly on its temperature. The cooler flame emits relatively more infrared light and less visible light (particularly blue light) than the hotter filament.

Is it true that cold water in a pan will boil faster than hot water in a pan?

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Is it true that cold water in a pan will boil faster than hot water in a pan?

No. It takes more heat to bring a pan of cold water to boiling than it does to bring an equivalent pan of hot water to boiling. You can see that this must be true by noting that the cold water must first reach the temperature of the hot water, after which both pans will be equivalent. But there are a few interesting peculiarities with freezing and boiling water. One worth noting is that water that has recently been boiled will freeze more easily than water at an equal temperature that hasn’t been boiled. That’s because boiling drives the dissolved gases out of the water so that it can crystallize more easily. The ice that forms from boiled water tends to be unusually clear because it contains very few air bubbles. Water that contains lots of dissolved air traps those air bubbles as it freezes and the air bubbles slow the freezing process.

How does a UPC scanner work?

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How does a UPC scanner work?

UPC labels are the bar codes placed on consumer goods to identify them as they pass over a glass window containing a UPC scanner. Although UPC labels were first conceived by Norman Joseph Woodland in the late 1940’s, the scheme to read those codes required a very bright and narrow beam of light that could be scanned rapidly across the bars in order to measure their widths. Conventional light sources barely worked and the idea didn’t catch on until lasers became available. A modern UPC scanner begins with a laser that emits a tightly collimated beam of light. Early scanners used helium-neon lasers, but new scanners use cheaper and more reliable solid-state or diode lasers. In a typical scanner, the red beam from a laser is directed toward a spinning object—either a carefully faceted and mirrored disk or a flat disk containing a carefully designed hologram. Laser light that reflects from the spinning object emerges from the glass window above the scanner and sweeps rapidly through the space like a tiny searchlight. When this light beam encounters a UPC label, each dark bars absorbs the beam while each light bar reflects it. Thus as the beam scans across the UPC label, the amount of light the product reflects fluctuates up and down in a characteristic manner. When a photodetector in the UPC scanner detects such a fluctuating reflected light signal, it determines that the laser beam is hitting a UPC label. A computer studies the sequence of the light and dark bars to determine exactly what UPC label is being hit and identifies the product to the store’s computers.

How does an electric guitar amplify the sound from the strings?

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How does an electric guitar amplify the sound from the strings?

As the steel strings of an electric guitar vibrate, they move back and forth across electromagnetic pickups on the guitar’s surface. Each of these pickups consists of a coil of wire with a permanent magnet passing through its center. This permanent magnet has a north magnetic pole at one end and a south magnetic pole at the other end. Surrounding the permanent magnet are lines of magnetic flux that arc gracefully through space from the magnet’s north pole to its south pole. These magnetic flux lines are associated with the forces that magnets exert on one another. Some of these flux lines pass very near the permanent magnet on their way from the north pole to the south pole and thus pass inside the coil of wire around the magnet. Other flux lines arc far outward and pass outside the coil of wire around the magnet. And a few of the flux lines pass through the steel string that lies just above one pole of the permanent magnet. Steel is a ferromagnetic metal, meaning that it easily develops strong north and south poles of its own when exposed to another magnet. This ferromagnetism is the result of a remarkable ordering process that takes place among the electrons inside the steel. The steel string is magnetized by its proximity to the permanent magnet in the pickup and it interacts strongly with the magnetic flux lines that pass near it. Some flux lines leaving the north pole of the permanent magnet connect to the south pole of the magnetized string and an equal number of flux lines leaving the north pole of the magnetized string connect to the south pole of the permanent magnet. Thus when the steel string vibrates back and forth, it pulls some of the flux lines with it. The paths that these flux lines take shift back and forth rhythmically as the string vibrates.

Whenever magnetic flux lines move, they create electric fields. An electric field is a phenomenon that exerts forces on charged particles, such as the mobile electrons in the coil of wire around the permanent magnet. As the string vibrates and the magnetic flux lines shift back and forth with it, electric fields appear in the wire coil and begin to push electrons through that coil. These electrons flow back and forth in the wire as the string vibrates. Wires connecting the pickup’s coil to an electronic audio amplifier carry these moving electrons (actually an electric current) to the amplifier, where they are detected and used to control a much larger electric current. When this amplified current is sent through a speaker, the speaker produces a very loud sound that’s an amplified version of the sound that the string itself is making as it pushes weakly on the air.

When you freeze water, are the minerals separated from the molecules of the wate…

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When you freeze water, are the minerals separated from the molecules of the water? (When I freeze store-bought water and it then thaws out, there is a glob of nasty looking minerals that settle to the bottom of the bottle.)

When water freezes, it forms ice crystals. Crystals are very orderly arrangements of molecules in which each molecule has a particular position and orientation. Each crystal grows from a tiny initial seed crystal by adding one molecule after another to the surfaces of the crystal. Since each molecule that attaches to the crystal must fit into a particular position and have a particular shape and orientation, molecules that are different from those in the crystal tend to be excluded from the growing crystal. Thus an ice crystal that’s growing in dirty water will nonetheless consist almost exclusively of water molecules. Only if the water freezes very quickly will it trap large numbers of impurities by not giving them time to get out of the way. Even gases are excluded from the ice, which is why air bubbles often appear as water freezes into ice.

