Author: Lou Bloomfield

How can I check the magnetron in a home microwave oven? I have checked the HV (h…

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How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much louder than normal. — AA, Ontario, CA

While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail by shorting out, the result of electromigration of the filament material. The tube would then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you’ve paid for it. I’d suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.

If there were no friction or air resistance, would the bowling ball pendulum con…

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If there were no friction or air resistance, would the bowling ball pendulum continue in motion forever?

Yes. If the pendulum had no way to convert its energy into thermal energy (e.g., via friction) and no way to transfer that energy elsewhere (e.g., via air resistance), it would continue to swing forever. While its energy would transform from gravitational potential energy (at the ends of each swing) to kinetic energy (at the middle of each swing) and back again, over and over, the total amount of energy it has won’t change.

In class, you sat motionless on a cart with a ball in your lap. You said that yo…

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In class, you sat motionless on a cart with a ball in your lap. You said that your momentum was zero. You then threw the ball in one direction and you began moving in the other direction. You said that your momentum was still zero. How can your momentum be zero if you are moving?

In both cases, I was referring to the total momentum of the ball and me. The total momentum of the ball and me was zero before I threw the ball and it was still zero after I threw the ball. However, before I threw the ball nothing was moving and after I threw the ball the two of us were moving in opposite directions. It was our total momentum that was zero after the throw, not our individual momenta. While the ball and I each had a nonzero momentum after the throw, our momenta were equal in amount but opposite in direction—the ball’s momentum was exactly opposite mine. If you were to add our momenta together, they would sum to zero. Since momentum is conserved and we couldn’t exchange momentum with anything around us, the ball and I began and ended with the same total amount of momentum: zero.

Piling sandbags in the back of a truck would increase friction between the wheel…

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Piling sandbags in the back of a truck would increase friction between the wheels and the ground, but wouldn’t it also increase the truck’s inertia, making it harder to stop on an icy road?

Adding sandbags to the back of a pickup truck increases the truck’s traction and adds to the truck’s mass. Fortunately, the truck’s traction increases more dramatically than its mass and it becomes easier to start and stop the truck, rather than the reverse. That’s because even a modest amount of sand can double the force pressing the rear wheels against the road and thus double the frictional forces the wheels can experience. That same amount of sand won’t double the total mass of the truck.

What would things look like if I could see wavelengths of the spectrum other tha…

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What would things look like if I could see wavelengths of the spectrum other than just visible light (e.g., X-rays, radio waves, ultraviolet, infrared, gamma rays, etc.)? — SH, Hurricane, UT

As you looked around, you would see a general glow of radio waves, microwaves, and infrared light coming from every surface. That’s because objects near room temperature emit thermal energy as these long-wavelength forms of light. While we don’t normally see such thermal radiation unless an object is hot enough for some of it to be in the visible range, your new vision would allow you to see everything glow. The warmer an object is, the brighter its emission and the shorter the wavelengths of that emission. People would glow particularly brightly because of their warm skin.

You would also see special sources of radio waves, microwaves, and infrared light. Radio antennas, cellular telephones, and microwave communication dishes would be dazzlingly bright and infrared remote controls would light up when you pressed their buttons.

You would see ultraviolet light in sunlight and from the black lights in dance halls. But there wouldn’t be much other ultraviolet light around to see, particularly indoors. X-rays and gamma rays would be rare and you might only see them if you walked into a hospital or a dentist’s office. Gamma rays would be even rarer, visible mostly in hospitals.

How fast is the earth moving through space? Does this movement affect our percep…

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How fast is the earth moving through space? Does this movement affect our perception of time? — GR, Grabil, IN

Because there is no preferred reference frame for the universe, we can only talk about the earth’s speed in reference to other objects. For example, the earth is moving at about 5 kilometers per second relative to the sun and about 30,000 kilometers per second relative to the center of the galaxy. These speeds do affect our perceptions of time, so that times passes at a different rate for us than for someone closer to the sun or to the galactic center. However, gravitational wells also affect the perception of time, so that the effects are complicated. The earth is also receding extremely rapidly from objects at the far side of the universe; so fast that time passage is dramatically affected. Those distant objects appear to be aging very slowly and their light is shifted substantially toward the red.

