Electricity

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ELECTRICITY

Electrical and electronic devices and machines have become an integral part of contemporary life, ranging from household appliances and computers to huge industrial machines. When home and business owners pay the monthly bill from the electrical power company for the use of all of these items, they are paying for energy very conveniently delivered over electrical wires from the power company.

Although the delivery of electricity to homes and businesses has been possible only during the past century, static electricity was observed by the ancient Greeks over 2,000 years ago. They also noticed natural magnets, called lodestones, found near the town of Magnesia. These were important discoveries because scientists now know that electricity and magnetism are intimately related. Magnetism is used by power companies to produce the electricity used every day.

ELECTRIC CHARGES AND ELECTRIC FORCE

If the humidity is low, it is common to experience a shock when touching a metal doorknob after walking across a carpeted floor. This is static electricity, and it can be studied by rubbing a hard rubber rod on some fur, and then touching the rod to a small metal ball that is suspended on the end of a silk thread. The ball quickly bounces away from the rod and is repelled by it, as shown in Figure 1a. The rod and the ball are electrified or charged with the same type of (net) charge. Further, when a glass rod is rubbed with silk and touched to a second similar ball, the second ball is repelled by the glass rod, shown in Figure 1b. But when the two balls are brought near each other (without touching), they are clearly attracted to each other as shown in Figure 1c.

In the mid-1700s Benjamin Franklin proposed that there are only two kinds of electrical charge, which he called plus and minus. He defined the net charge on the rubber rod to be negative, and the charge on the glass rod to be positive. Further, charges of the same kind are repelled from each other, while opposite charges are attracted to each other. The amount of electrical charge (often represented by the letter q or Q) is measured in coulombs (abbreviated as C) in the Standard International system of units, called S.I. units.

CHARGES IN THE STRUCTURE OF AN ATOM

Electrons particles in matter with a negative charge were discovered around 1900. All matter is made of tiny atoms packed closely together. The structure of the atom was proposed to be like a tiny solar system, with negative electrons revolving in orbits around a very tiny positive nucleus (see Figure 2). The charge ("e") of the electron is a certain fixed amount: only a millionth of a trillionth of 0.16 coulombs (or 0.16 × 10 ^-18 C). The total negative charge of all of the electrons in an atom is exactly the same (but of opposite sign) as the charge of the positive nucleus; thus an atom taken as a whole is normally uncharged or "neutral."

Since a piece of matter (e.g., a piece of rubber or copper) is made up of a great many neutral atoms, the piece is itself normally neutral or uncharged. When a rubber rod is rubbed with fur, some electrons are pulled from the fur onto the rod, giving it extra negative charge (the fur is then deficient of electrons so it is positively charged).

ELECTRIC CIRCUITS AND CURRENT FLOW

There were few applications of electricity (or of magnetism) before the invention of the battery by Alessandro Volta in 1800. A battery can cause charges to move for long periods of time. The movement or "flow" of electrical charge is called a current. A basic battery cell consists of two different metals (called electrodes) immersed in an acid or salt solution (called an electrolyte). Through chemical interactions, one electrode develops extra electrons and becomes negatively charged, while the other develops a deficiency of electrons and becomes positively charged; these are respectively labeled the negative (-) and positive (+) battery cell terminals.

Work (W) is done by a battery whenever it pushes a positive charge (+q) away from the (+) terminal (through space outside the battery) to the (-) terminal. The "potential" or "emf" or "voltage" (V) of a battery is defined as the work done in this process divided by the charge: V= W/q. In S.I. units the voltage V is in volts V to honor Volta. A battery cell often used in flashlights is the "carbon-zinc dry cell." If unused and fresh, the carbon-zinc cell has a voltage of about 1.5 V. Many flashlights use two carbon-zinc cells placed end-to-end or in "series." Strictly speaking, the two cells together constitute a "battery," and the voltage of this battery is 3.0 V. The common 12 V "lead-acid" battery used in automobiles has six cells connected in series with it; each lead-acid cell has a voltage of 2.0 V.

Suppose a long thin metal wire is connected by a pair of thick wires between the terminals of a battery. This is a basic "electric circuit" as shown in Figure 3a. In all metals, each atom permits roughly one of the outer electrons to move quite freely in the material; these are called the "free electrons." In contrast, all electrons of the atoms of good electrical "insulators," such as glass, rubber, and air, are tightly bound to the atoms and are not free to move through the body of the material. The free electrons are repelled from the negative battery terminal and attracted toward the positive terminal, so that a continuous movement of charge (an electrical current) results around this complete path or "circuit." But there are frictional effects (electrons bumping into atoms as they move along) that resist the movement or flow of electrons through the wires, especially in the thin wire. The frictional effects result in the electrical "resistance" (R) of the wire, and cause the wires to heat up, especially the long thin wire.

