Why does a gas lantern use a silk mantle? How does it produce such intense light…

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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

What causes a dropped ball to bounce? – MK

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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.

I just bought a set of nice chrome wheels with low profile tires for my car. Sin…

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I just bought a set of nice chrome wheels with low profile tires for my car. Since these 4 wheels are 40 pounds heavier than the old ones, I removed 40 pounds of weight from the body of the car to compensate. My acceleration times and braking distances have increased dramatically. Why? — DTS, Shawnee, Kansas

When you accelerate forward from a stop, the car’s kinetic energy is increasing. The time it takes you to reach cruising speed is largely determined by how fast the car’s engine can increase the car’s kinetic energy. Stopping speed is similarly determined by how quickly the brakes can remove the car’s kinetic energy. While your car still has the same mass that it had before you changed wheels, and thus would seem to require the same transfers of energy to start and stop, that’s not the case. Transferring mass from the car’s body to its wheels has substantially increased the amount of kinetic energy the car has when it’s moving at cruising speed. That’s because each spinning wheel has two forms of kinetic energy. First, its center of mass is heading forward at cruising speed, so it has a translational (motion along a line) kinetic energy proportional to its mass. Second, it is spinning about its center of mass, so it has a rotational kinetic energy proportional to its moment of inertia (the rotational equivalent of mass). If most of each wheel’s mass is located near its periphery, its rotational kinetic energy will be roughly equal to its translational kinetic energy. The 40 pounds you transferred to the wheels is counting twice as much as before! You’ve effectively added 40 pounds to the mass of your car. Your new wheels and tires are demanding far more energy from your car’s engine and delivering far more energy to your car’s brakes than the old wheels did and you’ll have to remove an additional 40 pounds from the car’s body to compensate.

A man falls into the center of the earth

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A man falls into the center of the earth — how much does he weight? Which way is space bent in the center of the earth? — JW, Virginia Beach, VA

At the center of the earth, the man would be truly weightless and the space around him would be exactly flat (no curvature due to gravity). This special situation occurs because the gravitational effects of the earth around the man are perfectly balanced. With equal amounts of the earth’s mass on each side, there is no special direction in which the man would accelerate.

If space is curved and gravity is not really a force (as per Einstein), how is i…

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If space is curved and gravity is not really a force (as per Einstein), how is it that an object can slingshot around a planet and gain kinetic energy? Where is the extra energy coming from? Which object converts mass to energy; the object or the planet? — EM, Redmond, WA

When a small object such as a satellite arcs around the back side of forward moving planet, the satellite’s speed and energy increase while the planet’s speed and energy decrease. The planet has given some of its energy to the satellite. Viewed in terms of curved space, the satellite follows a curved path because of the planet’s presence and the planet follows a curved path because of the satellite’s presence. The satellite’s effect on space is very small, but it is enough to change the planet’s path slightly. The planet arcs toward the satellite and gives up a small amount of its speed and energy in the process. This energy is transferred to the satellite as the satellite arcs toward the planet. Overall, the planet loses a little of its kinetic energy and the satellite gains an equal amount of kinetic energy. However, neither the planet nor the satellite experience any changes in rest mass. Both objects still have the same numbers of atoms as before and both still have their original masses.

How can one measure the vapor pressure of mercury? If it is amalgamated, what is…

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How can one measure the vapor pressure of mercury? If it is amalgamated, what is the relationship of vapor pressure with respect to temperature, material content in the amalgam, and free mercury? — BS, Erwin, TN

The vapor pressure of mercury is quite low at room temperature so you’d need a very sensitive pressure gauge and a vacuum system in order to measure it. You’d have to evacuate all of the air from the gauge and expose the empty gauge to a saturated vapor of mercury (mercury vapor that’s in contact with liquid mercury) alone. While the pressure will only be a few thousandths of a millimeter of mercury, there are a number of pressure gauges that are capable of measuring pressures in this range.

