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Current Electricity
If two equally and oppositely charged bodies are connected by a metallic conductor such as a wire, the charges neutralize each other. This neutralization is accomplished by means of a flow of electrons through the conductor from the negatively charged body to the positively charged one. (Electric current is often conventionally assumed to flow in the opposite direction-that is, from positive to negative; nevertheless, a current in a wire consists only of moving negatively charged electrons.) In any continuous system of conductors, electrons will flow from the point of lowest potential to the point of highest potential. A system of this kind is called an electric circuit. The current flowing in a circuit is described as direct current (DC) if it flows continuously in one direction, and as alternating current (AC) if it flows alternately in each direction.
Three interdependent quantities characterize direct current. The first is the potential difference in the circuit, which is sometimes called the electromotive force (emf) or voltage. The second is the rate of current flow. This quantity is usually given in terms of the ampere, which corresponds to a flow of about 6.24 × 1018 electrons per second past any point of the circuit. The third quantity is the resistance of the circuit. Under ordinary conditions all substances, conductors as well as non-conductors, offer some opposition to the flow of an electric current, and this resistance necessarily limits the current. The unit used for expressing the quantity of resistance is the ohm, which is defined as the amount of resistance that will limit the flow of current to 1 ampere in a circuit with a potential difference of 1 volt. The symbol for the ohm is the Greek letter W, omega. The relationship may be stated in the form of the algebraic equation E = I × R, in which E is the electromotive force in volts, I is the current in amperes, and R is the resistance in ohms. From this equation any of the three quantities for a given circuit can be calculated if the other two quantities are known. Another formulation is I = E/R. See Electric Circuit; Electric Meters.
Ohm's law is the generalization that for many materials over a wide range of circumstances, R is constant. It is named after the German physicist Georg Simon Ohm, who discovered the law in 1827.
When an electric current flows through a wire, two important effects can be observed: the temperature of the wire is raised, and a magnet or a compass needle placed near the wire will be deflected, tending to point in a direction perpendicular to the wire. As the current flows, the electrons making up the current collide with the atoms of the conductor and give up energy, which appears in the form of heat. The amount of energy expended in an electric circuit is expressed in terms of the joule. Power is expressed in terms of the watt, which is equal to 1 J/sec. The power expended in a given circuit can be calculated from the equation P = E × I or P = I 2 × R. Power may also be expended in doing mechanical work, in producing electromagnetic radiation such as light or radio waves, and in chemical decomposition.
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