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Electrical Measurements |
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III. Electrical Measurements  The flow of charge, current, in a wire is measured in terms of the number of coulombs per second going past a given point on a wire. One coulomb/sec equals 1 ampere, a unit of electric current named for the French physicist André Marie Ampère. See description under Current Electricity, below. When 1 coulomb of charge travels across a potential difference of 1 V, the work it does equals 1 joule, a unit named for the English physicist James Prescott Joule. This definition facilitates transitions from mechanical to electrical quantities. One joule equals 107 ergs. A widely used unit of energy in atomic physics is the electron volt (eV). This is the amount of energy gained by an electron that is accelerated by a potential difference of 1 V. This is a small unit and is frequently multiplied by 1 million or 1 billion, the result being abbreviated 1 MeV or 1 GeV. IV. 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. (In some branches of electrical engineering, electric current has been conventionally assumed to flow in the opposite direction, that is, from positive to negative.) 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 current. 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 either direction. Three interdependent quantities determine the flow of direct currents. 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 6,250,000,000,000,000,000 electrons per sec past any point of the circuit. The third quantity is the resistance of the circuit. Under ordinary conditions all substances, conductors as well as nonconductors, 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 (V), which is defined as the amount of resistance that will limit the flow of current to 1 amp, in a circuit with a potential difference of 1 V. This relationship is known as Ohm's law and is named after the German physicist Georg Simon Ohm, who discovered the law in 1827. Ohm's law 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 of Ohm's law is I = E/R. See Electric Circuit; Electric Meters. 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, which is equivalent to 0.738 ft-lb. Power expended is measured by the watt, which is equal to 1 J/sec. The power in a given circuit can be calculated from the equation P = E × I or P = 12 × R. Power may also be expended in producing mechanical work, electromagnetic radiation such as light or radio waves, and chemical decomposition. |
