. What causes a dropped ball to bounce? - MK
When you lift a ball off the floor, you transfer energy to it. This energy is stored in the gravitational force between the ball and the earth and is called gravitational potential energy. When you release the ball, its weight makes it accelerate downward and its gravitational potential energy gradually becomes kinetic energy, the energy of motion. When the ball hits the floor, both the ball's bottom surface and the floor's upper surface begin to distort and the ball's kinetic energy becomes elastic potential energy in these two distorted surfaces. The ball accelerates upward during this process and eventually comes to a complete stop. When it does, most of the energy that was initially gravitational potential energy and later kinetic energy has become elastic potential energy in the surfaces. However, some of the original energy has been converted into thermal energy by internal frictional forces in the ball and floor. The distorted ball and floor then push apart and the ball rebounds into the air. Some or most of the elastic potential energy becomes kinetic energy in the ball, and the rising ball then converts this kinetic energy into gravitational potential energy. But the ball doesn't reach its original height because some of its original gravitational potential energy has been converted into thermal energy during the bounce.
. Why does a gas lantern use a silk mantle? How does it produce such intense light — BW, Santa Clara, CA
The mantle of a lantern is actually a ceramic ash. The silk itself burns away completely and leaves behind only of the oxides of materials that were incorporated in the silk mantle when it was manufactured. The principal oxide formed when the standard Welsbach mantle is burned is thorium oxide, with a few percent of cerium oxide and other oxides. This use of thorium oxide or thoria, is a rare example of a radioactive element (thorium is radioactive) permitted in common household use. Thoria glows brightly when heated because it can tolerate extremely high temperatures without melting and because it is a very effective emitter of thermal radiation at temperatures of roughly 2200° C.
The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.
. Why does an object like metal give off light when it is heated? — ER, Fresno, CA
All objects emit thermal radiation—electromagnetic waves that are associated with the transfer of heat. That's because all objects contain electrically charged particles and whenever electrically charged particles accelerate, they emit electromagnetic waves. Since all objects have thermal energy in them, their electrically charged particles are always undergoing thermal motion and their thermally induced accelerations cause them to emit electromagnetic waves.
At normal temperatures, the electromagnetic waves of thermal radiation are too low in frequency and too long in wavelength for us to see. But when an object's temperature exceeds about 500° C, the object emits a dim glow. By 1800° C, the object emits the yellowish glow of a candle. By 2700° C, the object emits the yellowish-white light of an incandescent bulb. By 5800° C, the object emits the white light of the sun.
. Is there water on the moon? — JB, Edmonton, Canada
Recent radar studies of the moon's surface have indicated that water may be present at the bottoms of deep craters near the moon's north and south poles. Because sunlight never reaches into these craters, they have cooled by radiating their heat into the empty space overhead and are now extremely cold. They're so cold that water deposited there, probably by comet impacts, has remained as ice for millions of years. While the ice in your freezer slowly disappears because the water molecules sublime—become water vapor—at normal freezer temperatures, extremely cold ice barely sublimes at all and can exist in a vacuum almost indefinitely.
. How can an insulator carry a charge if it cannot conduct electricity? How can one charge an insulator? Can an insulator be charged by induction? — VV, Washington, DC
While charge can't move through an insulator, there is nothing to prevent charge from being placed on its surface or injected inside it. If you rub the surface of an insulator with a piece of silk, sliding friction will push electrons onto or off of its surface and leave its surface electrically charged. With no way for that charge to move about, the insulator's surface retains the charge indefinitely. A beam of fast moving electrons or other charged particles can be injected into an insulator and will become trapped inside it. Once again, the charges can't move around after the injection. Since charges can't flow in the insulator, you can't charge it by induction—a process in which proximity to a nearby charged object rearranges the charges in a conductor and allows you to trap those charges in a nonuniform arrangement.
