. Why is alternating current better than direct current? — MK, California
The genius of George Westinghouse and Nikola Tesla in the late 1800's was to realize that producing alternating current made it possible to transfer power easily from one electric circuit to another with the help of an electromagnetic device called a transformer. When an alternating electric current passes through the primary wire coil of a transformer, the changing magnetic and electric fields that this current produces transfer power from that primary current to the current passing through another coil of wire—the secondary coil of the transformer. While no electric charges move between these two wires, electric power does. With the help of a transformer, it's possible for a generating plant to move power from a large current of relatively low energy electric charges—low voltage charges—to a small current of relatively high-energy electric charges—high voltage charges. This small current of high voltage electric charges can move with relatively little power loss through miles and miles of high voltage transmission lines and can go from the generating plant to a distant city without wasting much power. Upon arrival at the city, this current can pass through the primary coil of another transformer and its power can be transferred to a large current of relatively low voltage charges flowing through the secondary coil of that transformer. The latter current can then deliver this electric power to your neighborhood. A transformer can't transfer power between two circuits if those circuits operate with direct current. Edison tried to use direct current in his power delivery systems and fought Westinghouse and Tesla tooth and nail for years. Edison even invented the electric chair to "prove" that alternating current was much more dangerous than direct current. Still, Westinghouse and Tesla won out in the end because they had the better idea.
. I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. - M
A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate rapidly around the motor's rotating central component—its "rotor." The rotor consists of a cylindrical aluminum metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications, but you wouldn't want it in an elevator—the sudden starting of the elevator car that would accompany the sudden starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced by a group of wires that are connected by way of metal ring (the "slip rings") and some stationary conductive brushes to some components outside the rotor. During the starting process, the currents that are induced in the rotor's wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully. When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted together so that the rotor behaves like the aluminum cage of a normal induction motor.
. Is there an inexpensive device for detecting leaks from a microwave oven?
Yes. You can get one from a hardware or appliance store for about $5 to $30. ComfortHouse.com sells one on-line at www.comforthouse.com
. While I have tended to downplay the leakage issue in the past, I bought a tester and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class demonstrations, so it had been abused and the door wasn't properly aligned any more. I retired it. Incidentally, the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters, and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.
. What causes things to glow in the dark? Why does phosphorus glow? Why does the glow die? — DB & EB
Most glow-in-the-dark materials store energy when they are exposed to visible light and then glow dimly as this stored energy is gradually converted back into light. In such a material, exposure to light promotes some of the electrons in the atoms or molecules to excited states and these electrons become trapped in lower-energy excited states from which they have trouble escaping. It takes a very long time for each of these trapped electrons to return to their original states by emitting light. Since that return is a random process, a glow-in-the-dark object glows with an ever diminishing light as the excited electrons return at random moments to their original states. Eventually almost all the electrons have returned and the glow weakens to essentially nothing.
White phosphorus also glows in the dark, but not for the same reason. You don't need to expose white phosphorus to light to make it glow; you need to expose it to air. The chemical reaction between phosphorus and oxygen causes the phosphorus to emit light. This reaction can also cause the white phosphorus to burst into flames. Because of its dangerous flammability and its toxicity, white phosphorus isn't something you want to have around.
. Why does a spring want to come back to the original position it started at? — JF, Hazen, ND
When you stretch or bend a spring, you are displacing atoms within the crystals that make up that spring. Each atom in those crystals moves a tiny bit nearer or farther from its neighbors and it begins to experience tiny forces that would push it back toward its original position if you let go of the spring. When you do let go of the spring, these tiny forces act together to return the spring to its original shape while returning the individual atoms in the crystals back to their original positions. However, if you bend a spring too far, the atoms begin to slide across one another and they can no longer find their way back to their original positions. In that case, the spring has become permanent bent and won't return to its original shape when you let go. Good spring materials are those that can tolerate a substantial amount of stretching or bending without allowing their atoms to slide across one another. Many common metals don't make good springs because this sliding occurs much too easily.
