Category: electric power distribution

How does an electric welder work? — JE

An electric welder sends an electric current through an ionized gas, forming a pattern of current flow through the gas that is known as an arc. The ionized gases in this arc consist of electrons that are negatively charged and atoms or molecules that have lost electrons to become positively charged. The electrons flow toward the positively charged metal at one end of the arc while the positively charged ion flow toward the negatively charged metal at the other end of the arc. As these charged particles move, they collide frequently with one another and with gas atoms or molecules along their paths, and they convert some of their electric energies into thermal energy. These collisions also produce additional ions. The enormous amounts of thermal energy produced by collisions as the charged particles flow through the arc melts the metals at the ends of the arc so that these metals can be fused together.

What is the formula for finding the power in an AC circuit?

If an appliance receiving power from an AC power source behaves as an electric resistor—meaning that the current passing through it is proportional to the voltage drop across it—then it’s easy to calculate the power being consumed by this appliance. You simply multiply the voltage drop across the appliance (measured in volts) by the current passing through the appliance (measured in amperes) to obtain the power (measured in watts). The voltage drop across the appliance indicates how much energy the appliance extracts from each unit of charge pass through it and the current passing through the appliance is the measure of how many units of charge are passing through the appliance each second. Thus the product of voltage drop times current gives the energy that the appliance extracts from the current each second, which is the power extracted by the appliance. On the other hand, if the appliance behaves like an inductor or capacitor—meaning that the current passing through it isn’t proportional to the voltage drop across it—it’s much harder to calculate the power that the appliance is consuming.

How do I make a battery that will charge using wind power? — K

Any rechargeable battery will do for this job, although I’d recommend using a lead-acid battery. To charge it, you need a wind-powered DC generator. You can make such a generator by attaching a DC motor to the blades of a fan and providing some weather-vane mechanism to ensure that the fan always points into the wind. The wind will then cause the fan to spin, and with it the motor. Wind energy will become mechanical energy and that will in turn become electric energy. The DC motor will act as a generator and will produce electric power.

To make this generator recharge the battery, you first need to ensure that the motor can generate a voltage that’s at least 20% higher than the voltage of the battery while the wind is blowing at its usual rate. If it can’t, you need a higher voltage motor or a lower voltage battery. Now you should connect the negative output wire of the generator to the negative terminal of the battery and use a power rectifier (a power diode) to connect the positive output wire of the generator to the positive terminal of the battery. You need this diode to prevent the battery from sending its power into the motor and making the fan turn when the wind isn’t blowing hard. If the fan starts turning when you’ve inserted the diode, you have it installed backward. When correctly inserted, the diode will prevent the battery from operating the fan so that the fan can only charge the battery. When the wind starts blowing and the fan starts turning, it will charge the battery.

How does a transformer lessen voltage? — C

When you send an alternating current through the primary coil of wire in a transformer, that current produces a magnetic field in the transformer. Because the current in the primary coil is changing with time—it’s an alternating current—this magnetic field is changing and changing magnetic fields are accompanied by electric fields. In the transformer, this electric field pushes electric charges around the secondary coil of wire in the transformer. Since these electric charges are pushed in the direction they are traveling, work is being done on them and their energies are increasing. However, in the transformer you mention, the secondary coil of wire has fewer turns in it that the primary coil of wire. As a result, the charges don’t receive as much energy per charge (as much voltage) as the charges in the primary coil are giving up. This type of transformer, in which the secondary coil has fewer turns of wire than the primary coil, is called a step-down transformer and reduces the voltage of an alternating current.

How does electricity get to my home?

The electricity you receive comes from a distant power plant. A generator in that power plant produces a substantial electric current of medium high voltage electric charge. This current is alternating, meaning that its direction of flow reverses many times a second—120 reversals per second or 60 full cycles of reversal (over and back) in the United States. This alternating electric current flows through the primary coil of wire in a huge transformer at the power plant, where it produces an intense alternating magnetic field. When a magnetic field changes with time, it produces an electric field and, in the transformer, this electric field pushes electric charges around a second coil of wire in the transformer, the secondary coil. The effect of this transformer is to transfer power from the current in the primary coil of the transformer to the current in the secondary coil of the transformer. Thus the generator’s electric power moves along to the current passing through the secondary coil of the transformer. However, the secondary coil has far more turns of wire than the primary coil and this gives each charge passing through that coil far more energy than the charges had in the primary coil. Although the current passing through that secondary coil is relatively small, it acquires an enormous voltage by the time it leaves the secondary coil. The transformer has produced this high voltage power needed for efficient power transmission to a distant city.

