What would be a legitimate form of propulsion for magnetic trains?

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What would be a legitimate form of propulsion for magnetic trains? — DS, Kenton, OH and MB, Willows, CA

The most sensible propulsion system for a magnetically levitated train would be a linear electric motor. This motor would consist of electromagnets on the train and electromagnets on the track. By turning these electromagnets on and off at carefully chosen moments, they can be used to pull or push the train forward for propulsion or backward for breaking. The timing is important because, for propulsion, the magnet on the train must always be attracted toward the track magnet in front of it and repelled by the track magnet behind it. For breaking, this relationship must be reversed.

What is a superconducting magnet?

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What is a superconducting magnet? — JS, Montreal, Quebec

Electric currents are magnetic. That’s the basis for electromagnets—if you run an electric current around a coil of wire, that coil of wire will develop a north magnetic pole at one end and a south magnetic pole at the other end. But an electromagnet made with normal copper wires consumes electric power all the time. The current passing through those wires wastes energy because of friction-like effects in the copper and the wires become hot. The electromagnet also needs a power source to keep its current flowing.

However, a superconducting electromagnet is one in which the wires are superconducting—the current passing through them doesn’t waste any power. Once a current has been started in a coil of superconducting wire, it flows forever. Since it doesn’t waste any power, that current needs no source of power and produces no thermal energy. In fact, you can buy superconducting magnets with the current already started at the factory. As long as the wires are kept cold (as they must be to remain superconducting), the current will continue to flow and the coil will remain magnetic forever.

What causes different types of lightning?

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What causes different types of lightning? — CR, Tokyo, Japan

Cloud to ground lightning is caused by difference in electric charges between the cloud and the ground. Cloud to cloud lightning is caused by difference in electric charges among the clouds themselves. While I’m not at all expert on the subject, I would guess that the various different types of lightning discharges are caused by differences in the distances between charged objects, by variations in the local electric conductivity of the air and clouds during a discharge, and by the sizes and shapes of the clouds and ground.

Why is it that the times at which the moon sets and rises change so much from da…

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Why is it that the times at which the moon sets and rises change so much from day to day?

The moon orbits the earth about every 27.3 days, so its position relative to the sun changes from day to day. Because of the moon’s movement around the earth, the moon rises and sets about 1 hour later every day. When the moon is on the sun side of the earth, it rises at sunrise and sets at sunset. Fourteen days later, when the moon is on the side opposite the sun, it rises at sunset and sets at sunrise.

What makes a soap bubble round?

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What makes a soap bubble round? — MZ, Massachusetts

The molecules in a liquid are touching one another and this touching reduces the molecules’ potential energies. Separating the molecules and reducing the extent of their touching requires energy and is something that the liquid won’t normally do on its own. The molecules at the surface of a liquid have fewer neighbors than they would have if they were in the body of the liquid. Those molecules thus have higher potential energies than they would have in the body of the liquid. To minimize the overall potential energy of a liquid, it naturally tends to minimize its surface area.

When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny droplet. The air keeps bubble large and it can’t avoid having lots of molecules on its surfaces, where they have higher than normal energies. But what the soap solution can do to minimize its total potential energy is to minimize the number of its molecules that are on the surface. The soap solution experiences what is called “surface tension”—an elastic tightening of its surface. This surface tension tends to minimize the surface area of the soap solution to minimize its potential energy. The soap solution minimizes its surface area around the trapped air by forming a spherical shape. A spherical shell makes the most efficient use of its surface area in enclosing a volume.

Could you describe the process of an ice cube melting only from ambient (room) t…

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Could you describe the process of an ice cube melting only from ambient (room) temperature? — JAS, Malta, NY

An ice cube is a crystal of water molecules. It is only stable up to a temperature of 32° F (0° C). When you place it in ambient temperature, it gradually warms until it reaches 32° F and then its surface begins to melt. As heat from the room flows into the ice cube, its molecules begin to separate briefly from one another and to exchange neighbors. These molecules lose their crystalline rigidity and structure and to become liquid. The liquid that forms is still at 32° F, but it has less order than the crystalline ice had.

As more heat flows into the mixture of ice and water, the ratio of solid ice to liquid water gradually changes and the fraction of liquid water increases. But only after all the ice has converted to water does the temperature of the water begin to rise significantly above 32° F.

How do bicycle shocks and suspension affect the performance of a bicycle?

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How do bicycle shocks and suspension affect the performance of a bicycle? — D

When the wheels of a bicycle are attached directly to the frame of a bicycle, the wheels and frame must move together. When one of the wheels hits a bump, both that wheel and the frame must accelerate upward together. When this happens, the bump exerts a huge upward force on the wheel and everything, including the unfortunate rider, experiences a sudden upward acceleration. A sudden jolt of this sort is unpleasant—the seat of the bicycle pushes upward violently on the rider and the rider feels large forces throughout his or her body. Each body part pushes upward on the body part above it so that everything leaps upward.

