Author: Lou Bloomfield

How does an electromagnetic doorbell work?

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How does an electromagnetic doorbell work? — SH, Sault Ste. Marie, Ontario

When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete, current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the coil of wire is an iron rod. When this the coil of wire—also called a solenoid—becomes magnetic, so does the iron rod. The iron rod becomes magnetic in such a way that it’s attracted toward and into the solenoid, and it accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid, but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of the solenoid that it strikes a bell on the far side—the doorbell! The rod rebounds from the bell and reverses is motion. It has traveled so far out the other side of the solenoid that it’s attracted back in the opposite direction. The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the solenoid and doesn’t move again until you stop pushing the button. Once you release the button, the current in the circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its original position at one end of the solenoid.

How does a siren work?

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How does a siren work? — MM, Waterloo, Iowa

A siren uses a perforated disk or drum to alternately block and unblock a stream of air. The classic siren has a spinning disk with a pattern of holes around its periphery. This disk is spun in front of a jet of air, producing pressure pulses that we hear as sound. A more modern siren has a spinning centrifugal fan that propels air radially outward through a pattern of holes in a drum around the fan. This centrifugal siren is much louder than the disc siren because the centrifugal system pushes large pulses of air through many openings at once, whereas the disc siren only has one pulsed source of air.

How are magnets made and what are they made of?

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How are magnets made and what are they made of? — S, San Francisco, CA

The strongest modern magnets are made by assembling lots of tiny magnetic particles into a solid object. These magnetic particles are “intrinsically” magnetic, meaning that the atoms from which the particles are formed retain their magnetism in coming together as a solid. Electrons are naturally magnetic and most atoms exhibit the magnetism of their electrons. But as these atoms come together to form a solid, most of them lose their magnetism. For example, copper, aluminum, gold, and silver are all nonmagnetic solids built from magnetic atoms. There are only a few materials that don’t lose their atomic magnetism and might be suitable for making permanent magnets. However, most of these magnetic materials only exhibit their magnetism when exposed to other magnets—when they’re alone, their magnetism is mostly hidden. For example, iron and steel are magnetic materials but they only appear strongly magnetic when you bring a permanent magnet near them.

To make a strong permanent magnet, you must find a material that is both intrinsically magnetic and that is able to stay magnetic when it’s by itself. Materials that hide their magnetism when alone do this by allowing their magnetic structure to break up into tiny pieces that all point in different directions. Each of these tiny magnetic pieces is called a magnetic domain, and iron and steel are normally composed of many magnetic domains. A good permanent magnet material is one that is intrinsically magnetic and that resists the formation of randomly oriented magnetic domains. A very effective way to make such permanent magnet materials is to assemble lots of tiny magnetic particles. Each of these particles is shaped in a way that makes one of its ends a north pole and its other end a south pole, and that makes it extremely hard for these two poles to exchange places. The particles are then aligned with one another and bonded together to form a permanent magnet. To make sure that the particles all have their north poles at one end and their south poles at the other end, the finished magnet is exposed to an extremely strong magnetic field—one so strong that it flips any misaligned magnetic particles into alignment with the others. After being magnetized in this manner, the permanent magnet is very hard to demagnetize, which is just what you want in a permanent magnet.

The most common magnet materials are Ferrite and Alnico. Ferrite magnets are made from a mixture of iron oxide and barium, strontium, or lead oxide. Alnico magnets are made from aluminum, nickel, iron, and cobalt, and consist of tiny particles of an iron-nickel-aluminum alloy inside an iron-cobalt alloy. But the strongest modern magnets are made from an iron-neodymium-boron alloy. The latter magnets are very resistant to demagnetization and the forces they exert on one another are amazingly strong.

How does a rail gun work?

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How does a rail gun work?

