Author: Lou Bloomfield

How does a transistor amplify an input signal in an audio amplifier?

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How does a transistor amplify an input signal in an audio amplifier? — AR, Pierrefonds, Quebec

The answer depends a little on which type of transistor is used, so I’ll consider only an audio amplifier based on MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). One of these three-electrode devices allows a tiny electric charge on its gate electrode to control a substantial current flowing between its source and drain electrodes. In a typical amplifier, the current flowing in the input circuit is allowed to deposit or remove electric charge from the gate electrode(s) of one or more MOSFETs. This action dramatically changes how much current flows in a second circuit. This second circuit is ultimately responsible for the current that passes out of the amplifier and through the speakers that reproduce sound. As the current in the input circuit fluctuates to represent a particular musical passage, the charges on the gates of the MOSFETs also fluctuate and the MOSFETs vary the current through the output circuit and the speakers. Because MOSFETs are so sensitive to even a tiny amount of charge, it doesn’t take much current in the input circuit to cause large changes in the current of the output circuit.

What’s energy?

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What’s energy?

Formally defined as “the capacity to do work”, energy is a measure of an object’s ability to make things happen. It is interesting to physicists for one important reason: it is a conserved physical quantity. By “conserved physical quantity”, I don’t mean that it’s something that we try not to waste. I mean that the amount of energy in an isolated system can’t change—energy can’t be created or destroyed, it can only be transferred from one object to another or converted from one form to another. Because you can’t make it or consume it, energy is an important characteristic of objects and systems. You can often watch it move from object to object and observe the consequences of this movement. For example, the energy that I’m using now to type at my keyboard arrived at the earth’s surface as sunlight, was used by plants to build new molecules that eventually become part of my breakfast this morning and are now being combined with oxygen in my body to allow me to move my fingers. Nowhere along this chain was energy created or destroyed—it simply moved about and changed forms. It will still be here tomorrow, and then next day, and even the day after that.

What is the approximate terminal velocity for a spent falling bullet that was fi…

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What is the approximate terminal velocity for a spent falling bullet that was fired into the air? Is this velocity sufficient to kill someone? – M

A bullet’s terminal velocity is the downward speed at which the upward force of air resistance acting on it balances its downward weight. Once the falling bullet reaches this speed, it coasts downward at a steady rate. Because air resistance depends largely on surface area while weight depends on volume, larger bullets will drop faster than smaller bullets (just as a piece of chalk drops faster than chalk dust). While I am not sure of the exact speed of a dropping bullet, I expect it to be several hundred miles per hour. As to whether or not it can kill someone, the answer is most definitely yes. In fact, a distant cousin of mine was killed several years ago during Mardi Gras when a falling spent bullet pierced her brain. Firing bullets into the air is an extraordinarily foolish and inconsiderate action. In cultures where it’s common to fire guns during celebrations, innocent people are frequently killed by these descending “party favors.” If you ever see people shooting guns into the air, you should immediately seek cover in a basement. Their bullets will return to earth in less than thirty seconds and will be just as deadly when they arrive as if they had been shot right at you.

If increasing the power demand on a generator that is turning at a steady rate s…

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If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep that generator turning, why do brownouts occur?

As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its alternating current won’t change. When the homes powered by the generator draw more current, then the generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it. But suppose that the turbine can’t exert any more torque on the generator. In that case, the power company can either shut down the generator or it can reduce the strength of the generator’s rotating magnet. This rotating magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a period of excessive current demand, the power company may choose to weaken the rotating electromagnet to prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow relatively dimly—a brown-out.

Why does an artificial sponge absorb more water than a natural sponge?

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Why does an artificial sponge absorb more water than a natural sponge? — JH, Angleton, TX

Water is drawn into a sponge in part because of an attraction between the water molecules and the sponge’s surface and in part because of water’s tendency to minimize its own surface area. When you put a drop of water on a waxy surface, the water beads up. That’s because water and wax don’t bind well to one another and the water molecules pull toward one another instead. The water droplet tries as best it can for form a sphere, since a sphere has the smallest surface area that a given volume of water can occupy. These forces that pull water’s surface inward are called surface tension.

But when you put a drop of water on real cellophane (a smooth form of cellulose), the water spreads out. That’s because water and cellulose bind strongly to one another and the water will permit its surface area to increase somewhat if that increase allows it to attach to more cellulose. Similarly, water binds well with other forms of cellulose, including paper, cotton, and Rayon. I think that most artificial sponges are either cellulose or a close chemical relative of cellulose.

A sponge absorbs water by allowing that water to cling to an extensive surface that binds well with water. The water spreads out along that surface while trying to minimize the surface area of any water that isn’t touching the sponge. The surface of a natural sponge interacts well with water (the sponge lives in water after all), but a natural sponge can’t compete with modern technology. A company that makes artificial sponges can adjust the chemical structure of the sponge’s plastic so that it binds nicely to water molecules; it can adjust the sizes of the holes in the sponge to attract the water as efficiently as possible with a given mass of plastic; and it can tailor wall thickness to give the sponge the right elasticity. Furthermore, some of the water is brought right into the plastic and that water softens or “plasticizes” the plastic. That’s why a sponge is hard when dry and soft when wet—the water molecules are effectively lubricating the plastic molecules so that they can slide past one another.

