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Electrical Power:
When voltage is applied to a conductor, current will flow and the amounts of current flow and voltage represent a quantity of power. Such electric power can be used for heating purposes, for operating a motor or in other applications of electric energy. Since we cannot get power for nothing, a battery or other power source must be used to generate the energy needed. The electric power symbol (P) is measured by the amount of voltage multiplied by the quantity of current flow. P = EI The unit of power is the Watt, named after the Scottish inventor James Watt(1736 -1819). One Watt of power is equal to one ampere of current flow produced by one volt of electric pressure. Because the watt unit relates to electric power, the symbol VA (Volt Amp) has been used as well as W (Watt). When power is calculated in terms of time, the unit of energy is the Joule. This is also known as the Watt-second and represents one watt of power for one second. In the measurement of ordinary electric power consumed in homes, the KilowattHour (KWhr) is utilized and this refers to 1000 Watts for 1 hour. In many electronic applications, however, only fractional power units are used, and the term milliwatt (mW) is then utilized for convience, to express one-thousandth of a watt. Thus, 0.0005-W = 0.5 mW. The formula P = EI solves for the amount of energy consumed in terms of unit watts. Therfore if 20 volts are present across a resistance and 2 amps of current flow, the amount of energy consumed equals 40 watts. If E voltage is unknown but I (amps) and R (resistance) are known, the following formula is used: P = I squared R Thus if 2 amps of current are flowing and the resistance is 10 ohms the amount of power is P = 2x2x10 = 40 watts. Power can also be found by dividing the voltage squared by the resistance: P = E squared / R. Because power is related to Ohm's law, the amount of power used can be used in formulas for finding unknown values of current and voltage. I = P/E R = P/I squared E = the square root of PxR I = the square root of P/R (power divided by resistance) When dealing with large amounts of electrical power, it may be required that you be able to determine the cost of the power consumed. You will be dealing with units of kilowatts and kilowatt-hours (kwh), which means the number of kilowatts used per hour. Thus, 25 kwh is equivalent to 25 kilowatts used for 1 hour. To find the cost of an electric usage bill, the formula is: watts x hours used x rate per kwh Cost = ----------------------------------- 1,000