How Everything Works   Page 135 of 160 (1595 Questions and Answers) Printer Friendly Version

1341. If you walk up 10 steps, one by one, do you exert the same amount of energy if you walk up the same set of 10 steps two by two? How are energy and effort related, or are they?
Ideally, it doesn't matter how many steps you take with each step—the work you do in lifting yourself up a staircase depends only on your starting height and your ending height (assuming that you don't accelerate or decelerate in the overall process and thus change your kinetic energy, too). But there are inefficiencies in your walking process that lead you to waste energy as heat in your own body. So the energy you convert from food energy to gravitational potential energy in climbing the stairs is fixed, but the energy you use in carrying out this procedure depends on how you do it. The extra energy you use mostly ends up as thermal energy, but some may end up as sound or chemical changes in the staircase, etc.

1342. Is hydroplaning a form of sliding friction?
Not exactly. Sliding friction refers to the situation in which two surfaces slide across one another while touching. In hydroplaning, the two surfaces are sliding across one another, but they aren't touching. Instead, they're separated by a thin layer of trapped water. While hydroplaning still converts mechanical energy into thermal energy, just as sliding friction does, the lubricating effect of the water dramatically reduces the energy conversion. That's why you can hydroplane for such a long distance on the highway; there is almost no slowing force at all.

Dan Barker, one of my readers, informed me of a NASA study showing that there is a minimum speed at which a tire will begin to hydroplane and that that speed depends on the square root of the tire pressure. Higher tire pressure tends to expel the water layer and prevent hydroplaning, while lower tire pressure allows the water layer to remain in place when the vehicle is traveling fast enough. As Dan notes, a large truck tire is typically inflated to 100 PSI and resists hydroplaning at speed of up to about 100 mph. But a passanger car tire has a much lower pressure of about 32 PSI and can hydroplane at speeds somewhat under 60 mph. That's why you have to be careful driving on waterlogged pavement at highway speeds and why highway builders carefully slope their surfaces to shed rain water quickly.

1343. Is it possible to create a magnet with more north poles than south poles? — GS
Yes, but only if some of the poles are weaker than other so that when you sum up the total north pole strength and the total south pole strength, those two sums are equal. For example, you can make a magnet that has two north poles and one south pole if the north poles are each half as strong as the south pole. All magnets that we know of have exactly equal amounts of north and south pole. That's because we have never observed a pure north or a pure south pole in nature and you'd need such a pure north or south pole to unbalance the poles of a magnet. A

The absence of such "monopoles" is an interesting puzzle and scientists haven't given up hope of finding them. Some theories predict that they should exist, but be very difficult to form artificially. There may be magnetic monopoles left over from the big bang, but we haven't found any yet.

1344. What is heat? What actually flows from a hot body to a cold body? — AW, Pakistan
Heat is thermal energy that is flowing from one object to another. While several centuries ago, people thought heat was a fluid, which they named "caloric," we now know that it is simply energy that is being transferred. Heat moves via several mechanisms, including conduction, convection, and radiation. Conduction is the easiest to visualize—the more rapidly jittering atoms and molecules in a hotter object will transfer some of their energy to the more slowly jittering atoms in molecules in a colder object when you touch the two objects together. Even though no atoms or molecules are exchanged, their energy is. In convection, moving fluid carries thermal energy along with it from one object to another. In this case, there is material exchanged although usually only temporarily. In radiation, the atoms and molecules exchange energy by sending thermal radiation back and forth. Thermal radiation is electromagnetic waves and includes infrared light. A hotter object sends more infrared light toward a colder object than vice versa, so the hotter object gives up thermal energy to the colder object.

1345. In alternating current, current reverses directions rapidly between the two wires, white and black. Why is it that only the black wire is "hot"?
When you complete a circuit by plugging an appliance into an electrical outlet, current flows out one wire to the appliance and returns to the electric company through the other wire. With alternating current, the roles of the two wires reverse rapidly, so that at one moment current flows out the black wire to the appliance and moments later current flows out the white wire to the appliance. But the power company drives this current through the wires by treating the black wire specially—it alternately raises and lowers the electrostatic potential or voltage of the black wire while leaving the voltage of the white wire unchanged with respect to ground. When the voltage of the black wire is high, current is pushed through the black wire toward the appliance and returns through the white wire. When the voltage of the black wire is low, current is pulled through the black wire from the appliance and is replaced by current flowing out through the white wire.

The white wire is rather passive in this process because its voltage is always essentially zero. It never has a net charge on it. But the black wire is alternately positively charged and then negatively charged. That's what makes its voltage rise and fall. Since the black wire is capable of pushing or pulling charge from the ground instead of from the white wire, you don't want to touch the black wire while you're grounded. You'll get a shock.

1346. How does one "pull up their legs"? Wouldn't you have to jump in some way or another?
It is possible to simply pull up your legs. When you do that, you reduce the downward force your feet exert on the ground and the ground responds by pushing upward on your feet less strongly. With less upward force to support you, you begin to fall.

1347. Can the electric current be taken out of the metal where the charge will not carry?
While charges can move freely through a metal, allowing the metal to carry electric current, it's much harder for charges to travel outside of a conductor. Charges can move through the air or through plastic or glass, but not very easily. It takes energy to pull the charges out of a metal and allow them to move through a non-metal. Most of the time, this energy requirement prevents charges from moving through insulators such as plastic, glass, air, and even empty space.

1348. Does an MRI work in the same way as a copier (or puts you in a magnetic field and copies an image of your body)?
No, an MRI uses a very different technique for imaging your body. A copier uses light to examine the original document while an MRI machine uses the magnetic responses of hydrogen atoms to map your body.

1349. How do dryer sheets diminish the clothes' static?
They leave a layer of conditioning soap on the clothes and this soap attracts moisture. The moisture conducts electricity just enough to allow static charge to dissipate.

1350. How do you get static out of hair?
If you put a conditioner on your hair, it will attract enough moisture to allow static charge to dissipate.

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