Electricity

Electricity

Electrical charge

Electric fields

Coulomb’s law and the forces between electrical charges

Current

Voltage

Resistance

Ohm’s law

Electrical power

Alternating current and direct current

Resources

Electricity consists of all phenomena resulting from electrical charges at rest and in motion. Our present-day understanding of electrical principles has developed from a long history of experimentation. Electrical technology, essential to modern society for energy transmission and information processing, is the result of our understanding of electricity.

Electrical charge

Electrical charge is a property possessed by a few of the fundamental particles that make up matter. Electrical charge is either positive or negative. Charges with the same sign repel while unlike charges attract. The unit of electrical charge is the coulomb, named for Charles Coulomb, an early authority on electrical theory.

The most obvious sources of electric charge are the negatively-charged electrons from the outer parts of atoms and the positively-charged protons found in atomic nuclei. Electrical neutrality is the most probable condition of matter because most objects contain nearly equal numbers of electrons and protons. Physical activities that upset this balance will leave

an object with a net electrical charge, often with important consequences.

Excess static electric charges can accumulate as a result of mechanical friction, as when someone walks across a carpet. Friction transfers charge between shoe soles and carpet, resulting in the familiar electrical shock when the excess charge sparks to a nearby person.

Many semiconductor devices used in electronics are so sensitive to static electricity that they can be destroyed if touched by a technician carrying a small excess of electric charge. Computer technicians often wear a grounded wrist strap to drain away an electrical charge that might otherwise destroy sensitive circuits they touch.

Electric fields

Charged particles alter their surrounding space to produce an effect called an electric field. An electric field is the concept we use to describe how one electric charge exerts force on another distant electric charge. Whether electric charges are at rest or moving, they are acted upon by a force whenever they are within an electric field. The ratio of this force to the amount of charge is the measure of the field’s strength.

An electric field has vector properties in that it has both a unique magnitude and direction at every point in space. An electric field is the collection of all these values. When neighboring electrical charges push or pull each other, each interacts with the electric field produced by the other charge.

Electric fields are imagined as lines of force that begin on positive charges and end on negative charges. Unlike magnetic field lines, which form continuous loops, electric field lines have a beginning and an ending. This makes it possible to block the effects of an electric field. An electrically conducting surface surrounding a volume will stop an external electric field. Passengers in an automobile may be protected from lightning strikes because of this shielding effect. An electric shield enclosure is called a Faraday cage, named for Michael Faraday.

Coulomb’s law and the forces between electrical charges

Force, quantity of charge, and distance of separation are related by a rule called Coulomb’s law. This law states that the force between electrical charges is proportional to the product of their charges and inversely proportional to the square of their separation.

The Coulomb force binds atoms together to form chemical compounds. It is this same force that accelerates electrons in a TV picture tube, giving energy to the beam of electrons that creates the television picture. It is the electric force that causes charge to flow through wires.

The electric force also binds electrons to the nuclei of atoms. In some kinds of materials, electrons stick tightly to their respective atoms. These materials are electrical insulators that cannot carry a significant current unless acted upon by an extremely strong electrical field. Insulators are almost always nonmetals. Metals are relatively good conductors of electricity because their outermost electrons are easily removed by an electric field. Some metals are better conductors than others, silver being the best.

Current

The basic unit of electric current is the ampere, named for the French physicist Andre Marie Ampere. One ampere equals 1 coulomb of charge drifting past a reference point each second.

Voltage

Voltage is the ratio of energy stored by a given a quantity of charge. Work must be performed to crowd same-polarity electric charges against their mutual repulsion. This work is stored as electrical potential energy, proportional to voltage. Voltage may also be thought of as electrical pressure.

The unit of voltage is the volt, named for Alessandro Volta. One volt equals one joule for every coulomb of electrical charge accumulated.

Resistance

Ordinary conductors oppose the flow of charge with an effect that resembles friction. This dissipative action is called resistance. Just as mechanical friction wastes energy as heat, current through resistance dissipates energy as heat. The unit of resistance is the ohm, named for Georg Simon Ohm. If 1 volt causes a current of 1 ampere, the circuit has 1 unit of resistance. It is useful to know that resistance is the ratio of voltage to current.