An important property of this or any electrical circuit is the rate that charge moves past a place in the circuit (e.g., out from or into a battery terminal). The electrical current (I) is defined to be the charge (Q) that flows, divided by the time (t) required for the flow: I = Q/t. In S.I. units the current (I) is in amperes (A).

In the early 1800s, Georg Ohm studied the effect on current of changing the battery voltage as well as the length, cross-sectional area, and material of the wire in an electrical circuit like Figure 3a. He found that the current flow (I) was proportional to the voltage (V) and inversely proportional to the resistance (R) of the wire to this flow. The resistance (R) is quite constant for a given piece of metal wire; Ohm's law states that we may write that I=V/R. The S.I. units of resistance are ohms (?). The resistance to current flow of a wire is larger if the wire is longer, but smaller if the wire is thicker (larger cross-sectional area).

The smallest diameter copper wire generally permitted within the walls for U.S. household wiring is called no. 14 wiring; such wire has a diameter of about 1/16 inch (1.63 mm). A 10 foot length (about 3 meters) of no. 14 copper wire has a resistance of about 0.025 ohms; note that a 100 foot length (about 30 meters) has a resistance of 0.25 ohms.

Figure 3b shows the basic electrical circuit of Figure 3a as it is drawn using conventional electrical symbols. Note the symbol for a battery. The connecting wires with negligible resistance are drawn as lines, while an element with significant resistance is drawn as a zigzag line. An electrical switch is a length of metal that can be moved so that a space of air appears in the circuit; the air space has a large resistance (perhaps billions of ohms) so that virtually no current flows through it when it is "open" (as represented by the dashed line in Figure 3b. When the switch is closed the circuit path is complete and current flows around the "closed circuit." It is conventional to draw the current flow direction as from the (+) battery terminal to the (-) terminal. This is the direction that (+) charges would move if they were moving within the wire, and this was assumed to be the case from about 1800 to 1900. It is now known electrons move in the opposite direction, but the old conventional direction is almost universally used. In any case, the current flows in only one direction in this circuit; this situation is called "direct current" or "d.c."

ELECTRICAL POWER AND ELECTRICAL ENERGY

As electrical current flows through a wire of significant resistance, the wire is heated and the wire gives off heat energy. The wire in an incandescent light bulb is heated so much that it glows and gives off light energy as well as heat energy. This energy is supplied by a battery or other source of emf. Further, "power" is the rate that energy is being supplied, or that work is being done: power = energy/time, or power = (work done)/time. In S.I. units power is measured in watts. Using the definition of voltage and of current one can show that when a current flows from a battery (or any source of emf), the battery is delivering power at the rate of the voltage times the current: P = VI. For example, suppose a 115 V battery is supplying 0.87 amperes of current to a light bulb connected to the battery; then the power being used by the light bulb is equal to (115V) × (0.87A) = 100 watts. This particular bulb is a "100 watt light bulb." Note that because 1,000 watts equals one kilowatt, this bulb is using a tenth of a kilowatt or 0.10 kilowatts (0.10 kW) of power. ^F0^Since power = energy/time, it follows that energy = power × time; that is, energy supplied is equal to the power multiplied by the time-interval. The unit of electrical energy unit used by the electrical utility company is (kilowatts) × (hours), or kWh. Suppose a 100 W bulb is turned on for one day or 24 hours. The energy used by the bulb is (0.10 kW) × (24 h) = 2.4 kWh. A typical electrical energy cost in the United States might be 8 cents per kWh, so the cost for using this bulb for an entire 24 hour day would be (2.4 kWh) × (8 cents/kWh) = 19.2 cents.

Note that electrical energy used (and paid for) depends not only on the power consumption by a device or appliance, but also the length of time that it is used in a month. Therefore people are sometimes surprised to discover that there is a modest electric bill even after they have been gone from home on vacation and "everything" had been turned off. However, a number of devices around the home were probably operating, such as electric clocks, the electric motor on the furnace, the refrigerator, and the freezer. There are also many applicances operating on standby mode, such as televisions, answering machines, and cordless appliance. These are sometimes called "phantom power" devices.

MAGNETS, MAGNETIC FIELD, AND ELECTROMAGNETS

Magnets also repel or attract each other. A magnet always has both a north pole (N) and a south pole (S). Again, "opposites attract and likes repel" so that two magnets with north poles close to each other are repelled from each other, even without touching. The influence in the region near a magnet is often pictured as a "magnetic field"; invisible "magnetic field lines" are imagined, and often drawn in a diagram of a magnet. Figure 4a illustrates a steel bar "permanent magnet" with its magnetic field lines.

