Month: January 1997

Why do we see colors when light strikes atoms?

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Why do we see colors when light strikes atoms? — GN, Marine City, MI

When white light strikes a molecule, that molecule may absorb some of the light. Light interacts with molecules as particles called “photons” and whether a particular photon is absorbed depends on the structure of the molecule and the color of the photon. Each molecule has the ability to absorb only certain colors of light. For example, a particular molecule may absorb only red photons. As a result, your eye will see only green and blue light photons coming from that molecule when it’s exposed to white light and you will perceive that molecule as having a blue-green color known as cyan. In general, the colors that you see coming from molecules that are illuminated by white light are the colors of light that the molecules don’t absorb.

On really cold winter days at temperatures well below zero, I’ve noticed that su…

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On really cold winter days at temperatures well below zero, I’ve noticed that sunlight is brighter and whiter than on days that are a little below freezing. Why does this happen? — CP, Madison, WI

The colder the air is, the less humidity it can hold. That’s because at low temperature, water molecules in the air are much more likely to land on a surface and stick than they are to break free from a surface and enter the air. Thus cold air is relatively free of water molecules. Water molecules in the air tend to bind together briefly and form tiny particles that scatter light. The sky is blue because of such scattering from tiny particles. With less water in the air, there is less scattering of sunlight. As a result, the sky is a darker blue, almost black, and the sunlight that reaches you directly from the sun retains a larger fraction of its blue light. The sun appears less red and more blue-white than on a warmer, more humid day.

Does an electric blanket produce enough EMF to affect the body and possible incr…

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Does an electric blanket produce enough EMF to affect the body and possible increase the risk of cancer? – FL

The electromagnetic fields (EMF) produced by the currents in an electric blanket are very weak and it takes a pretty sensitive electronic device to detect them. You body is not nearly so sensitive and I still haven’t seen any credible explanation for how these fields could cause any injury to biological tissue. I strongly suspect that all the concern about EMF is just hysteria brought about by a few epidemiological flukes or mistakes.

How do an ammeter and voltmeter work? Why must the former be connected in series…

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How do an ammeter and voltmeter work? Why must the former be connected in series while the latter goes in parallel? — SK, New Haven, CT

The answer is somewhat different for older electromechanical meters than for modern electronic meters. I’ll start with the electromechanical ones and then briefly describe the electronic ones. An electromechanical meter has a coil of wire that pivots in a nearly friction-free bearing and has a needle attached to it. This coil also has a spring attached to it and that spring tends to restore the coil and needle to their zero orientation. Because the spring opposes any rotation of the coil and needle, the orientation of the needle depends on any other torque (twist) experienced by the coil of wire—the more torque the spring-loaded coil experiences, the farther the coil and needle will turn away from the zero orientation. The needle’s angle of deflection is proportional to the extra torque on the coil.

The extra torque exerted on the spring-load coil comes from magnetic forces. There is a permanent magnet surrounding the coil, so that when current flows through the coil it experiences a torque. Because a current-carrying coil is magnetic, the coil’s magnetic poles and the permanent magnet’s magnetic poles exert forces on one another and the coil experiences a torque. This magnetic torque is exactly proportional to the current flowing through the coil. Because the torque on the coil is proportional to the current and the needle’s angle of deflection is proportional to this torque, the needle’s angle of deflection is exactly proportional to the current in the wire.

To use such a meter as a current meter (an ammeter), you must allow the current flowing through your circuit to pass through the meter. You must open the circuit and insert this ammeter in series with the rest of the circuit. That way, the current flowing through the circuit will also flow through the meter and its needle will move to indicate how much current is flowing.

To use such a meter as a voltage meter (a voltmeter), some current is divert from the circuit to the meter through an electric resistor and then returned to the circuit. The amount of current that follows this bypass and flows through the electric resistor is proportional to the voltage difference across that resistor (a natural phenomenon described by Ohm’s law). The voltmeter system thus diverts from the circuit an amount of current that is exactly proportional to the voltage difference between the place at which current enters the voltmeter and where it returns to the circuit. The needle’s movement thus reflects this voltage difference.

In an electronic voltmeter, sensitive electronic components directly measure the voltage difference between two wires. Virtually no current flows between those two wires, so that the meter simply makes a measurement of the charge differences on the two wires. An electron ammeter uses an electronic voltmeter to measure the tiny voltage difference across a wire that is carrying the current. Since the wire also obeys Ohm’s law, this voltage difference is proportional to the current passing through the wire.

How does a dehumidifier know when to turn on and off? The one I bought from Sear…

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How does a dehumidifier know when to turn on and off? The one I bought from Sears doesn’t use the “wet-bulb/dry-bulb” method (of which I could use a better understanding, too). How does its on-off switch work? — JS, Amherst, NY

Most humidity sensing switches or “humidistats” use the expansion or contraction of certain materials to measure humidity. The more humid the air is, the more water molecules there will be in those materials and their shapes and sizes will be affected. For example, human hair becomes longer when wet and it makes an excellent humidity sensor. On a dry day, a hair will contain relatively few water molecules and its length will be shorter. On a humid day, the hair will contain more water molecules and its length will be longer.

