Month: December 1969

How do photocells work?

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How do photocells work?

A photocell is just a diode that is specialized to turn light into separated electrical charge. When light hits the “n-type” side of this diode, it adds energy to the valence level electrons there and moves them to the empty conduction levels. These electrons may even have enough energy to leap across the p-n junction into the “p-type” material. Once they get there, they cannot return because of the depletion region and the one-way effect of the diode. Instead, they are collected by wires attached to the “p-type” material, flow out through some electrical circuit, and return to the “n-type” material through another set of wires.

How do diodes work?

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How do diodes work?

Diodes are made of semiconductors, which are essentially the same as photoconductors. These materials normally have electrons filling all of the valence levels and empty conduction levels. The empty conduction levels are at energies well above those of the valence levels so that electrons cannot easily shift from a valence level to a conduction level, a shift that is necessary for the material to conduct electricity. Thus semiconductors are normally insulating. But when the semiconductor is mixed or “doped” with other atoms, it can become conducting. A doping that removes electrons from the valence levels and leaves some of those levels empty produces “p-type” semiconductor. A doping that adds electrons to the conduction levels produces “n-type” semiconductor. Both “n-type” and “p-type” semiconductors can conduct electricity. But when the two materials touch, the form a non-conducting “depletion” region, where all of the conduction electrons in the “n-type” material near the junction have wandered into the “p-type” material to fill the empty valence levels there. This p-n junction or diode can only carry current in one direction. If you add electrons to the “n-type” side of the junction, they will push into the depletion region and can cross over into the “p-type” side. Thus electrons can flow from the “n-type” side to the “p-type” side; current can flow from the “p-type” side to the “n-type” side. But if you add electrons to the “p-type” side, they fill in empty valence levels in that “p-type” material and make the depletion region even larger. The diode cannot conduct current from the “n-type” side to the “p-type” side. Thus the diode is a one-way device for current.

How can current alternate

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How can current alternate — why doesn’t it cancel itself out.

Actually, it does cancel out on the average. When you plug a toaster into the AC power line and turn it on, current begins to flow back and forth through that toaster. At first it flows out one wire of the outlet, through the toaster, and returns into the other wire of the outlet. About 1/120th of a second later, the current has reversed direction and is now flowing out of the second wire of the outlet, through the toaster, and into the first wire. It continues flowing back and forth so that, on the average, it heads nowhere. But the toaster receives energy with every cycle of the current so that there is a net flow of power to the toaster even if there is no net flow of current through it.

Are there any objects that use compressed air to create electricity?

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Are there any objects that use compressed air to create electricity?

Moving air is used to create electricity: wind-powered generators. Compressed air is usually created with electrical power, so using it to generate electricity would be inefficient. But wind-powered generators are a common sight in some parts of the country. The wind blows on the turbine blades, doing work on them and providing the mechanical power needed to turn a generator. The generator converts this mechanical work into electrical energy.

Why is direct current so much better than alternating current?

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Why is direct current so much better than alternating current?

It depends on the situation. You cannot use a transformer with direct current, so in that sense, alternating current is better. But many electronic devices need direct current because they require a steady flow of charges that always head in the same direction. So there are times when you need DC and times when you need AC.

Why does less current flow through a longer wire?

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Why does less current flow through a longer wire?

Wires obey Ohm’s law: the current flowing through them is proportional to the voltage drop across them. But the precise relationship depends on the wire’s length. A short wire will carry a large current even when the voltage drop across it is small because that wire has a small electrical resistance; it does not impede the flow of electricity very much. But a long wire has a large electrical resistance and will only carry a large current if the voltage drop across it is large. If you do not change the source of electrical power (e.g. a battery) and replace short wires with long wires, those wires will not be able to carry as much current.

Why does a high voltage transformer make ozone?

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Why does a high voltage transformer make ozone?

High voltages involve large accumulations of like electric charges. These charges repel one another ferociously and can leap off into the air near sharp points and edges. They produce sparks and corona discharges. While these discharges are useful in some devices (e.g. copiers and air cleaners), they tend to transfer energy to air molecules and can break up those air molecules. When normal oxygen molecules (which each contain 2 oxygen atoms) break up, the resulting oxygen atoms can stick to other oxygen molecules to form ozone molecules (which each contain 3 oxygen atoms). That is why you can often smell ozone near electrical discharges, high voltage power lines, and after thunderstorms.

Why do north and south poles on magnets change back and forth?

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Why do north and south poles on magnets change back and forth?

Only electromagnets can change back and forth and then only when they are connected to a supply of alternating current. A permanent magnet, such as that used to hold notes to a refrigerator, has permanent poles that do not change. But an AC powered electromagnet, such as that found in a transformer, does have poles that change back and forth.

Why are there danger signs around high voltage equipment?

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Why are there danger signs around high voltage equipment?

Your body is a relatively good conductor of electricity and it is easily damaged by currents flowing through it. Your body uses electricity to control its functions and an unexpected current of as little as a few hundredths of an ampere can interrupt those functions. In particular, your heart can stop beating properly. Fortunately, your skin is a pretty good insulator so it is hard to get any current to flow through you. But high voltages can push current so hard that it punctures your skin and begins to flow through you. While the current is actually what injures you, the high voltage is what breaks down your protective skin and allows that current to flow through you.

Where does the exact reversal occur in an alternating current circuit (where doe…

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Where does the exact reversal occur in an alternating current circuit (where does the energy diminish completely and then turn the opposite way)?

The reversal of the current in an alternating current (AC) circuit occurs everywhere in the circuit at once. The whole current gradually slows to a stop and then heads backward. At the moment it comes to a complete stop, the electric power company isn’t supplying any power at all and the circuit isn’t consuming any. Because the power delivery pulses on and off in this manner, devices that operate on AC power are designed to store energy between reversals. Motors store their energy as rotational motion. Stereos store energy as separated electric charge in devices called capacitors, or as magnetic fields in devices called inductors.