. There is an experiment involving grapes and microwaves that we found on the internet. If a grape is cut in half—with a piece of skin attached between the two halves—and it is then microwaves, sparks are produced. What is happening? — GB, Antioch, CA
This experiment is described in Fun with Grapes - A Case Study
. While I haven't tried it yet myself, I believe I know why it works. Grape juice is somewhat able to conduct electricity and the two halves of the grape are connected by a weak conducting path: the skin bridge. When the microwave oven is turned on, the microwaves not only heat the water in the grapes, they also push a few mobile electric charges back and forth through the skin bridge from one side of the grape to the other. This current releases energy as it passes through the narrow bridge and it heats the bridge extremely hot. The bridge soon catches fire and the electric current driven by the microwaves begins to pass through the flame. When current passes through a gas, it tends to ionize that gas (remove electrons from the gas atoms) so that the gas itself begins to conduct electricity. When current flows through atmospheric pressure air, it forms a brilliant arc. In this case, the arc that you see is powered by the microwaves as they push electric charges back and forth from one side of the grape to the other. An excellent set of movies showing this and other microwave oven experiments appears at http://www.physics.ohio-state.edu/~maarten/microwave/microwave.html
. Why don't microwaves get stuck in the food we put in the microwave oven?
Microwaves are like light—both are electromagnetic waves and both move extremely quickly. While it is possible to trap a light wave briefly between two mirrors, that wave will eventually be absorbed or released. The same is true of a microwave. It's almost impossible to trap a microwave for more than 1 second, even in very exotic enclosures, so you needn't worry about them becoming trapped in food. The food simply absorbs them and turns their energy into thermal energy.
. How does a hydrogen bomb work? How does it differ from the atomic bomb besides the simple difference of fusion and fission? — KS, Lake Oswego, OR
A hydrogen bomb uses the heat from a fission bomb (a uranium or plutonium bomb, sometimes called an atomic bomb) to cause hydrogen nuclei to collide and fuse, thereby releasing enormous amounts of energy. While a fission bomb can initiate its nuclear reactions at room temperature, fusion reactions won't begin until the nuclei involved have been heated to enormous temperatures. That's because the nuclei are all positively charged and repel one another strongly up until the moment they stick. Only at enormous temperatures (typically hundreds of millions of degrees) will the nuclei collide hard enough to stick and release their nuclear energy. A typical hydrogen bomb (also called a fusion bomb or thermonuclear bomb) uses a fission trigger to initiate fusion in a mixture of deuterium and tritium, the heavy isotopes of hydrogen. These neutron-rich isotopes fuse much more easily than normal hydrogen. Because deuterium and tritium are both gases, and because tritium is unstable and gradually decays into the light isotope of helium, some hydrogen bombs form the tritium during the explosion by exposing lithium nuclei to neutrons from the fission trigger. Thus the "fuel" for many thermonuclear bombs is actually lithium deuteride, which becomes a mixture of tritium and deuterium during the explosion and then becomes various helium nuclei through fusion.
. Do you think it will ever be possible to build/create different atoms up to carbon or perhaps even gold (the alchemist's dream)? You would have to use fusion, wouldn't you? Would this be a good source of energy? — JB, Norman, OK
As you noted, this process of sticking together smaller atomic nuclei or nuclear fragments to form larger atomic nuclei is called fusion. Many smaller nuclei release energy when they grow via fusion, so long as the resulting nuclei are no larger than 56Fe (the nuclei of a normal iron atom). Above that size, energy is consumed in the process of sticking the nuclei together. So building carbon nuclei would release energy and building gold atoms would require energy. But while it's possible to construct atomic nuclei up to carbon or even gold, it isn't very practical. It's very difficult to bring atomic nuclei close to one another because they are all positively charged and repel one another fiercely. Because the nuclear energy these nuclei release during fusion only emerges at the moment they actually touch, something must push them together for that to occur. The nuclei can be pushed together by (1) nuclear fission reactors, (2) particle accelerators, (3) thermonuclear weapons, (4) giant lasers, or (5) thermal fusion reactors. None of these systems is ready to synthesize large quantities of normal atoms in a cost effective manner (although nuclear fission reactors do produce useful quantities of radioactive isotopes) and none is ready to produce practical energy from fusion processes.
