Category: electric power distribution

Is not the current used in Europe direct current? If so, do they use transformers or do their lines get very hot? Why do our appliances not work there?

Europe uses alternating current, just as we do, however some of the characteristics of that current are slightly different. First, Europe uses 50 cycle-per-second current, meaning that current there reverses directions 100 times per second. That’s somewhat slower than in the U.S., where current reverses 120 times per second (60 full cycles of reversal each second or 60 Hz). Second, their standard voltage is 230 volts, rather than the 120 volts used in the U.S.

While some of our appliances won’t work in Europe because of the change in cycles-per-second, the biggest problem is with the increase in voltage. The charges entering a U.S. appliance in Europe carry about twice the energy per change (i.e. twice the voltage) and this increased “pressure” causes about twice the number of charges per second (i.e. twice the current) to flow through the appliance. With twice the current flowing through the appliance and twice as much voltage being lost by this current as it flows through the appliance, the appliance is receiving about four times its intended power. It will probably burn up.

How are you “shocked”?

Your body is similar to salt water and is thus a reasonably good conductor of electricity. Once current penetrates your skin (which is insulating), it flows easily through you. At high currents, this electricity can deposit enough energy in you to cause heating and thermal damage. But at lower currents, it can interfere with normal electrochemical and neural process so that your muscles and nerves don’t work right. It takes about 0.030 amperes of current to cause serious problems for your heart, so that currents of that size can be fatal.

How come the flashlight works when you switch the batteries but my walkman or gameboy doesn’t?

The bulb in a battery doesn’t care which way current flows through it. The metal has no asymmetry that would treat left-moving charges differently from right-moving charges. That’s not true of the transistors in a walkman or gameboy. They contain specialized pieces of semiconductor that will only allow positive charges to move in one direction, not the other. When you put the batteries in backward and try to propel current backward through its parts, the current won’t flow and nothing happens.

If only electrons move around, why do you keep using positive charges in the demos?

It’s useful to describe moving electric charges as a current and for that current to flow in the direction that the charges are moving. Suppose that we define current as flowing in the direction that electrons take and look at the result of letting this current of electrons flow into a charge storage device. We would find that as this current flowed into the storage device, the amount of charge (i.e. positive) charge in that device would decrease! How awkward! You’re “pouring” something into a container and the contents of that container are decreasing! So we define current as pointing in the direction of positive charge movement or in the direction opposite negative charge movement. That way, as current flows into a storage device, the charge in that device increases!

Why are batteries so expensive?

They contain highly purified and refined chemicals and are actually marvels of engineering. It’s more surprising to me that they are so cheap, given how complicated they are to make.

In alternating current, current reverses directions rapidly between the two wires, white and black. Why is it that only the black wire is “hot”?

When you complete a circuit by plugging an appliance into an electrical outlet, current flows out one wire to the appliance and returns to the electric company through the other wire. With alternating current, the roles of the two wires reverse rapidly, so that at one moment current flows out the black wire to the appliance and moments later current flows out the white wire to the appliance. But the power company drives this current through the wires by treating the black wire specially—it alternately raises and lowers the electrostatic potential or voltage of the black wire while leaving the voltage of the white wire unchanged with respect to ground. When the voltage of the black wire is high, current is pushed through the black wire toward the appliance and returns through the white wire. When the voltage of the black wire is low, current is pulled through the black wire from the appliance and is replaced by current flowing out through the white wire.

The white wire is rather passive in this process because its voltage is always essentially zero. It never has a net charge on it. But the black wire is alternately positively charged and then negatively charged. That’s what makes its voltage rise and fall. Since the black wire is capable of pushing or pulling charge from the ground instead of from the white wire, you don’t want to touch the black wire while you’re grounded. You’ll get a shock.

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 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-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 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.