The values of current, voltage, and resistance are all related
because the amount of current that flows is dependant on the amount of
electric pressure (emf) applied and the opposition (resistance) encountered
by the electron movement. These relationships were established by 
George Ohm (1787 - 1854) and were set down as specific formulas
whereby an unknown quantity of R, I, or E can be found when the values of
any two of these are known. One of these formulas shows how the value of
current(I) can be ascertained when voltage (E) and resistance (R) are known.

                I=E/R or V/R

        This formula indicates that the amount of current flowing in a 
circuit is equal to the quotient obtained when the voltage value is divided
by the resistance value. Thus, if there are 10 volts impressed across a 
5 ohm resistor, the current flow through the resistor is 2 amps.

        An unknown value of either voltage or resistance can also be 
found by rearrangement of the formula. Voltage, for example, can be
obtained if the values of current and resistance are known.

                V or E = IR

        As this formula shows, an unknown voltage can be found by multiplying
the current by the resistance value. Thus, if the current flow through
a 3 ohm resistance is 20 amps, the voltage across the resistance is 60 volts.

        If the value of the resistance is desired, the known values of 
current and voltage are used for solving the problem by use of the formula:

                R = E/I or V/I

        As an example, assume that the voltage across a resistance is 50
volts and the value of the current flow is 10 amps. Dividing the E value
by the I value indicates a resistance of 5 ohms.

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	


	Energy can be changed from one form to another but can never
be destroyed. Therefore, we may readily change electrical power into
mechanical power; the converse is also true. The usual method of 
referring to mechanical power is in terms of units of horsepower;
	

		One horsepower is equal to 746 watts.

	This equality is valid when considering that the equipment used
to produce one horsepower operates at 100% efficiency, which is not
possible, since there is always some power lost in the form of friction
heat and other losses.


	A wattmeter is an instrument that is designed to indicate
directly the active power in an electric circuit. It consist of a 
coil connected in series with the circuit, such as an ammeter, and
a coil connected in parallel with the circuit, such as a voltmeter.
Both coils actuate the same meter, thereby giving the measurement of
both the current and voltage affecting one meter, which may be calibrated
in watts, kilowatts or megawatts.

	Power is expressed in DC circuits and AC circuits that contain
only resistance by the formula:

	P (watts) = E x I
	
			  E x I
	P (kilowatts) = ---------
			 1,000


	Power is expressed in an AC circuit that contains Inductive and/or
Capacitive Reactance by the formulas:

	VA (volt-amperes) = E X I

				  E X I
	KVA (kilovolt-amperes) = --------
				  1,000


	P (watts) = E X I X Power Factor


	Power Factor is determined by the formula:

			True Power (watts)
	Power Factor = --------------------------
			Apparent power (E X I)


			  W           KW
	Power Factor = --------  = -------
		        E X I	     KVA


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