Category: electric power generation

How does a photocell absorb light and turn it into power? — MR

A photocell is actually a large diode—a one-way device for electric current. Like most diodes, the photocell consists of two different layers of chemically altered or “doped” semiconductors, the anode layer and the cathode layer, and the junction between these two layers has the peculiar property that it normally allows electrons to cross it in only one direction. There is what’s called a “depletion region” at the junction, a very thin insulating layer with two electrically charged surfaces—the surface on the cathode side is positively charged and the surface on the anode side is negatively charged.

When an electron, which is negatively charged, approaches the depletion region from the anode side, it first encounters the depletion region’s negatively charged surface and is repelled. But when the electron approaches from the cathode side, it first encounters the depletion region’s positively charged surface and is attracted. If it has enough energy when it approaches the depletion region from the cathode side, the electron can cross the depletion region to reach the anode layer. Thus electrons can move relatively easily from the photocell’s cathode layer to its anode layer but they can’t go back.

When a photocell is exposed to light, some of the light particles (photons) are absorbed in the diode’s cathode layer. When such an absorption occurs, the photon’s energy may be transferred to an electron in the cathode, giving that electron the energy it needs to cross the depletion region and reach the anode. But once the electron has arrived at the anode it can’t return to the cathode directly across the depletion region. Instead, it must flow through an external circuit in order to return to the cathode. As that electron flows through the external circuit, it can give up some of its energy, obtained from the light photon, to devices in that circuit. In that manner, light energy has provided energy to an electrically powered device.

If I measure current from a photocell, am I indirectly measuring power as well? — MR

As long as current is free to flow from one end of the photocell to the other, the amount of current flowing through that circuit is almost exactly proportional to the number of light particles (photons) striking the photocell each second. Since the rate at which photons strike a photocell is generally proportional to the light power striking that photocell, you can use a measurement of current to make a measurement of light power. While there are a few subtle details that you must be careful about, particularly changes in the light spectrum and unanticipated impediments to the free flow of current through the circuit, this relationship between the current and the light power is very useful. For example, most camera light meters use photocells to determine exposures.

How do long range metal detectors work? — AS

In general, metal detectors find metal objects by looking for their electromagnetic responses. For example, you can tell when an iron or steel object is nearby by waving a magnet around. If you feel something attracting the magnet, you can be pretty sure that there is a piece of iron or steel nearby. Similarly, if you wave a strong magnet rapidly across an aluminum or copper surface, you’ll feel a drag effect as the moving magnet causes electric currents to flow in the metal surface—electric currents are themselves magnetic.

Of course, a real metal detector is much more sensitive than your hands are, but it’s using similar principles to detect nearby metal. Most often, a metal detector uses a coil of wire with an alternating current in it to create a rapidly changing magnetic field around the coil. If that changing magnetic field enters a piece of nearby metal, the metal responds. If the metal is ferromagnetic—meaning that it has intrinsic magnetic order like iron or steel—it will respond strongly with its own magnetic field. If the metal is non-ferromagnetic—meaning that it doesn’t have the appropriate intrinsic magnetic order—it will respond more weakly with magnetic fields that are caused by electric currents that begin to flow through it.

In a short range metal detector, the detector looks for the direct interaction of its magnetic field and a nearby piece of metal. That nearby metal changes the characteristics of the detector’s wire coil in a way that’s relatively easy to detect. But in a longer-range metal detector, the electromagnetic coil must actually radiate an electromagnetic wave and then look for the reflection of this electromagnetic wave from a more distant piece of metal. That’s because the magnetic field of the coil doesn’t extend outward forever—it dies away a few diameters of the coil away from the coil itself. For the metal detector to look for metal farther away, it needs help carrying the magnetic field through space. By combining an electric field with the magnetic field, the long-range metal detector creates an electromagnetic wave—a radio wave—that travels independently through space. Electromagnetic waves reflect from many things, particularly objects that conduct electricity. So the long-range metal detector launches an electromagnetic wave and then looks for the reflection of that wave. This wave reflection technique is the basis for sonar (sound waves) and radar (radio waves), and it can be used to find metals deep in the ground. Unfortunately, the ground itself conducts electricity to some extent, so it becomes harder and harder to distinguish the reflections from metal from the reflections from other things in the ground.

How do windmills work to generate electricity? — KT, Aurora, Ontario

Windmills extract energy from the wind by rotating as the wind twists them. Whenever an object rotates in the same direction as the torque (the twist) being exerted on it, mechanical work is done on that object. In this case, wind exerts a torque on the windmill’s blades and they rotating in the direction of that torque, so the wind is doing work on the blades. Work is the mechanical transfer of energy, so the wind is transferring some of its energy to the blades.