In the early 1800s H. C. Oersted discovered that an electric current flowing through a coil of wire produces a magnetic field, as shown in Figure 4b. A long coil of wire, often wound on an iron core to enhance the magnetic field, is called a "solenoid" or "electromagnet" and attracts iron or steel objects, just like a permanent magnet.

ELECTRIC GENERATOR

Shortly after Oersted's discovery, Michael Faraday found that changing the number of magnetic field lines within a coil of wire induces an emf or voltage in the coil so that a current flows if there is a closed circuit. Thus not only can a current produce a magnetic field, but a changing magnetic field can generate a current. Figure 5a shows the basic idea of an electric generator. A coil consisting of a single turn of wire is pictured; this is twirled (rotated) in front of a bar magnet so that the magnetic field lines pass through the coil, first from one side and then the other side. The changing magnetic field within the coil results in an alternating emf or voltage that is measured between the terminals A and B, and a current that reverses directions (i.e., an alternating current or a.c.) flows through the attached "load" resistance.

The alternating voltage V _AB measured between terminals A and B is graphed in Figure 5b. If the coil is turned at 60 cycles per second, then the frequency of alternation is 60 cycles per second (called 60 Hz in S.I. units). This is the standard frequency for commercial a.c. power in the United States (in Europe the standard is 50 Hz). By increasing the number of turns (or loops) in the coil, or by increasing the magnetic field strength, it is possible to increase the amount of emf or voltage generated (e.g., the peak voltage V _peak illustrated in Figure 5b). A typical power company generator might generate 10,000 V _rms. This is much too large to be safe for household use, so the generated voltage must be "transformed" down to about 115 V _rms that is present at a regular household outlet. (But first it is actually transformed up to much higher voltage to reduce energy lost in transporting the electrical energy.)

Note that a.c. voltages (also a.c. currents) are usually measured and quoted as "r.m.s." The r.m.s. value is 70.7 percent of the peak value. This is done so that the formula to calculate electrical power in the a.c. case is the same as for the d.c. case stated earlier: power is the voltage times the current (provided both are rms values).

It is important to realize that it requires effort to turn the coil of an electrical generator if current is being supplied by the generator; that is, work must be done to turn the generator coil. Conservation of energy requires that the energy used to turned the coil (i.e., mechanical energy input to the generator) is at least as much as the electrical energy produced by the generator in a given period of time. Electrical generators have relatively high efficiency, so that the electrical power output is perhaps 90 percent of the mechanical energy input, with the remaining 10 percent lost in various heating effects in the generator. Most commercial (utility company) electrical generator coils in the United States are turned by steam engines that burn coal to obtain the energy to generate the electricity.

TRANSFORMERS AND DELIVERY OF ELECTRIC POWER

Alternating voltage (called a.c. voltage) can be quite readily changed to a different value through the use of an electrical transformer. Note that an electrical transformer will not transform a d.c. voltage to another d.c. voltage value; this is a principal reason why commercial electricity in the United States (and most of the world) is almost always a.c. rather than d.c. A transformer consists basically of two coils of wire, with one coil often wound on top of the other. Electrical voltage is supplied to the coil called the "primary" coil, while voltage is output from the "secondary" coil. Then the basic transformer relationship is that the ratio of the secondary voltage to primary voltage is the same as the ratio of secondary-coil-turns to primary-coil-turns.

Suppose 10,000 V _rms (e.g., from an electrical power plant generator) is input to a transformer where the number of secondary turns is 75 times more than the number of primary turns. Then the voltage from the secondary will be 75 × 10,000 V _rms = 750,000 V _rms. This very high voltage level is actually produced by the secondary of the transformers at a modern commercial electrical power plant; the 750,000 V _rms is connected to the high voltage transmission lines (thick wire cables) that are used to transport the electrical power over long distances (perhaps hundreds of miles). The high voltage is used to reduce the energy lost to heating of the transmission wire resistance by the electrical current. The power lost to heating of the transmission wire is proportional to the square of the current flowing, so that the losses mount rapidly as the current flowing increases. But the power transmitted is equal to the transmission line voltage times the current flowing. For a given power, the current flowing will be much less (and energy lost in the transmission process is much less) if the transmission line voltage is very large. At the output end of the transmission line, a transformer is used again to step the voltage down to a level appropriate for local distribution (e.g., 2,400 V _rms), and a neighborhood transformer is finally used to step the voltage down to 220V _rms and 110 V _rms for use in homes and businesses.

Dennis Barnaal

See also: Electric Power, Generation of; Electric Power Transmission and Distribution Systems; Electricity, History of; Transformers.