Author: Lou Bloomfield

There is an experiment involving grapes and microwaves that we found on the inte…

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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?

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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.

Please explain how the different welding systems work, (Arc, TIG, MIG, and Oxy-A…

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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.

If time passes more slowly for someone who is moving quickly and enormous speeds…

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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.

How fast does sound travel through the telephone? – T

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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.

Do you think it will ever be possible to build/create different atoms up to carb…

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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 does a hydrogen bomb work? How does it differ from the atomic bomb besides t…

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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.

What types of sound can humans hear? What types of materials are soundproof? How…

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What types of sound can humans hear? What types of materials are soundproof? How is the volume of a sound changed? Is the speed of sound the same in all types of media, such as water or air? — JM, Fairfax, VA

In air, sounds are disturbances that consist of compressions and rarefactions—the air molecules are packed either more tightly or less tightly than normal. These regions of too high or too low pressure and density move through the air at about 330 meters per second—the speed of sound and when they pass our ears, we may hear them as sound. As a particular sound passes our ears, the air pressure rises and falls and then rises again, over and over. The number of full cycles—a pressure rise then a pressure fall—that pass our ears each second determines the pitch of the sound we hear. The lowest pitch that our ears are sensitive to is about 20 cycles per second and the highest pitch that we can detect is about 20,000 cycles per second. While other pitches are possible, we simply can’t hear them with our ears.

A sound’s volume is determined by the extent to which the air pressure fluctuates as the sound passes. A loud sound involves a stronger pressure fluctuation than a soft sound. Soundproof materials are ones that decrease the volume of the sound passing through them by weakening the pressure fluctuations. There are two ways to decrease the volume of sound passing through a material: by absorbing the sound or by reflecting it. Soft materials such as carpet or foam rubber absorb sound by allowing the sound’s pressure fluctuations to waste their energies bending the materials. The sound’s energy is converted into thermal energy. Hard, dense materials reflect sounds by making the sounds change speed. Sound travels quickly through most solids and liquids—typically about 5 to 10 kilometers per second. Whenever a wave changes speed in passing from one medium to another, part of that wave is reflected. Thus as sound speeds up in entering a hard surface from the air and as that sound slows down when reentering the air, much of the sound reflects.

What are gas permeable contact lenses made from and what do they use to pigment …

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What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK

A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear’s eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or “vulcanized” to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I’m not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.

How does the auto-focusing system on a camera work?

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How does the auto-focusing system on a camera work? — RM, Lititz, PA

There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera’s lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it’s confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That’s why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.