A wet-bulb/dry-bulb system measures humidity by looking at the temperature drop that occurs when water evaporates. As water evaporates from the bulb of the wet thermometer and the bulb’s temperature drop, the rate at which water molecules leave the bulb’s surface decreases. The bulb temperature drops until the rate at which water molecules leave the bulb is equal to the rate at which water molecules return to the bulb from the air. At that point, there is no net evaporation going on. In humid air, water molecules return to the bulb more often so that this balance is reached at a higher temperature than in dry air. The wet bulb temperature is thus warmer on a humid day than it is on a dry day.

How does a thermostat regulate temperature?

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How does a thermostat regulate temperature? — TF, Auburn, WA

A typical thermostat turns on the furnace whenever the temperature falls below a certain temperature and turns the furnace off whenever the temperature rises above another temperature. Those two temperatures are slightly separated so that the furnace doesn’t turn on and off too rapidly. In a typical home thermostat, a bimetallic coil tips a small mercury-filled glass bottle. The bimetallic coil is made from two different metal strips that have been sandwiched together and then rolled into a coil. As the temperature changes, the two metals expand differently and the coil winds or unwinds. As it does, it tips the glass bottle and the mercury rolls from one end of the bottle to the other. When the mercury falls to one end, it allows an electric current to flow between two wires and the furnace turns on. When the mercury falls to the other end of the bottle, the current stops flowing and the furnace turns off. So the winding and unwinding of the coil controls the furnace and the home temperature tends to hover at the point where the bottle of mercury is almost perfectly level. When you adjust the set point of the thermostat, you tilt the whole coil and bottle so that the average temperature in your home must shift in order for the bottle to be almost level.

Some friends and I are having a debate. They maintain that if a person sleeps on…

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Some friends and I are having a debate. They maintain that if a person sleeps on an unheated waterbed, heat might be drawn from their body to the point that hypothermia would occur. Is it possible for a waterbed to do this? — JS, College Park, MD

The answer depends on how cold you allow room temperature to become. Without a heater, the water temperature in the bed will be very close to room temperature. When you then lie on the bed, you will be in contact with a surface that’s at room temperature and heat will flow out of you and into the water. Your heat will warm the water and it will tend to float upward and remain at the top surface of the waterbed, forming an insulating layer that will slow your heat loss. However, heat will continue to diffuse into the water as a whole and you will continue to lose heat. As long as the water isn’t too cold, your metabolism will be able to replace the lost heat and you’ll stay warm. But if the room and waterbed are very cold, your temperature will begin to drop. I’m not sure how cold the water would have to be for this to happen, but if the room and water were almost ice cold, you’d probably have trouble.

In “Empire Strikes Back”, when Luke learns that Darth Vader is his father, he …

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In “Empire Strikes Back”, when Luke learns that Darth Vader is his father, he falls/jumps off a platform in Cloud City without his hand. Given the fact that objects reach terminal velocity, which would have a faster terminal velocity and which would hit the ground first if in the movie that fell from a height of 1000 meters?

Luke would probably reach the ground before his hand. An object reaches a terminal velocity as it fall because the upward force of air resistance becomes stronger as the object’s downward speed increases and this upward force eventually stops the object from accelerating downward. The object’s downward speed at the point when it stops accelerating is its terminal velocity. Since air resistance is what sets this terminal velocity, an object that experiences a great deal of air resistance relative to its weight will have a smaller terminal velocity than an object that experiences relatively little air resistance relative to its weight. Because Luke is much larger than his hand, he has lots of weight relative to his surface area. Since surface area largely determines air resistance, he experiences relatively little air resistance relative to his weight. His hand has less weight relative to its surface area and it experiences a lot of air resistance relative to its weight. So Luke’s terminal velocity is larger than that of his hand. He reaches the ground first. This tendency for large objects to descend faster than small objects explains why small animals, such as insects, can fall from incredible heights without injury. They reach their terminal velocities quickly and descend rather slowly to the ground.

You said that if I push on a friend they will push back (even if they are asleep…

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You said that if I push on a friend they will push back (even if they are asleep). But if I push hard enough, they will fall to the ground, whereas I will not. Therefore, I don’t see how the reaction is equal. Can you please explain this? – JK

Newton’s third law only observes that the forces two objects exert on one another are equal in amount but opposite in direction. The law doesn’t make any statement about the consequences of those forces on the objects involved. Moreover, it doesn’t say that those forces are the only forces on the objects. When you push on an awake friend, your friend will obtain additional forces from the ground or a nearby wall, and will manage to avoid falling over. Even though you push your friend away from you, your friend will see to it that the ground pushes them toward you. As a result, they will probably stay in one place. But when your friend is asleep, they won’t be able obtain the additional forces necessary to compensate for the force you exert on them and they may accelerate away from you or fall over.

Would a small-mass hammer that accelerated rapidly exert more horizontal force o…

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Would a small-mass hammer that accelerated rapidly exert more horizontal force on a nail than a large-mass hammer that didn’t accelerate very much?

Yes. Since the only horizontal force acting on the hammer is that exerted on it by the nail, the hammer’s acceleration is entirely determined by that force. The force on the hammer is equal to the hammer’s mass times the hammer’s acceleration (Newton’s second law). If both hammers experienced the same acceleration, then the large-mass hammer would have to be experiencing the larger force from the nail and would therefore be exerting the larger force on the nail. But because the small-mass hammer is experience a larger acceleration, the force that the nail is exerting on it may be quite large. If the small-mass hammer’s acceleration is large enough, the force on it may exceed the force on the large-mass hammer.