To reduce the upward acceleration that the rider experiences, the direct connection between the bicycle wheels and the frame can be replaced by a spring suspension. When the wheel of a bicycle with a spring suspension encounters a bump, the springs compress and the force on the frame and rider is much smaller. The rider still accelerates upward, but not as rapidly as the wheel and without the abrupt jolt of a suspensionless bicycle. In fact, by the time the rider has begun to rise much, the wheel will probably have rolled back off the bump and the spring will return to its original shape. Overall, the rider will barely move at all and will hardly notice the bump.

But a spring suspension isn’t perfect by itself. Suppose that the bicycle rolled over a curb and onto a sidewalk. This bump doesn’t end—the pavement level rises permanently. When the wheel hits the curb, it rises suddenly and compresses the spring. But since the wheel never drops back to its original height, the only way for the spring to decompress back to its original shape is for the frame and rider to rise. And that’s what happens. But the frame and rider don’t stop moving once the spring has reached its original shape. They have upward momentum and they continuing rising. The spring begins to stretch upward now. Eventually the frame and rider stop rising and begin to descend again, but they continue to bounce up and down as though they were on a pogo stick. In effect, they are on a pogo stick. When a spring is compressed or stretch, it stores energy. If there is nothing to get rid of the energy stored in the bicycle’s compressed or stretched spring, the frame and rider will continue to bounce up and down indefinitely.

To stop the bouncing (and prevent most of it in the first place), a bicycle with a spring suspension also has shock absorbers. These devices waste energy whenever the wheel and frame move relative to one another. Whether the spring is compressing or stretching, the shock absorber extracts energy from the wheel, frame, and spring, and turns that energy into thermal energy. As a result, the frame and rider don’t bounce significantly after the wheel rides up and onto the curb. Similar issues occur in cars, where shock absorbers damp out the bouncing that can occur because the car body is suspended above the wheels on springs.

How does one prove that the earth is round?

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How does one prove that the earth is round?

There are many possibilities, so I’ll suggest an intriguing method that is familiar to surveyors. While the overly simple technique I suggest isn’t particularly practical, it is closely related to surveying techniques that are practical.

Take a very long string, say about 20 miles long, and attach one end of the string to a post. Now draw the string taut and walk all the way around the post while holding on to the other end of the string. If you measure the distance you walked while completing one full trip around the post, you would expect it to be related to the length of the string by a factor of 2 times pi because you learn in grade school that the circumference of a circle is 2 times pi times the radius of that circle. However, that relationship is only true if you’re working on a flat surface. Since the earth is curved, the circumference of the circle around which you walk will be somewhat less than 2 times pi times the radius of the circle. That result is enough to prove that you’re on a curved surface.

You can see this effect by performing the experiment I just suggested on the surface of a basketball. Take a short length of string and use it, together with a pin and a pencil, to draw a circle on the surface of the ball. If you measure the circumference of that circle and compare it to 2 times pi times the length of the string, the circle’s circumference will be a bit shorter than expected. As with the earth, the basketball is a curved surface. The larger the circle you try to draw in this manner, the greater the discrepancy between 2 times pi times the radius and the actual circumference of the circle.

How dangerous are plastics for storing and reheating food? I remember hearing th…

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How dangerous are plastics for storing and reheating food? I remember hearing that plastic containers can release carcinogenic materials when reheating food in the microwave. I also heard that plastics can release “plasticizers” into food even when cold. What studies exist about these dangers? — CVL, Fairfax, VA

While I’m not up to date on actual studies, I would think that most food storage plastics introduce very little contamination into the foods stored in them. We have become so concerned as a society about toxic chemicals in recent years that we tend to overreact much of the time. While the actual polymer molecules in most plastics are relatively inert and harmless, plastics inevitably contain some small molecules, either by accident or by design, that work their way into food. Even if some of these molecules are toxic or carcinogenic, the quantities involved are almost certainly insignificant. Modern chemical testing can detect incredibly small quantities of various chemicals and we panic every time we find them in our environment. But the societal cost of banning or avoiding all contact with or use of these chemicals may have hidden costs that are worse than the problem we’re trying to solve. Moreover, I’ll bet that many of the foods put in plastic containers are greater health hazards than the containers themselves.

Does food coloring change the color of food?

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Does food coloring change the color of food?

Food coloring is a solution of dye molecules—molecules that absorb light of certain wavelengths extremely efficiently. When a particle of light—a photon—of the right wavelength encounters one of these dye molecules, an electron in the molecule uses the photon’s energy to shift from one quantum level to another. The photon vanishes and the molecule is placed in an electronically excited state. The dye molecule’s electron quickly returns to its original quantum level by releasing this extra energy as thermal energy within the molecule and its surroundings. Overall, the photon has vanished and the dye has become warmer. When you add these dye molecules to food, the dye gives the food a color by preventing that food from transmitting or reflecting certain colors of light. The dye simply absorbs those colors.