## How does current flow and return in a home electric hot water heater? I only see…

#### How does current flow and return in a home electric hot water heater? I only see two black hot wires and no white return wire. — DT, Waianae, HI

Your hot water heater is powered by 240 volt electric power through the two black wires. Each black wire is hot, meaning that its voltage fluctuates up and down significantly with respect to ground. In fact, each black wire is effectively 120 volts away from ground on average, so that if you connected a normal light bulb between either black wire and ground, it would light up normally. However, the two wires fluctuate in opposite directions around ground potential and are said to be “180° out of phase” with one another. Thus when one wire is at +100 volts, the other wire is at -100 volts. As a result of their out of phase relationship, they are always twice as far apart from one another as they are from ground. That’s why the two wires are effectively 240 volts apart on average.

Most homes in the United States receive 240 volt power in the form of two hot wires that are 180° out of phase, in addition to a neutral wire. 120-volt lights and appliances are powered by one of the hot wires and the neutral wire, with half the home depending on each of the two hot wires. 240-volt appliances use both hot wires.

## I cannot understand a step-up transformer. Why is the voltage doubled when we do…

#### I cannot understand a step-up transformer. Why is the voltage doubled when we double the secondary turns? What isn’t it possible to have a dc transformer; since the law of induction says that when a current passes through a conductor it provides a magnetic field, isn’t it the same as ac? — C

A transformer only works with ac current because it relies on changes in a magnetic field. It is the changing magnetic field around the transformer’s primary coil of wire that produces the electric field that actually propels current through the transformer’s secondary coil of wire.

When dc current passes through the primary coil of wire, the coil does have a magnetic field around it, but it doesn’t have an electric field around it. The electric field is what pushes electric charges through the secondary coil to transfer power from the primary coil to the secondary coil. In contrast, when ac current passes through that primary coil of wire, the magnetic field around the coil flips back and forth in direction and this changing magnetic field gives rise to an electric field around the coil. It is this electric field that pushes on electrically charged particles—typically electrons—in the secondary coil of wire. These electrons pick up speed and energy as they move around the secondary coil’s turns. The more turns these charged particles go through, the more energy they pick up. That’s why doubling the turns in a transformer’s secondary coil doubles the voltage of the current leaving the secondary coil.

## I heard on a news report that there is a paint that will generate heat from a 12…

#### I heard on a news report that there is a paint that will generate heat from a 12-volt battery. What can you tell me about this subject? — JF

Generating heat from a battery is relatively easy. All you need is a material that conducts electricity only moderately well and you’re in business. If you allow current to flow through that material from the battery’s positive terminal to its negative terminal, the current will lose energy as it struggles to get through the material and the current’s lost energy will become thermal energy in the material. The only difficult part of this task is in choosing the right material so that it doesn’t produce too much or too little heat. In short, the electric resistance of the finished material has to be in the right range. For a solid system that you can cut and tailor, that’s not much of a problem. But for a paint, it could be tricky. To make an inexpensive paint, it would probably need to use carbon powder as the electric conductor. A thin layer of carbon granules held in place by a plastic of some sort would probably provide a suitable conducting surface that would become warm when you allowed current to flow through it from a battery. There are copper and silver conducting paints that might also work, but these are rather expensive and I’m not sure how they behave at elevated temperatures.

## What are the frequency characteristics of transformers? Are they related to the …

#### What are the frequency characteristics of transformers? Are they related to the circuit components and the ratio of primary to secondary turns around the iron core? — JM, Lakewood, Colorado

The frequency characteristics of a transformer are determined principally by the materials in the transformer’s core. Power flows from the primary circuit to the secondary circuit by way of the magnetization of the transformer’s core. With each half-cycle of the alternating current in the primary circuit, the transformer’s core must magnetize and demagnetize. A transformer core’s ability to magnetize and demagnetize properly depends on the frequency of the alternating current in the transformer’s coils. If that frequency is too low, the core may saturate—reach its maximum possible magnetization—during the half-cycle. In that case, the core will not be able to transfer the requisite amount of energy to the secondary coil and the power transferred between the two coils will be inadequate. That’s why low frequency transformers often contain huge iron cores—cores that avoid saturation by spreading out the magnetization and stored energy over large volumes of iron.

On the other hand, if the frequency of current in the primary is too high, the core may be unable to magnetize and demagnetize fast enough to keep up with it and the power transfer will again be inadequate. The core may also become hot due to friction-like losses in the core material. That’s why high frequency transformers use special core materials such as ferrite powders or even air. Although air (or really empty space) can’t store large amounts of energy in small volumes when it magnetizes, it can respond extremely quickly. Air-core transformers operate well at extremely high frequencies.

## How does an electric welder work?

#### 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?

#### 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?

#### 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?

#### 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?

#### 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. W…

#### 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.