Once the mercury is amalgamated with other metals, its vapor pressure drops substantially. The mercury atoms bind so strongly into the amalgam that they can remain in it for years, centuries, or even millennia. Mercury’s vapor pressure in this bound form is exceedingly low. To measure it, you’d need a mass spectrometer that’s capable of counting the atoms in the vapor above the amalgam.

What is the scientific explanation of a rainbow?

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What is the scientific explanation of a rainbow? — RS, Salinas, CA

A rainbow is caused by three important optical effects: reflection, refraction, and dispersion, all working together. The rainbow forms when sunlight passes over your head and illuminates falling raindrops in the sky in front of you. This sunlight enters each spherical raindrop, partially reflects from the back surfaces of the raindrop, and then leaves the raindrop and heads toward you. The raindrop helps some of the sunlight make a near U-turn. But the path that the light follows after it enters the raindrop depends on its color. Light bends or “refracts” as it changes speed upon entering water from air and the amount it bends depends on how much its speed changes. Since violet light slows more than red light, a phenomenon called “dispersion,” the violet light bends more than the red light and the two colors begin to follow different paths through the drop. All the other colors are spread out between these two extremes.

The colored rays of light then partially reflect from the back surface of the raindrop because any change in light’s speed also causes partial reflection. Now the various colors are on their way back toward you and the sun. The light bends again as it emerges from the raindrop and the various colors leave it traveling in different directions. Only one color of light will be aimed properly to reach your eyes. But there are other raindrops above and below it that will also send light backward and some of that light will also reach your eyes. But this light will be a different color. What you see when you observe the rainbow is the lights that many different raindrops send back toward your eyes. The upper raindrops send their red light toward your eyes while the lower raindrops send their violet light toward your eyes. You see a series of colored bows from these different raindrops.

In a microwave oven, does food cook from the inside out or outside in?

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In a microwave oven, does food cook from the inside out or outside in? — KS, Essex, England

If the piece of food isn’t too large, it all cooks at once. The microwaves that heat the food pass deep into it and they deposit energy in every part of the food simultaneously. Only if the piece of food is so large that an appreciable amount of microwaves are absorbed before they reach the center will the center cook more slowly than the outside. I doubt that this shielding of the center is a problem with foods small enough to fit inside a normal microwave oven. However, the microwaves in a microwave oven aren’t perfectly uniform, so that some parts of a meal will cook a bit faster than others. That’s why it’s important to move the food about during cooking to achieve uniform heating throughout.

I recently acquired a microwave that “doesn’t cook as fast as it used to.” Doe…

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I recently acquired a microwave that “doesn’t cook as fast as it used to.” Does this sound right? What type of service might need to be performed? – W

It is possible for a microwave to lose cooking speed. If the microwave source isn’t able to produce as intense microwaves as before or if it doesn’t turn on reliably and steadily, it won’t cook as fast. For the source to produce less intense microwaves, the high voltage power supply would probably have to be weak. Its storage capacitor could have failed or one or more of its high voltage diodes could have burned out. According to a reader, the most likely cause of weak cooking in a microwave oven is a failed capacitor—with no ability to store separated charge in its capacitor, the oven produces pulsing rather than steady microwaves and delivers less average power. I suppose that the magnetron itself could be dying, with the most common failure (according to that same reader) being shorting out, the result of electromigration of the filament material. For the source to not turn on reliably, it would probably have to have a bad connection to the power line. One good possibility is that the relay that turns on power to the high voltage power supply is not making good contact.

Listen to the microwave as it operates on a medium setting. It should cycle on and off every five or ten seconds. You should hear it hum softly during the on half of the cycle and then stop humming during the off half of the cycle. Different power levels simply vary the fractions of on time and off time. If you don’t hear the hum or the hum is intermittent, then something is probably wrong with the power relay or with something else in the high voltage power supply. If the relay is flaky, a little cleaning of its contacts may cure the problem. Be careful of the high voltage capacitor, which can store a lethal charge even when the unit is unplugged.