a. Ohm's law states that the current flowing in a DC circuit is directly proportional

to the EMF and inversely proportional to the resistance of the circuit. Ohm's law may be

expressed by anyone of three equations. In common usage, I is current expressed in

amperes, E is EMF expressed in volts, and R is resistance expressed in ohms.

I = _E_

R = _E_

E=IxR

R

I

Thus, to find a resistance when E and I are known, use R = E/I (volts divided by

amperes) to find the voltage. When I and R are known, use E = I X R. If any two of

these values are known, it is possible to find the value of the unknown by using one of

these formulas. A simple aid to memorizing Ohm's law formulas is to cover the

unknown factor in the formula __E__ and see what mathematical manipulations

IxR

are required to find it. For example, if it is desired to find the resistance (R) when the

applied voltage (E) is 12 volts and the current (I) is 3 amperes--cover R and note that E

should be divided by I. Thus 12 volts 3 amperes = 4 ohms resistance.

a. A given amount of electricity can do a definite amount of work. There is a

simple relationship between the work produced and the voltage and amperage in a

circuit.

(1)

Power is the rate at which work is done per unit of time.

(2) The watt (W) is the unit of electrical power. It is equivalent to one

ampere of current flowing at a pressure (EMF) of one volt.

(3) The power rule formula determines the power of continuous DC (the

amount of work the current can do per second):

P=IxE

power in watts = amperage times voltage

(4) The power of most electrical machinery is measured in terms of a unit

called a horsepower: 746 watts are equal to one horsepower.

b. The most common loss of power in electrical work is due to the heat

developed when current is flowing through a resistance. The greater the resistance of

the circuit, the greater is the rate of heat production. A combination of the power

formula and Ohm's law (E = I X R) gives the power loss due to heat production of a

MD0950

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