Category: electric power generation

How can you make electricity with magnets? – AL

You can make electricity by moving a magnet past a wire. The magnet has a magnetic field around it—something that exerts forces on magnetic poles. If you move the magnet and its magnetic field, you create an electric field—something that exerts forces on electric charges. That’s because whenever a magnetic field changes with time, it creates an electric field. This electric field will push on the mobile electrons in a wire. So when you move a magnet past a wire, you are producing a changing magnetic field in the wire. This changing magnetic field produces an electric field and the electric field makes the electrons in the wire accelerate. The moving electrons are electricity. Generators move magnets past wires (or wires past magnets) to produce electricity.

How does an internal voltage regulator type auto alternator work and are they any better than an external regulator type? – H

An alternator is a device that uses rotary motion to generate electricity. As the car engine turns, it spins a magnet (the rotor) in the alternator and this spinning magnet induces electric currents in a set of stationary wire coils (the stator). The alternator’s ability to generate electric currents by spinning a magnet past stationary wires is an example of electromagnetic induction. Induction is a general phenomenon in which a moving or changing magnetic field creates an electric field, which in turn pushes electric charges through a conducting material. Overall, some of the engine’s mechanical energy is converted into electric energy.

The amount of energy given to each electric charge that flows through the wires in the stator depends on the speed with which the magnet turns and the strength of that magnet. Whether it’s internal or external, the voltage regulator monitors this energy per charge—also known as the voltage—to make sure that it’s correct. If not, it adjusts the strength of the alternator’s magnet. It can do this because the alternator’s magnet is actually an electromagnet and its strength depends on how much current is flowing through its wire coils. The voltage regulator carefully adjusts the current flowing through the electromagnet in order to obtain the proper output voltage from the alternator. Actually, the alternator itself produces alternating current, so a set of solid-state diodes converts this alternating current into direct current. A car’s electric system, particularly its battery, operates on direct current. Since the alternator’s operation is the same whether the voltage regulator is inside it or external to it, neither version should be better than the other.

How does waterpower work? – MA

By “waterpower” I assume that you mean hydroelectric power. In that case, water from an elevated source enters a pipe and travels downhill to a generating plant. As the water descends, its gravitational potential energy (the stored energy associated with height and the earth’s gravity) becomes pressure potential energy (the stored energy associated with pressure) and kinetic energy (the energy of motion). By the time the water reaches the generating plant, it has enormous pressure and a modest speed.

This moving, high-pressure water is then sent through a fan-like turbine. As the water moves toward the low pressure beyond the turbine, it does work on the turbine’s rotating blades and its energy is transferred to those blades. The water gives up its energy and the turbine takes away this energy in its rotary motion. The turbine is attached to an electric generator, which uses moving magnets and wire coils to turn the turbine’s rotary energy into electric energy. The electric energy is carried away on wire to be used elsewhere. Overall, the water’s gravitational potential energy has become electric energy.

Can you get electricity or some sort of energy or power from fruit? — J, Embrun, Ontario

The answer is yes, but the method may not be what you had in mind. While it’s possible to make a battery by inserting two dissimilar metal strips into the fruit, the battery that results is really powered by the metals themselves. The fruit juice just acts as an “electrolyte”—an electrically conductive liquid that facilitates the movement of electric charges. Claiming that the fruit is responsible for the energy is like claiming that the stone in “stone soup” (an old tale about a beggar who tricks the villagers in a community into contributing vegetables to spice up the soup that he’s making with his magic stone) is really the basis for the soup.

The best way to obtain energy from the fruit is to eat it! The sugars and starches in the fruit have plenty of chemical potential energy that’s released when those chemicals are oxidized in your body. This released energy is what allows you to live, work, and play.

How efficient are solar energy cells and windmills in producing energy for everyday use? — JJ, San Antonio, TX

There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you’re probably most interested in their cost efficiency—in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants. As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.

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 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.

I have an old car that has a generator instead of an alternator, so I assume it runs DC. What about newer cars? They still use a DC battery right? So what about the alternator? Doesn’t that produce AC current? How does that work in a DC circuit?

Generators can produce either DC or AC power, depending on how they’re arranged. A car generator was one that produced DC power. An alternator produces AC power. Since all cars operate on DC power (they use a battery, after all), the AC power is always converted to DC power. In modern cars, this is done with electronic devices, similar to those used in electronic equipment such as stereos and televisions. Converting DC to AC or vice versa is no big deal anymore. In the old days, it was harder and they used DC generators.

If you connect two direct current motors so that the current flowing through one also flows through the other, then turning one motor will cause the other motor to turn as well. If you reverse the direction of rotation, the other motor will also reverse its direction of rotation. Why does this happen?

DC motors turn in a direction that depends on the direction of that current. If you reverse the direction of current flowing through the motor, its direction reverses, too. When you use one DC motors as a generator, it produces DC current! The direction of that current depends on which way you turn the motor. Thus as you turn the first motor clockwise, it generates current in a particular direction through the circuit connecting the two motors and the second motor also turns clockwise. If you then reverse the first motor, the current in the circuit reverses and so does the second motor.

What happens to the current when it “stops”?

Current refers to moving charged particles. In most solids, the particles that do the moving are negatively charged electrons that move in the opposite direction from the way we say that current is flowing. These charged particles are the components of atoms and molecules, so they are always there inside a wire or the filament of a light bulb, even if they are not moving. Thus when the current “stops”, these electrically charged particles simply stop moving. You can imagine a pipe full of water. The water can be flowing to the right or left (a current) or it can be standing still (no current). The water itself, like the charged particles, doesn’t disappear when the flow stops.