The minerals that you see in the thawed bottle of water were originally dissolved or suspended in the water. But as the water froze, the ice crystals excluded those impurities and they remained in the liquid portion of the water. Eventually the liquid portion of the water dwindled away and the minerals were forced to come out of solution as solid particles. When the water thawed, those minerals failed to redissolve (they’re often only weakly soluble in water and have great difficulty redissolving).

This phenomenon whereby crystallizing a liquid separates out its impurities is very useful in chemistry—many important chemicals, notably medicines, are purified in this manner. Similarly, freezing water is an important way of purifying it in some locations—native people in cold countries have used sea ice (the pure ice that forms when seawater freezes) as a source of fresh water for centuries. And you may have noticed that when you eat frozen juice, you can suck away the sweet flavored portion and leave behind only the pure ice portion—because the sugar and flavors have been excluded from the pure ice crystals during freezing.

Why did Fahrenheit choose 32° for the freezing point of water and 212° f…

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Why did Fahrenheit choose 32° for the freezing point of water and 212° for the boiling point of water? These seem like such awkward numbers to use.

Daniel Gabriel Fahrenheit chose as the zero of his temperature scale the temperature at which ice melts when it’s mixed 50/50 with salt. He then set the temperature at which pure ice melts to be 30° above zero and normal body temperature to be 90° above zero. These values were adjusted several times over the years as temperature measurements became more accurate and are now 32° and 98.6° respectively. Having established the temperature scale based on these various situations, he had no choice about water’s boiling temperature. Water’s boiling temperature at normal atmospheric pressure simply turns out to be roughly 212° on his temperature scale.

Are divining rods and their abilities to locate ground water fact or myth?

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Are divining rods and their abilities to locate ground water fact or myth?

I’m afraid that I think they’re myth. Despite extensive searches, physicists have found only four forces in nature: gravity, the electromagnetic force, the strong force, and the weak force. Of these, only gravity and the electromagnetic force are noticeable outside of atoms. Since ground water has no electric charge, it can’t affect a divining rod through the electromagnetic force. That leaves only gravity as a possibility and the gravity between modest sized objects such as a stick and a pool of water is so incredibly weak that I can’t imagine anyone detecting it with their hands. Having eliminated all the possible external forces that would bend a stick downward when it’s near water, it’s clear that this bending is done by the hands of the person holding it. Perhaps a good dowser can see features in the environment that prompts the dowser, consciously or unconsciously, to believe that water is nearby. In short, I think that there are people who are good at identifying signs that indicate ground water is present and who can find that water. The divining rod itself is unimportant.

Why is it that when you have water on your skin and an air current travels over …

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Why is it that when you have water on your skin and an air current travels over it, your skin gets cold?

Whenever water is exposed to air, the water and air begin to exchange water molecules. By that, I mean that water molecules leave the surface of the liquid water to become water vapor in the air and water molecules that are already vapor in the air leave the air to become liquid water. If the relative humidity of the air is less than 100% (meaning that the air can still hold more water vapor), more water molecules will leave the liquid water than will return to it and the liquid water will gradually evaporate into water vapor. If the relative humidity of the air is greater than 100% (meaning that the air is holding more water vapor than it can tolerate), more water molecules will return to the liquid water than leave it and the water vapor will gradually condense into liquid water.

For a water molecule to leave the surface of liquid water, it needs a substantial amount of energy because it must break several hydrogen bonds which are holding it to its neighbors. It obtains this extra energy from nearby molecules and they become colder. Whenever a water molecule returns to the surface of liquid water, it returns this energy to the nearby molecules and they become hotter. Thus whenever liquid water is evaporating, the water molecules that leave the liquid water are taking away its energy so that it becomes colder. And whenever water vapor is condensing, the water molecules that return to the liquid water are giving it energy so that it becomes hotter.

When your skin is wet and water is evaporating from it, your skin also becomes colder. Blowing additional air across your skin prevents any build-up of humid air near its surface so that far more water molecules leave your skin than return to it. The evaporation then proceeds rather quickly and your skin feels quite cold.

Both hydrogen and oxygen fuel flame, but together they make water and that can p…

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Both hydrogen and oxygen fuel flame, but together they make water and that can put out a flame. Why?

In a sense, water is the “ash” that forms when hydrogen burns in oxygen. Like all fully burned materials, water can’t burn any further. When you put cold water on a fire, it extracts heat from the fire because the water is much colder than the fire and heat naturally flows from hotter objects to colder objects. Since heating the water doesn’t cause the water to burn (it can’t burn), the heat that’s lost by the fire doesn’t create new fire (as would be the case if you threw gasoline on the fire instead of water). So the water gradually cools down the fire until the fire no longer has enough thermal energy to sustain its own chemical reactions. The fire then goes out.