You have mentioned the relationships between electric fields, magnetic fields, a…

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You have mentioned the relationships between electric fields, magnetic fields, and current. Which causes which? Does current cause a magnetic field, in turn, causing flow in the next circuit and so forth? What is this order of occurrence? — BJ

Those three items, electric fields, magnetic fields, and currents, are strongly interrelated. Here are some of those relationships: (1) currents cause magnetic fields, (2) currents that change with time cause magnetic fields that change with time, (3) magnetic fields that change with time cause electric fields, (4) electric fields cause currents to flow in electric conductors. From these relationships, you can see that any time you have a changing current through one circuit, you can end up with a current flowing through another nearby circuit. Power moves from the first circuit to the second circuit with the help of a magnetic field and an electric field. A moving magnet also produces a magnetic field that changes with time and it can send a current through a nearby circuit, too.

How does luminol work?

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How does luminol work? — CW, San Antonio, TX

Luminol produces light during a chemical reaction with either molecular oxygen or a mixture of potassium ferricyanide and hydrogen peroxide and is probably the basis for most light sticks. In an alkaline (basic) solution, the luminol molecule becomes a dianion, a molecule with two negative charges on it. In this dianion form, the molecule has two nitrogen atoms exposed to the solution and these nitrogen atoms are easily replaced by two oxygen atoms. When that exchange takes place, a molecule of nitrogen gas is released and the final oxidized luminol is left in an electronically excited state. This molecule quickly gets rid of its excess energy by emitting light.

Why is alternating current better than direct current?

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Why is alternating current better than direct current? — MK, California

The genius of George Westinghouse and Nikola Tesla in the late 1800’s was to realize that producing alternating current made it possible to transfer power easily from one electric circuit to another with the help of an electromagnetic device called a transformer. When an alternating electric current passes through the primary wire coil of a transformer, the changing magnetic and electric fields that this current produces transfer power from that primary current to the current passing through another coil of wire—the secondary coil of the transformer. While no electric charges move between these two wires, electric power does. With the help of a transformer, it’s possible for a generating plant to move power from a large current of relatively low energy electric charges—low voltage charges—to a small current of relatively high-energy electric charges—high voltage charges. This small current of high voltage electric charges can move with relatively little power loss through miles and miles of high voltage transmission lines and can go from the generating plant to a distant city without wasting much power. Upon arrival at the city, this current can pass through the primary coil of another transformer and its power can be transferred to a large current of relatively low voltage charges flowing through the secondary coil of that transformer. The latter current can then deliver this electric power to your neighborhood. A transformer can’t transfer power between two circuits if those circuits operate with direct current. Edison tried to use direct current in his power delivery systems and fought Westinghouse and Tesla tooth and nail for years. Edison even invented the electric chair to “prove” that alternating current was much more dangerous than direct current. Still, Westinghouse and Tesla won out in the end because they had the better idea.

I would like to know a little more about the ac slip ring motor and its uses, pa…

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I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. – M

A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate rapidly around the motor’s rotating central component—its “rotor.” The rotor consists of a cylindrical aluminum metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications, but you wouldn’t want it in an elevator—the sudden starting of the elevator car that would accompany the sudden starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced by a group of wires that are connected by way of metal ring (the “slip rings”) and some stationary conductive brushes to some components outside the rotor. During the starting process, the currents that are induced in the rotor’s wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully. When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted together so that the rotor behaves like the aluminum cage of a normal induction motor.

In the simplest terms, how does a basic electrical circuit work?

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In the simplest terms, how does a basic electrical circuit work? — CC, Port St. Joe, FL

An electric circuit is racetrack for electric charges. It must be a complete loop—a “circuit”—so that the charges don’t pile up somewhere along the track. The simplest circuit has a source of energy for the electric charges (e.g., a battery) and a device that takes energy away from the electric charges (e.g., a light bulb). When the charges are in motion through the circuit, they are an electric current. By convention, current points in the direction of positive charge flow, so you can imagine a stream of positive charges circling this circuit over and over again, with current pointing always in the direction that those positive charges are moving. As the current passes through the battery, entering it at the battery’s negative terminal and leaving it at its positive terminal, the charges pick up energy. The battery is converting some of its stored chemical potential energy into electric energy and giving that energy steadily to the current flowing through it. The battery is “pumping” the charges from its negative terminal to its positive terminal. The current continues around the circuit and then passes through the light bulb. In the light bulb, the charges give up most of their energies to the filament and the filament becomes white hot. The current continues out of the bulb and returns to the negative terminal of the battery to pick up more energy. This simple circuit is present in a flashlight. The same charges complete this circuit millions of times each second, shuttling energy from the battery to the bulb.