. You have mentioned the relationships between electric fields, magnetic fields, and current. Which causes which? Does current cause a magnetic field, in turn, causing flow in the next circuit and so forth? What is this order of occurrence? — BJ
Those three items, electric fields, magnetic fields, and currents, are strongly interrelated. Here are some of those relationships: (1) currents cause magnetic fields, (2) currents that change with time cause magnetic fields that change with time, (3) magnetic fields that change with time cause electric fields, (4) electric fields cause currents to flow in electric conductors. From these relationships, you can see that any time you have a changing current through one circuit, you can end up with a current flowing through another nearby circuit. Power moves from the first circuit to the second circuit with the help of a magnetic field and an electric field. A moving magnet also produces a magnetic field that changes with time and it can send a current through a nearby circuit, too.
. If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep that generator turning, why do brownouts occur?
As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its alternating current won't change. When the homes powered by the generator draw more current, then the generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it. But suppose that the turbine can't exert any more torque on the generator. In that case, the power company can either shut down the generator or it can reduce the strength of the generator's rotating magnet. This rotating magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a period of excessive current demand, the power company may choose to weaken the rotating electromagnet to prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow relatively dimly—a brown-out.
. How does luminol work? — CW, San Antonio, TX
Luminol produces light during a chemical reaction with either molecular oxygen or a mixture of potassium ferricyanide and hydrogen peroxide and is probably the basis for most light sticks. In an alkaline (basic) solution, the luminol molecule becomes a dianion, a molecule with two negative charges on it. In this dianion form, the molecule has two nitrogen atoms exposed to the solution and these nitrogen atoms are easily replaced by two oxygen atoms. When that exchange takes place, a molecule of nitrogen gas is released and the final oxidized luminol is left in an electronically excited state. This molecule quickly gets rid of its excess energy by emitting light.
. Why does an artificial sponge absorb more water than a natural sponge? — JH, Angleton, TX
Water is drawn into a sponge in part because of an attraction between the water molecules and the sponge's surface and in part because of water's tendency to minimize its own surface area. When you put a drop of water on a waxy surface, the water beads up. That's because water and wax don't bind well to one another and the water molecules pull toward one another instead. The water droplet tries as best it can for form a sphere, since a sphere has the smallest surface area that a given volume of water can occupy. These forces that pull water's surface inward are called surface tension.
But when you put a drop of water on real cellophane (a smooth form of cellulose), the water spreads out. That's because water and cellulose bind strongly to one another and the water will permit its surface area to increase somewhat if that increase allows it to attach to more cellulose. Similarly, water binds well with other forms of cellulose, including paper, cotton, and Rayon. I think that most artificial sponges are either cellulose or a close chemical relative of cellulose.
A sponge absorbs water by allowing that water to cling to an extensive surface that binds well with water. The water spreads out along that surface while trying to minimize the surface area of any water that isn't touching the sponge. The surface of a natural sponge interacts well with water (the sponge lives in water after all), but a natural sponge can't compete with modern technology. A company that makes artificial sponges can adjust the chemical structure of the sponge's plastic so that it binds nicely to water molecules; it can adjust the sizes of the holes in the sponge to attract the water as efficiently as possible with a given mass of plastic; and it can tailor wall thickness to give the sponge the right elasticity. Furthermore, some of the water is brought right into the plastic and that water softens or "plasticizes" the plastic. That's why a sponge is hard when dry and soft when wet—the water molecules are effectively lubricating the plastic molecules so that they can slide past one another.
Formally defined as "the capacity to do work", energy is a measure of an object's ability to make things happen. It is interesting to physicists for one important reason: it is a conserved physical quantity. By "conserved physical quantity", I don't mean that it's something that we try not to waste. I mean that the amount of energy in an isolated system can't change—energy can't be created or destroyed, it can only be transferred from one object to another or converted from one form to another. Because you can't make it or consume it, energy is an important characteristic of objects and systems. You can often watch it move from object to object and observe the consequences of this movement. For example, the energy that I'm using now to type at my keyboard arrived at the earth's surface as sunlight, was used by plants to build new molecules that eventually become part of my breakfast this morning and are now being combined with oxygen in my body to allow me to move my fingers. Nowhere along this chain was energy created or destroyed—it simply moved about and changed forms. It will still be here tomorrow, and then next day, and even the day after that.