Electrical Principles

Electrical Principles

Resistance, Voltage, and Current

Electricity comes in two forms: alternating current (AC) and direct current (DC). We will first consider the effects of DC on various electrical components and identify the three main characteristics of any electrical circuit. Figure 2-1 illustrates the basic atomic structure—where it all begins.

When we take a close look at nature,we find that all matter is composed of atoms. In the basic structure of an atom, we find that the nucleus is at the center, surrounded by one or more orbiting electrons. This structure is replicated many times for any material. If the material is an insulator, the orbiting electrons do not move from place to place or from atom to atom.

For the purposes of discussion, we will consider aconductor as an atom with three or less orbiting electrons in the outer shell. An atom with five or more orbiting electrons will be considered an insulator. An atom with four orbiting electrons is considered a semiconductor and will be discussed later in this chapter.

Electrons in the outer orbiting rings find it easy to move from atom to atom whenever they are forced to do so. The force that tends tomove

electrons is called voltage (electrical pressure in a circuit). Voltage is basically the force that causes electrons to travel from atom to atom. As you would expect, the higher the voltage, the more force that is available to move electrons. Some textbooks use the term electromotive force (EMF) when describing voltage. Figure 2-2 shows how voltage “forces” electrons to move from atom toatom.

As shown in Figure 2-2, electrons move from atom to atom to take up a spot vacated by the previous electron. Electrons flow in an orderly manner through a conductor. A typical comparison is to compare water flow in a pipe with that of electron flow in a conductor (Figure 2-3).

When you turn on a water faucet, a certain amount of water pressure forces water through the pipe and out theend of the faucet. The exact same phenomena holds true for electricity.

When you turn on a light, you allow voltage (force) to push electrons (current) through the wire and cause the light to illuminate. At this point, the obvious question is, Why is it necessary to move electrons in the first place?

The reason is fundamental: every electrical user (light bulb, TV, toaster, etc.) hasresistance, measured in ohms (O). The user of electricity is called an electrical load. Figure 2-4 shows a simple fluid and electrical circuit and illustrates the relationship of voltage, current, and resistance.

As seen in the figure, a simple fluid circuit consists of a pump to supply the source of water pressure. The water fountain is considered the load. The pipes provide the path for the water toflow and also provide a certain amount of resistance to flow.

A simple electrical circuit consists of a source of electrons (battery), a load (light bulb), and conductors (wires) to complete the circuit.

Several devices have been added to measure what is happening in the fluid and electrical circuits. In the fluid circuit, a flow meter is measuring how many gallons per minute are beingpumped. A pressure meter is also used to measure water pressure is in the system.

In the electrical circuit, an ammeter is used to measure the rate of electron flow (ampere flow per second or how many electrons are used). A voltmeter is used to measure the electrical pressure available in the circuit. The basic principle is that it takes voltage (electrical pressure) to force current (electrons) toovercome resistance. Essentially, the more the restriction in the water nozzle of the fountain, the more water pressure is needed to overcome the resistance. Similarly, the more electrical resistance in the circuit, the more electrical pressure (voltage) is needed to overcome the resistance and light the bulb.

With this general background, we will now look at a DC voltage waveform and review...