Month: December 1996

How can an insulator carry a charge if it cannot conduct electricity? How can on…

Lou Bloomfield Leave a comment

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.

Is there water on the moon?

Lou Bloomfield Leave a comment

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.

Why does an object like metal give off light when it is heated?

Lou Bloomfield Leave a comment

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.

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

Lou Bloomfield Leave a comment

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

Lou Bloomfield Leave a comment

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…

Lou Bloomfield Leave a comment

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

Lou Bloomfield Leave a comment

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…

Lou Bloomfield Leave a comment

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…

Lou Bloomfield Leave a comment

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.