Can you explain how the telephone wiring in my home works for the telephone? My …

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Can you explain how the telephone wiring in my home works for the telephone? My touch-tone phone has 4 wires, but I understand that only 2 wires are used. Does the phone use the other 2 wires for the light on the phone pad, etc.? — DS, Larkspur, CA

Your telephone performs all of its functions using only those 2 wires. The 2 extra wires are virtually never used by a single-line telephone. The only exception that I’m aware of is the old “Princess Telephone,” which had a special light powered by the extra pair of wires. In most telephones, even the power for the lighted keys comes from the 2 main wires. While the telephone is off the hook, the telephone company sends a constant DC current through those two wires. This current powers the telephone’s electronics and its lights. When you talk, the microphone causes the telephone’s electric impedance to fluctuate up and down and this variation causes sound to be reproduced in your friend’s earpiece. Pressing the dialing buttons causes similar fluctuations in impedance and the telephone company uses these tones to make the proper connections. When the telephone company rings your telephone, they send a higher voltage AC current through the two wires and the telephone’s bell rings.

Radioactive elements’ half-lives are fixed and they decay at a constant rate. Th…

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Radioactive elements’ half-lives are fixed and they decay at a constant rate. Their decay rates have been determined thanks in part to our nuclear weapons research. Under what circumstances can a radioactive element have its decay rate changed? Can the element’s radioactivity be destroyed (cancelled) by applying high temperatures? If so, how high would the temperature have to go to achieve this? — RD, Humble, TX

Since radioactivity is a feature of atomic nuclei, the only way to alter radioactivity is to alter atomic nuclei. But there aren’t many ways to change atomic nuclei. Of various atomic and subatomic particles, only a neutron can enter a nucleus easily and cause it to rearrange. However, it’s more common for a neutron to increase radioactivity than to destroy it, so that’s not a good approach. Furthermore, the only practical way to obtain neutrons is with radioactivity.

Heating a collection of nuclei can cause them to collide and rearrange. However, this process is also fraught with problems. The products of the fusion and fission events that occur when nuclei collide will probably be radioactive themselves, so that it’s unlikely that heating radioactive materials will make them less radioactive. Instead, it’s likely that heating radioactive materials will make them more radioactive. Furthermore, the temperatures at which nuclei will begin to collide are extraordinarily high. Even the smallest nuclei repel one another fiercely so that they need temperatures of 100 million degrees C or more to begin colliding effectively. Larger nuclei, such as those common in nuclear wastes, won’t collide until their temperatures exceed 1 billion degrees C. The only way to reach these temperatures is with nuclear weapons and they certainly don’t reduce the radioactivity of nearby materials. In short, the only way to get rid of radioactivity is by waiting patiently.

Why do colors fade in the sun? – RD

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Why do colors fade in the sun? – RD

While light travels as electromagnetic waves, it’s emitted and absorbed as particles called “photons.” Each photon carries with it a tiny bit of energy. The amount of energy in a photon depends on the wavelength of the light associated with it. While a photon of red light contains too little energy to cause chemical processes to occur in most molecules, a particle of violet or ultraviolet light contains enough energy to cause significant chemical damage to a typical molecule. Since sunlight contains a substantial amount of violet and ultraviolet lights, it can cause a fair amount of chemistry to occur in the molecules that absorb it. That’s why colors often fade in sunlight. Many colored molecules are relatively fragile and are damaged by photons of ultraviolet light. The portion of a dye molecule that gives it its color is called a “chromophore” and is usually the most fragile part of the molecule. Destroying its chromophore will often leave a dye molecule colorless. Exposure to sunlight was the traditional way to bleach fabrics and make them white.