. What is a short in the electrical system of a car? What causes shorts? — BM, Puyallup, WA
A short circuit is a conducting path that allows electric current to flow from its source (typically the positive terminal of a battery) to its destination (typically the negative terminal of that battery) without passing through the equipment that the current is supposed to operate. The conducting path is thus a short cut for the current that allows it to complete its circuit too quickly, hence the name "short circuit." In virtually all automobiles, the whole body of the car is connected to the negative terminal of the battery so that any accidental conducting path from the battery's positive terminal to the body of the car is a short circuit. Since a short circuit doesn't include a device that's designed to consume electric power, the wires of the short circuit must consume that electric power. They often become hot and may cause a fire.
. How can you make electricity with magnets? - AL
You can make electricity by moving a magnet past a wire. The magnet has a magnetic field around it—something that exerts forces on magnetic poles. If you move the magnet and its magnetic field, you create an electric field—something that exerts forces on electric charges. That's because whenever a magnetic field changes with time, it creates an electric field. This electric field will push on the mobile electrons in a wire. So when you move a magnet past a wire, you are producing a changing magnetic field in the wire. This changing magnetic field produces an electric field and the electric field makes the electrons in the wire accelerate. The moving electrons are electricity. Generators move magnets past wires (or wires past magnets) to produce electricity.
. Would extreme temperatures affect the strength of a magnet? — PL, Columbus, OH
Yes! High temperatures disorder materials and destroy magnetic order. Permanent magnets can be demagnetized by heating them, often to surprisingly modest temperatures. Many household magnets can be spoiled by putting them in a hot oven. Even electromagnets will lose most of their strength at very high temperatures because they rely on iron and iron undergoes several phase transitions at high temperatures that destroy its magnetic order. You can show that iron loses its magnetism at high temperatures by heating a steel nail red hot with a propane torch and then trying to pick it up with a magnet. Be careful not to burn yourself. The hot nail won't stick to the magnet because it won't have any magnetic order. Once the nail cools, its magnetic order will reappear.
. How do you calculate the path light takes after going through a lens and how do you measure the curvature of the lens? — AS, Champaign, IL
The surfaces of most lenses are shaped like the surfaces of spheres. Such "spherical" lenses can be characterized by a single distance: the focal length. For converging lenses, those with convex or outward-bulging surfaces, light from a distant object such as the sun will converge together after passing through the lens and will form an image of the object at a distance of the focal length from the center of the lens. You can find this "real" image by holding a sheet of white paper beyond the lens and looking for a clear pattern of light corresponding to the object. If the object is closer to the lens, the image will form a bit farther from the lens. The relationship between the distance to the object (the object distance or OD), the focal length of the lens (F), and the distance to the image (the image distance or OD) is given by a simple formula: 1/F = 1/OD + 1/ID.
This lens formula works for diverging lenses, too, but those lenses have negative focal lengths and produce their images on the object side of the lens. You can only view these "virtual" images by looking at them through the lens itself.
The easiest way of determining a lens's focal length is by measuring the distance between the lens and the real image it forms of a distant object. However, you can measure the curvatures of the lens's surfaces and calculate its focal length. Special gauges exist that touch the lens at several points, usually a circle and a central point, and determine how curved its surface is.
. How do you create sculptures out of glass? - RD
While I know how to work with glass in principle, I'm certainly not able to make sculptures. Although anyone can shape glass, doing so with artistry and precision requires great skill. In effect, glass is a frozen liquid. Its microscopic structure is very similar to that of a liquid and it softens with temperature rather than melting abruptly. If you heat a piece of glass carefully with a propane torch, it will begin to flow as a thick liquid (like cold honey). In that state, it can be reshaped rather easily. But making it take the shapes you want is a whole other story and something I know little about. I have bent lots of glass tubes in my day, but I often kink the tubing or smash it flat by accident. A skilled glassblower can do seemingly impossible things with glass. I should also note that glass can be cut or shaped by a water-cooled abrasive wheel. Again, anyone can slice and dice glass but it takes great skill to do something attractive. I usually chip the glass pieces that I try to cut.