. How fast does sound travel through the telephone? - T
When your voice travels through the telephone, it doesn't travel as sound. Instead, the microphone of your telephone unit produces an electric current that represents the sound of your voice. From there on until it arrives at the earpiece of your friend's telephone unit, your voice travels as an electromagnetic signal—either an electric current, a radio wave, or a light wave. Only when it reaches the earpiece is the electromagnetic signal used to recreate the sound itself. Since electromagnetic signals travel at or near the speed of light, your voice moves extremely quickly from your telephone unit to your friend's telephone unit. It would be quite easy, for example, for a friend living a few miles away to tell you about a nearby explosion or thunderclap and then have you hear that explosion or thunderclap yourself. Your friend's words would travel much more rapidly through the phone lines than the sound would travel over the countryside.
However, even the speed of light isn't fast enough in some cases. Shortly after the break-up of AT&T, new long-distance carriers began to appear. Some of these companies used geosynchronous satellites to handle the long distance calls. Because these satellites sit about 22,300 miles above the earth's equator, the travel time for radio waves to and from these satellites is a substantial fraction of a second. The delay between when you spoke and when your friend heard your voice was long enough that your friend might have begun talking, too. Those conversations were very awkward because you had to be very deliberate about starting and stopping your speech. You almost had to tell your friend when you were done talking so that they could begin. All modern long-distance calls are handled by surface links so that there is almost no delay, except perhaps when going to the other side of the earth.
. If time passes more slowly for someone who is moving quickly and enormous speeds are needed to explore distant space, is there any way to counteract this time/speed phenomenon so that those on earth will not die waiting for the "space travelers/explorers" to return? — BC, Ottawa, Canada
Unfortunately, no. Those of us who remained on earth would watch the explorers head off at enormous speeds toward the stars and would be old and gray before they returned. Even if the explorers could move at almost the speed of light, it would take them many years to reach nearby stars and many years to return. Since there is no way that they could travel even as fast as the speed of light, the absolute minimum time it would take for a round trip, from our perspective, would be the round trip distance to the stars divided by the speed of light.
But this brings up one of the peculiar results of special relativity. From our perspective on earth, the explorers are moving quickly as they head toward the stars and their clocks appear to be running slowly to us. But from their perspective, we are moving quickly in the other direction and our clocks appear to be running slowly to them. This apparent paradox is resolved by the fact that the explorers would not agree with us on the ordering of two events occurring at different locations—space and time appear differently to us; they are intermingled. However, when the explorers accelerate in order to turn around and headed back toward us, their perceptions of space and time undergo a radical change. They see our clocks zoom ahead while we continue to see their clocks running fairly slowly. When the explorers finally returned to earth, their clocks indicate that they had been gone only a short time. However our clocks indicate that they had been gone at least as long as the time it would take light to complete the roundtrip. This situation leads to the famous "twin paradox," in which one twin travels through space while the other remains at home. When the explorer twin returns to earth, the explorer twin is still young but the earthbound twin is very old. If near-light-speed travel were to become possible (a very remote possibility), such twin paradoxes would certainly occur.
. Please explain how the different welding systems work, (Arc, TIG, MIG, and Oxy-Acet) and why some types work with certain metals (steel, aluminum, titanium, and cast iron) and others don't? — DC, Ceder, MN
While I have very little experience welding myself, I can make a number of general observations about welding. All of the welding systems you mention are trying to join several pieces of metal by melting them together. In most cases, one of the pieces of metal is being used to form the joint and is sacrificed completely in the process (typically it's a welding rod made of a special metal that's good at forming a joint). How the melting and joining process proceeds depends on the welding system used.
An arc welder passes an electric current through the air from the pieces to be joined to a welding rod. The rod becomes so hot as the result of this arc that it melts and joins with the other pieces of metal, binding them together permanently. This scheme only works with relatively non-flammable metals such as steel. Aluminum or titanium will burst into flames when the arc starts. To joint these flammable metals, the arc has to be protected by a shroud of an inert gas such as argon or helium. TIG and MIG welding are based on this inert gas approach (the "IG" part of the names). In Tungsten-Inert-Gas (TIG) welding, an arc passes from the pieces being joined to a tungsten electrode. Tungsten has such as high melting point that it survives this arc and another piece of metal, the welding rod, is fed into the arc where it melts to form the joint. In Metal-Inert-Gas (MIG) welding, the arc passes from the pieces to a metal welding rod. This system resembles normal arc welding, in that the welding rod melts to form the joint, however now the arc is shrouded by a flow of inert gas so that there is no oxygen around to support combustion. Flammable metals can be welded with TIG or MIG welding and so can non-flammable metals.
