Section II. Electric System
1.14. GENERAL
In understanding the electric system, it may help to compare it to a water system such as the one
illustrated on the left in figure 1.10. The path of the water system, the pipe, compares to the wires of the
electric system, illustrated on the right in figure 1.10, which form a path called a circuit. In the water
system, the pump supplies energy and the turbine absorbs it. Similarly, in the electric system, the
generator supplies energy and the motor absorbs it.
In the water system, the pressure the pump supplies varies; the size of the pipe must allow for
this variance of pressure. Different size pipes offer different resistance so that the quantity of water
flowing through the pipe is affected. The characteristics of the water system are paralleled by like
factors in the electric system where they are called volts, ohms, and amperes. These and other basic
principles which govern the flow of electricity and its related magnetic effects are discussed in this
section.
1.15. VOLTAGE
Voltage is the pressure that forces current through a circuit. The pressure is supplied by an
electric generator or battery and is often called potential difference or electromotive force (emf). When
a circuit is available, the voltage causes a current to flow; when the circuit is closed or broken, the
current will not flow. In practice, a volt is defined as a potential difference of pressure that will cause
one ampere to flow through a resistance of one ohm. Voltage is the pressure that forces current through
a circuit.
1.16. RESISTANCE
Resistance is the property of a material, or conductor, which opposes the flow of current when
voltage is applied and which converts electrical energy into heat. An ohm is the unit used to measure
resistance. Very small resistances are measured in millionths of an ohm, called microhms. Large
resistances are measured in millions of ohms, called megohms.
Materials that offer very large resistance are called insulators or conductors. Conductors carry
electric current easily, while insulators offer more resistance to the flow of current. Although there are
no perfect insulators and conductors, organic and vitreous
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