A rail gun is a device that uses an electromagnetic force to accelerate a projectile to very high speeds. This acceleration technique is based on the fact that whenever an electrically charged particle moves in the presence of a magnetic field, it experiences a force that pushes it perpendicular to both its direction of travel and the magnetic field. In a rail gun, this perpendicular magnetic force—known as the Lorentz force—pushes the projectile along two metal rails and can accelerate it to almost limitless speeds.

The rail gun’s projectile must conduct electricity and it completes the electric circuit formed by two parallel metal rails and a high current power source. During the rail gun’s operation, current flows out of the power source through one rail, passes through the projectile, and returns to the power source through the other rail. As it passes through the two rails, the electric current produces an intense magnetic field between the rails. The projectile is exposed to this magnetic field and as charged particles pass through the projectile, they experience a Lorentz force that pushes them and the projectile in one direction along the rails. The projectile picks up speed as it travels along the rails and doesn’t stop accelerating until the current ceases or it leaves the rails. In practice, the power sources used in most rail guns is a large bank of capacitors. These devices store separated electric charge and supply enormous currents to the rails for a brief period of time.

How does a video recorder work?

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How does a video recorder work? — SH, Sault Ste. Marie, Ontario

A video recorder is much like a normal tape recorder, except that it records far more information each second. When you play an audiotape in a normal tape recorder, small magnetized regions of tape move past a playback head. This playback head consists of an iron ring with a narrow gap in it and there is a coil of wire wrapped around the ring. As the magnetized regions of the tape pass near the ring’s gap, they magnetize the ring. The ring’s magnetization changes as the tape moves and these changing magnetizations cause currents to flow in the coil of wire. These currents are amplified and used to reproduce sound. When you record the tape, the recorder sends currents through the wire coil, magnetizing the iron ring and causing it to magnetize the region of tape that’s near the gap in the ring.

In a video recorder, the tape moves too slowly to produce the millions of the magnetization changes needed each second to represent a video signal. So instead of moving the tape past the playback head, the video recorder moves the playback head past the tape. As the tape travels slowly through the recorder, the playback head spins past it on a smooth cylindrical support. The tape is wrapped part way around this support and two or more playback heads take turns detecting the patches of magnetization on the tape’s surface. The tape is tilted slightly with respect to the spinning heads so that the heads sweep both along the tape and across its width. That way, the entire surface of the tape is used to record the immense amount of information needed to reproduce images on a television screen. During recording, currents are sent through the heads so that they magnetize the tape rather than reading its magnetization.

How does an operational amplifier work?

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How does an operational amplifier work? — BR

An operational amplifier is an extremely high gain differential voltage amplifier—a device that compares the voltages of two inputs and produces an output voltage that’s many times the difference between their voltages. How the operational amplifier performs this subtraction and multiplication process depends on the type of operational amplifier, but in most cases two input voltages control how current is shared between two paths of a parallel circuit. Even a tiny difference between the input voltages produces a large current difference in the two paths—the path that’s controlled by the higher voltage input carries a much larger current than the other path. The imbalance in currents between the two paths produces significant voltage differences in their components and these voltage differences are again compared in a second stage of differential voltage amplification. Eventually the differences in currents and voltage become quite large and a final amplifier stage is used to produce either a large positive output voltage or a large negative output voltage, depending on which input has the higher voltage. In a typical application, feedback is used to keep the two input voltages very close to one another, so that the output voltage actually falls in between its two extremes. At that operating point, the operational amplifier is exquisitely sensitive to even the tiniest changes in its input voltages and makes a wonderful amplifier for small electric signals.

How do the display lasers used in sporting events work? I think it has something…

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How do the display lasers used in sporting events work? I think it has something to do with mirrors.

They do use mirrors. When you bounce a laser beam from a mirror, any small change in the mirror’s orientation can cause a large change in the beam’s final destination. Simple laser light shows bounce lasers from low-mass mirrors that are mounted on elastic membranes. As those membranes are driven into motion by sound waves, the mirrors tip and turn and the laser beams move around in beautiful patterns on a distant screen or wall. In laser light shows that produce specific shapes and images, the mirrors that steer the laser beams are driven by high-speed electromagnetic mechanisms that can change a mirror’s angle dramatically in thousandths of a second. With several of this electromagnetically controlled mirrors working together and guided by a computer, the beam can be steered to draw complicated shapes on a screen or other surface.