As for oxygen-acetylene welding, here a very hot flame is used to heat the pieces involved to very high temperatures. A welding rod that melts at a slightly lower temperature than the pieces themselves is then used to join the pieces. The advantage to using this system is that it doesn't pass a current through the pieces and doesn't rely on their electric properties. The current of an arc welder could damage thin materials but an oxygen-acetylene flame should not (assuming they are relatively non-flammable metals). I'm sure that the metallurgical characteristics of the joints vary from system to system, but I can't make any useful statements about this. For a more detailed discussion of when and where to use each technique, you'll need a more experienced person than me.
. Why is it impossible to make a wheel that turns forever, all by itself? — AWG, Karachi, Pakistan
While people are always trying to build perpetual motion machines, they will never succeed. All of these devices are intended to obtain useful energy—what physicists call "work"—from either nowhere or from less useful energy—what physicists often call "heat" or "thermal energy". Obtaining work from nowhere is really impossible; energy is a conserved quantity, meaning that it simply cannot be created or destroyed. For a machine to do work, it must obtain energy from somewhere or something else. So if anyone tries to sell you a car engine that doesn't take any fuel at all—thus creating work out of nowhere—don't buy it! It's a fraud.
As for machines that try to convert thermal energy completely into work, they are also impossible, but for a different reason. While they don't violate the conservation of energy, they do violate the laws of thermodynamics. Thermal energy is disordered energy—it is energy that has been distributed randomly among the individual atoms and molecules in an object so that it cannot be easily reassembled to do useful work. When you burn a candle, all of the energy the candle once had is still in the room, but it's much harder to use. Just as a coffee cup is much more useful before you drop it than after you drop it, so energy is much more useful before you disorder it than after you disorder it. The difficulty with reassembling thermal energy to do useful work is a statistical one: it's unlikely that this energy will spontaneously reassemble itself in a useful manner, just as its unlikely that a dropped coffee cup will spontaneously reassemble itself in a useful manner. The laws of mechanics don't prevent either of those reassemblies from occurring, but both reassemblies are statistically very unlikely to occur. How often have you dropped a broken cup and had it fall together rather than apart? So if someone tries to sell you a car engine that uses the thermal energy in the surrounding air as "fuel"—thus turning thermal energy completely into work—don't buy it! It's also a fraud.
. Why are there pistons in an engine? — T, Enola, PA
The pistons in a gasoline engine compress the fuel and air mixture before ignition and then extract energy from the burned gases after ignition. When the engine is operating, each piston travels in and out of a cylinder with one closed end many times a second. The piston makes four different strokes during its travels. In the first or "intake" stroke, the piston travels away from the closed end of the cylinder and draws the fuel and air mixture into the cylinder through an opened valve. During the second or "compression" stroke, the piston travels toward the closed end of the cylinder and compresses the fuel and air mixture to high pressure, density, and temperature. The spark plug now ignites the fuel and air mixture and it burns. During the third or "power" stroke, the piston travels away from the closed end of the cylinder and the expanding gases do work on the piston, providing it with the energy that propels the car forward. During the fourth or "exhaust" stroke, the piston travels toward the closed end of the cylinder and pushes the burned gases out of the cylinder through another opened valve.
. What happens to the temperature of water when ice is added? — OS, Havana, FL
Whenever a glass contains a mixture of both ice and water, the temperature of its contents will be 32° F (0° C). That's because liquid water isn't stable below this temperature and solid ice isn't stable above this temperature. The two can coexist stably only at 32° F.
When you first mix the water and ice, the water is likely to hotter than 32° F and the ice is likely to be colder than 32° F. Heat flows from the water to the ice—from hotter to colder—and soon one of them reaches 32° F. While heat may then continue to flow from the water to the ice, the one that's at 32° F won't change temperature any more. Instead, it will begin to turn into the other form. For example, if the ice reaches 32° F first, it will begin to melt as heat flows into it and turn into water at 32° F. Or if the water reaches 32° F first, it will begin to freeze as heat flows out of it and turn into ice at 32° F. Pretty soon, both the ice and the water will be at 32° F and there they will remain so long as both are still present in the glass.
From a molecular standpoint, ice is an orderly crystalline solid in which the water molecules are neatly arranged in rows, columns, and stacks. Liquid water is a disorganized soup of water molecules that are regularly changing neighbors and moving about, though always in contact with one another. As you add heat to cold ice, its molecules jiggle more and more vigorously against one another until they finally begin to move about as liquid water, a process we call "melting." As you remove heat from warm water, its molecules move about less and less rapidly until they finally begin to cling permanently to one another as solid ice, a process we call "freezing."