This high voltage electric current passes through the countryside on high voltage transmission wires. The value of using a small current of high voltage charges is that wires waste power in proportion to the square of the electric current they are carrying. Since the current in the transmission wires is small, they waste relatively little power.

When this current reaches your town, it passes through a second transformer, which transfers its power to yet another electric current. This current is large and, because it passes through a coil that has few turns of wire, it acquires only a medium high voltage when it flows through the secondary coil of the new transformer. Electricity from this second transformer flows toward your neighborhood through medium high voltage wires. Finally, near your home there is a third and final transformer that extracts power from the medium high voltage current and transfers that power to a very large current that acquires a low voltage when it flows through the secondary coil of the final transformer. It is this very large current of low voltage charges that flows through appliances in your home and those of your neighbors. That final transformer is often visible as a large gray drum on a utility pole or a green box in someone’s yard.

A charge coupled device converts light (photonic energy) into electric energy. What is the underlying mechanism that makes this happen? — PM, Belfast, Ireland

As in any photoelectric cell, the energy from a single particle of light—a photon—is used to raise the energy of an electron in a diode and to propel that electron from one side of the diode to the other. In this process, the light energy is partly converted to electrostatic potential energy and partly to thermal energy. Since a diode only carries current in one direction, the electron is unable to return to its original side. In a photoelectric cell, the electron flows through a circuit to return to the other side of the diode and provides energy to that circuit. In a charge coupled device, a complicated charge shifting system transfers the electrons to a detector that registers how much light was absorbed.

I’m doing a science fair project on electricity and I need to know how to make a homemade hot dog cooker. – BE

Although I have never done it myself, I understand that it is possible to run electric power directly from the power line through a hot dog and to use the resistive heating that occurs as electric current struggles to pass through the hot dog to cook that hot dog. While I can’t recommend doing this and caution anyone trying it to be extremely careful with the electricity (i.e. seek adult supervision from someone who is experienced with the safe handling of electricity), I believe that it can be done. My understanding is that you should carefully connect each wire of an electric power cord (unplugged!) to its own nail (choose an uncoated steel nail to avoid toxic materials). You should then insert one nails into each end of the hot dog and place that hot dog on a safe, nonconducting surface where no one and nothing can touch it. Finally, you should plug the electric cord into an electric socket that is properly connected to a working circuit breaker. I would recommend using a socket protected by a ground-fault interrupter (GFI) such as are used in modern bathrooms (the ones with a “test” and “reset” button). (As you can see, I don’t want anyone hurt!) I’m not sure how quickly the hot dog will cook, but I’d expect it to be quite fast. Be sure to unplug the cord before getting anywhere near the hot dog.

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.

If voltage shocks you, why does current kill you?

Your skin is a very good electric insulator and it prevents any current from passing through your body as long as that current doesn’t have much voltage. A higher voltage (the electric equivalent of “pressure”) is required to push charge through your skin. But once the charge is inside you body, it moves through you quite easily—your body fluids are essentially salt solutions and are relatively good conductors of electricity.

However, a small current passing through your body won’t cause injury. It takes about 0.030 amperes or 30 milliamperes to cause a life-threatening disturbance to your “electric system.” The small currents associated with static electricity are not enough to cause trouble, even through they easily pass through your skin. So high voltages are needed to break through your protective barrier—your skin—in order to give you a shock, but large currents are needed to injury you.

Is it safe to live near high-power lines?

It’s probably fine. While the high-tension wires do create modest alternating electric and magnetic fields, there is no credible evidence that these fields cause any injury and no one has proposed a convincing mechanism whereby those fields could affect biological tissue.