Basic Electrical Theory UNITS OF ELECTRICAL MEASUREMENTUsing Ohm’s Law for the value of voltage (E),E = I x Rand using substitution laws,P = I x ( I x R)power can be described as the current (I) in a circuit squared times the resistance (R) of thecircuit. Equation (1-7) is the mathematical representation of this concept.P = I^{2}R (1-7)InductanceInductanceis defined as the ability of a coil to store energy, induce a voltage in itself, andoppose changes in current flowing through it. The symbol used to indicate inductance inelectrical formulas and equations is a capital L. The units of measurement are called henries.The unit henry is abbreviated by using the capital letter H. One henry is the amount ofinductance (L) that permits one volt to be induced (V_{L}) when the current through the coil changesat a rate of one ampere per second. Equation (1-8) is the mathematical representation of the rateof change in current through a coil per unit time.(1-8)DIDtEquation (1-9) is the mathematical representation for the voltage V_{L} induced in a coil withinductance L. The negative sign indicates that voltage induced opposes the change in currentthrough the coil per unit time (DI/Dt).(1-9)V_{L}L^{}DIDtInductance will be studied in further detail later in this text.CapacitanceCapacitanceis defined as the ability to store an electric charge and is symbolized by the capitalletter C. Capacitance (C), measured in farads, is equal to the amount of charge (Q) that can bestored in a device or capacitor divided by the voltage (E) applied across the device or capacitorplates when the charge was stored. Equation (1-10) is the mathematical representation forcapacitance.(1-10)CQERev. 0 Page 17 ES-01