Mechanical friction can be desirable, as in automobile brakes, or undesirable, when friction creates unwanted energy loss. Resistance is always a factor in current electricity unless the circuit action involves an extraordinary low-temperature phenomenon called superconductivity. While superconducting materials exhibit absolutely no resistance, these effects are confined to temperatures so cold that it is not yet practical to use superconductivity in other than exotic applications.

Ohm’s law

Ohm’s law defines the relationship between the three variables affecting simple circuit action. According to Ohm’s law, current is directly proportional to the net voltage in a circuit and current is inversely proportional to resistance.

Electrical power

The product of voltage and current equals electrical power. The unit of electrical power is the watt, named for James Watt. One watt of electrical power equals 1 joule per second. If 1 volt forces a 1-ampere current through a 1-ohm resistance, 1 joule per second will be wasted as heat. That is, 1 watt of power will be dissipated. A 100-watt incandescent lamp requires 100 joules for each second it operates.

Electricity provides a convenient way to connect cities with distant electrical generating stations. Electricity is not the primary source of energy, rather it serves

KEY TERMS

Ampere— A standard unit for measuring electric current.

Conductors— Materials that permit electrons to move freely.

Coulomb force— Another name for the electric force.

Electric field— The concept used to describe how one electric charge exerts force on another, distant electric charge.

Generator— A device for converting kinetic energy (the energy of movement) into electrical energy.

Insulator— An object or material that does not conduct heat or electricity well.

Joule— The unit of energy in the mks system of measurements.

Ohm— The unit of electrical resistance.

Semiconductor devices— Electronic devices made from a material that is neither a good conductor or a good insulator.

Volt— A standard unit of electric potential and electromotive force

Watt— The basic unit of electrical power equal to 1 joule per second.

as the means to transport energy from the source to a load. Electrical energy usually begins as mechanical energy before its conversion to electrical energy. At the load end of the distribution system the electrical energy is changed to another form of energy, as needed.

Commercial electrical power is transported great distances through wires, which always have significant resistance. Some of the transported energy is unavoidably wasted as heat. These losses are minimized by using very high voltage at a lower current, with the product of voltage and current still equal to the power required. Since the energy loss increases as the square of the current, a reduction of current by a factor of 1/100 reduces the power loss by a factor of 1/10, 000. Voltage as high as 1, 000, 000 volts is used to reduce losses. Higher voltage demands bigger insulators and taller transmission towers, but the added expense pays off in greatly-reduced energy loss.

Alternating current and direct current

Direct current, or DC, results from an electric charge that moves in only one direction. A car’s battery, for example, provides a direct current when it forces electrical charge through the starter motor or through the car’s headlights. The direction of this current does not change.

Current that changes direction periodically is called alternating current, or AC. Our homes are supplied with alternating current rather than direct current because the use of AC makes it possible to step voltage up or down, using an electromagnetic device called a transformer. Without transformers to change voltage as needed, it would be necessary to distribute electrical power at a safer low voltage but at a much higher current. The higher current would increase the transmission loss in the power lines. Without the ability to use high voltages, it would be necessary to locate generators near locations where electric power is needed.

Southern California receives much of its electrical power from hydroelectric generators in the state of Washington by a connection through an unusually long DC transmission line that operates at approximately one million volts. Electrical power is first generated as alternating current, transformed to a high voltage, then converted to direct current for the long journey south. The direct-current power is changed back into AC for final distribution at a lower voltage. In certain applications, such as this one, the use of direct current more than compensates for the added complexity of the AC to DC and DC to AC conversions.

Resources

BOOKS

Gibilisco, Stan. Electricity Demystified. New York: McGraw-Hill Professional, 2005.

Keljik, Jeff. Electricity 3: Power Generation and Delivery. 8th ed. Clifton Park, NY: Thomson Delmar Learning, 2005.

Donald Beaty