Electricity is a combination of a force called VOLTAGE and the movement of invisible particles known as CURRENT. The force of voltage can be compared to the force generated by a water pump, which moves water through a distribution system, generally an arrangement of pipes. Voltage is the force that causes current to flow through a system of wires. Current is the movement of invisible particles that causes electrical devices to operate. We cannot see current, but we can determine its presence by the effects it produces Figure 2-7 for example, shows the effect of current. It shows how the voltage force from a battery causes electrical current to flow through wires and an electrical motor. The current is invisible, but it produces the effect of making the motor run. Current flows through the wires much the same way as water flows through pipes.

Current consists of electrons, which are invisible atomic particles. Voltage is the force that causes current, in the form of electrons, to move through wires and electrical devices. However, one important difference between current in wires and water in pipes is that water can flow out of a

Figure 2-3.-Different characteristics of current and water.

broken pipe, but current cannot flow out of a broken wire. When a wire is broken, the force of the voltage is removed from the motor, as shown in Figure 2-3. The circulating pump in the working system creates a force that moves hot water through the pipes and radiator. The battery creates a force that moves current through the wires and causes the motor to run. The wire and pipe are broken open in the broken system. In these instances, the circulating pump forces water to flow out of the pipe, but even though the battery still creates a voltage force, current does not flow out of the wire. You will find additional information on the basic principles of electricity in the NEETS, module 1, NAVEDTRA 172-01-00-88, chapter 1.

OHM'S LAW

In the early part of the 19th century, George Simon Ohm proved by experiment that a precise relationship exists between current, voltage, and

resistance. This relationship is called Ohm's law and is stated as follows:

I = E/R,

where: I = current in amperes,

E = voltage in volts, and

R = resistance in ohms.

As stated in Ohm's law, current is inversely proportional to resistance. This means, as the resistance in a circuit increases, the current decreases proportionately. In the equation I = E/R, if any two quantities are known, the third one can be determined.

NEWTON'S LAWS

Sir Isaac Newton was an English philosopher and mathematician who lived from 1642 to 1727 A.D. He was the formulator of the basic laws of modern philosophy concerning gravity and motion. Before we discuss motion and other related factors, you should be familiar with Newton's laws. These laws are the bases for the theories of physics that we describe in the following sections.