UNITS OF ELECTRICAL MEASUREMENT Basic Electrical TheoryCurrentElectron current, or amperage, is described as the movement of free electrons through aconductor. In electrical formulas, current is symbolized with a capital I, while in the laboratoryor on schematic diagrams, it is common to use a capital A to indicate amps or amperage (amps).ResistanceNow that we have discussed the concepts of voltage and current, we are ready to discuss a thirdkey concept called resistance. Resistance is defined as the opposition to current flow. Theamount of opposition to current flow produced by a material depends upon the amount ofavailable free electrons it contains and the types of obstacles the electrons encounter as theyattempt to move through the material. Resistance is measured in ohms and is represented by thesymbol (R) in equations. One ohm is defined as that amount of resistance that will limit thecurrent in a conductor to one ampere when the potential difference (voltage) applied to theconductor is one volt. The shorthand notation for ohm is the Greek letter capital omega (W). Ifa voltage is applied to a conductor, current flows. The amount of current flow depends upon theresistance of the conductor. The lower the resistance, the higher the current flow for a givenamount of voltage. The higher the resistance, the lower the current flow.Ohm’sLawIn 1827, George Simon Ohm discovered that there was a definite relationship between voltage,current, and resistance in an electrical circuit. Ohm’s Law defines this relationship and can bestated in three ways.1. Applied voltage equals circuit current times the circuit resistance. Equation (1-2) is amathematical respresentation of this concept.E = I x R or E = IR(1-2)2. Current is equal to the applied voltage divided by the circuit resistance. Equation(1-3) is a mathematical representation of this concept.(1-3)IERES-01 Page 14 Rev. 0
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