The blades don’t keep this newly acquired energy. Instead, they do work on a generator. The generator, which consists of a rotating magnet that spins within stationary coils of wire, uses this energy to generate electricity. The amount of power that a windmill generates depends on the wind speed and the windmill’s size, but large windmills can generate in excess of a million watts of electric power.

How do you make solar cells? — BP

Solar cells are made in the same way that semiconductor diodes are made. Two different types of semiconductor, p-type and n-type, are joined together to form a diode—a one-way device for electric current. When light energy is absorbed in the n-type portion of the diode, it can propel an electron across the p-n junction between the materials and into the p-type material. Since the electron can’t return across the p-n junction to its original location, it must flow through an external circuit to get back. Since it obtains energy from the light that sent it across the junction, the electron can provide that energy to the circuit. The solar cell is thus a source of electric power.

How would you construct and wire a battery recharger using solar panels as a voltage source? — JW, Kingston, Ontario

First, you would need to put enough solar panels in series to develop a voltage greater than that of your battery. For example, to recharge a 1.5 volt battery, you would probably have to attach three or four simple solar cells in series because each one only provides a current passing through it with about 0.5 volts of voltage rise. Having assembled enough solar cells, you should then attach the positive output terminal of the solar cell chain to the positive terminal of your battery and attach the negative output terminal of the solar cell chain to the negative terminal of your battery. When you put the solar cells in the light, they will begin to push electric current backward through the battery and the battery will recharge. Whenever you send current backward through a battery, its electrochemical reactions can run backward and it can recharge to some extent. Unfortunately, some batteries recharge more effectively than others—the bad ones just turn the recharging energy into thermal energy. The only real subtlety in this business is in stopping the charging when the battery is fully recharged. You should check the battery voltage periodically and when it’s close to the voltage of a new battery, it probably can’t take any more charging.

In making an electric generator, how do different aspects of the wire affect the total voltage and amperage? What are the effects of wire gauge, number of turns in the coils, and whether the magnets move past the coils or the coils past the magnets? — BLM, Houston, TX

First, it doesn’t matter when the magnet moves past the coils or the coils past the magnet; a generator will work the same way in either case. The voltage produced by the generator is determined by the number of turns in its coils, the strength of its magnet, and the rate at which its magnet turns. The more turns in the coils, the more work the generator does on each charge that passes through those coils and the more voltage the charges have when they leave the generator. The current that the generator can handle is limited by the power of its engine and by the wire’s ability to handle the current without wasting too much power. In general, a generator’s wire gauge is chosen to minimize power loss while keeping the coils reasonably small and light. If you try to send too much current through the generator, its engine may stall or its wires may overheat.

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 much water power do you need to turn on a light bulb? How much wind power does it take to turn on a light bulb? Can artificial light make a solar paneled car run? If so, how bright? — BB, Stafford Springs, CT

If you are trying to light a 60 watt bulb, you must deliver 60 watts of electric power to it (unless you are willing to have it glow relatively dimly). So the answers to your questions are 60 watts of waterpower and 60 watts of windpower. But you are probably more interested in how much water or wind is needed to run those power sources. An efficient water generator that produces 60 watts of power lowers about 6 liters (or one and a half gallons) of water about 1 meter (or 3 feet) each second. An efficient wind generator that produces 60 watts of power stops about 1 cubic meter (or 32 cubic feet) of air moving at 36 km/h (or 21 mph) each second. Finally, a solar powered vehicle needs at least several hundred watts of power to operate. Since solar panels are only about 20% energy efficient and artificial light sources are also only about 10 to 50% energy efficient, it would take thousands of watts of artificial lighting to operate a solar powered car. Not very practical.

How do steam generators produce electricity? — KA, North Platte, NE

In a steam generating plant, water is boiled in a confined container (a “boiler”) to produce very high-pressure steam. This steam is allowed to flow through a turbine to the low-pressure region beyond the turbine. A turbine resembles a fan, but one that is turned by the gas that flows through it rather than by a motor. The steam flows through the blades of the turbine and exerts forces on those blades to keep the turbine rotating. The steam loses energy as it twists the turbine around in a circle and this energy is transferred to the rotating turbine. The low-pressure steam is recovered from the end of the turbine. It is then condensed back into liquid water with the help of a cooling tower and then returned to the boiler for reuse.

The rotating turbine is connected to the rotating portion of a generator. This rotating component is an electromagnet and, as it spins, its magnetic field passes across a set of stationary wire coils. Whenever the magnetic field through a coil of wire changes, any current flowing through that coil experiences forces that may add or subtract energy from it. In this case, the rotating magnet transfers energy to the current passing through the wire coils and “generates” electricity. The current in these stationary wires carries away energy from the generator and it is this energy that eventually arrives in your home through the power lines. Overall, the energy flows from the boiler, to the steam, to the turbine, to the generator, to the current, and to your home.