What makes an airplane fly?

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What makes an airplane fly? — BO, Pemberton, MN

As an airplane’s wing moves through the air, the airstream approaching the wing separates into a flow over the top of the wing and a flow under the bottom of the wing. The wing is shaped and tilted so that the flow over the wing follows a longer path to arrive at the sharp trailing edge of the wing than the flow under the wing must follow. Because it has a shorter distance to travel, the flow under the wing initially arrives at the trailing edge of the wing first and flows up and around that trailing edge to meet the flow over the wing. This type of flow has a kink in it at the wing’s trail edge and is unstable. A few moments after the wing begins moving through the air, the kink at the trailing edge blows away from the wing altogether. This kink leaves as a vortex—a whirling cyclone of air—and as it does, it causes the flow over the wing to speed up so that the two airflows join together cleanly at the wing’s trailing edge. To increase its speed, the flow over the wing converts some of its pressure energy into kinetic energy. Because the flow over the wing has used up some of its pressure energy, and thus experienced a drop in pressure, there is an unbalanced pressure across the wing: the pressure beneath the wing is greater than the pressure above the wing. This imbalance in pressure leads to an overall upward force on the wing and this upward force is what supports the plane’s weight so that it remains suspended in the air. Overall, the airstream is deflected downward as the result of this complicated flow pattern around the wing and the air pushes the wing upward in response. A nice image of the airstream leaving a plane’s wings can be seen at the Canon website, http://www.usa.canon.com/explorers/flight.html.

A company claims that if you place their sealed liquid-filled plastic ball into …

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A company claims that if you place their sealed liquid-filled plastic ball into your washing machine, you can eliminate the need for caustic detergents, improving the ecology and saving the planet. The claim is that this ball changes the ionic charge of the water and “magically releases” the dirt from your clothing. Is it possible to use ions to clean as well or better than detergent? — RO, Garden City, MI

I’m afraid that this claim is nonsense and, like the stone in “stone soup,” the ball does nothing at all. The old-time medicine show didn’t really disappear, it just evolved into a more modern form. Since the ball doesn’t add or remove chemicals from the water, it can’t alter the numbers of neutral and ionic particles in the water. But ions have very little to do with how water cleans clothes anyway. Water is already a wonderful solvent for salts and sugars, so you can clean many soils from your clothes with just water alone. But water is a poor solvent for oils and fats because oil and fat molecules don’t bind well to water molecules. That’s where detergents come into play—they form shells called micelles around the oil and fat molecules and render those molecules soluble in water. Without detergents, you’ll have trouble cleaning oils and fats from your clothes. Since oils and fats aren’t affected one way or the other by ions, even the ball’s claimed activity won’t help them to dissolve in the water.

How do neon lights work?

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How do neon lights work? — MT, Cement City, MI

A neon light uses a high voltage transformer to place electric charges on the wires at each end of a neon-filled glass tube. One end of the tube receives positive charges and the other end receives negative charges. Since like charges repel one another, the vast numbers of like charges at each end push apart strongly and some of them leave the wire and enter the neon gas. Once they’re in the gas, these charges are draw quickly toward the opposite charge at the far end of the tube. As they travel through the tube, these moving charges pick up speed and kinetic energy but they occasionally collide with neon atoms as they travel and can transfer some of their kinetic energies to the neon atoms. The neon atoms retain this extra energy only briefly before getting rid of it in the form of visible light—the familiar red glow of a neon lamp. Overall, electric charges stream from one end of the tube to the other, frequently colliding with the neon atoms and causing those atoms to emit red light. If you look closely at a neon lamp, you’ll see that it is the gas itself that’s emitting the red light.