T.L.E 50

"BASIC ELECTRICITY"

A Basic Circuit

We begin our discussion with a simple example circuit – a flashlight (or “electric torch” as the Brits call it). This has three basic components: a battery, a switch, and a light bulb. For our purpose, the flashlight has two possible states: on and off. Here are two diagrams.

Light is Off Light is On

In the both figures, we see a light bulb connected to a battery via two wires and a switch. When the switch is open, it does not allow electricity to pass and the light is not illuminated. When the switch is closed, the electronic circuit is completed and the light is illuminated.

The figure above uses a few of the following basic circuit elements.

We now describe each of these elements and then return to our flashlight example. The first thing we should do is be purists and note the difference between a cell and a battery, although the distinction is quite irrelevant to this course. A cell is what one buys in the stores today and calls a battery; these come in various sizes, including AA, AAA, C, and D. Each of these cells is rated at 1.5 volts, due to a common technical basis for their manufacture. Strictly speaking, a battery is a collection of cells, so that a typical flashlight contains one battery that comprises two C cells or D cells. An automobile battery is truly a battery, being built from a number of lead-acid cells.

A light is a device that converts electronic current into visible light. Nothing surprising here. A switch is a mechanical device that is either open (not allowing transmission of current) or closed (allowing the circuit to be completed). Note that it is the opp door, which allows one to pass only when open.

The Idea of Ground

Consider the above circuit, which suggests a two-wire design: one wire from the battery to the switch and then to the light bulb, and another wire from the bulb directly to the battery. One should note that the circuit does not require two physical wires, only two distinct paths for conducting electricity. Consider the following possibility, in which the flashlight has a metallic case that also conducts electricity.

Physical Connection Equivalent Circuit

Consider the circuit at left, which shows the physical connection postulated. When the switch is open, no current flows. When the switch is closed, current flows from the battery through the switch and light bulb, to the metallic case of the flashlight, which serves as a return conduit to the battery. Even if the metallic case is not a very good conductor, there is much more of it and it will complete the circuit with no problem.

In electrical terms, the case of the battery is considered as a common ground, so that the equivalent circuit is shown at right. Note the new symbol in this circuit – this is the ground element. One can consider all ground elements to be connected by a wire, thus completing the circuit. In early days of radio, the ground was the metallic case of the radio – an excellent conductor of electricity. Modern automobiles use the metallic body of the car itself as the ground. Although iron and steel are not excellent conductors of electricity, the sheer size of the car body allows for the electricity to flow easily.

To conclude, the circuit at left will be our representation of a flashlight. The battery provides the electricity, which flows through the switch when the switch is closed, then through the light bulb, and finally to the ground through which it returns to the battery.

As a convention, all switches in diagrams will be shown in the open position unless there is a good reason not to.

The student should regard the above diagram as showing a switch which is not necessarily open, but which might be closed in order to allow the flow of electricity.

Voltage, Current, and Resistance

It is now time to become a bit more precise in our discussion of electricity. We need to introduce a number of basic terms, many of which are named by analogy to flowing water. The first term to define is current, usually denoted in equations by the symbol I. We all have an intuitive idea of what a current is. Imagine standing on the bank of a river and watching the water flow. The faster the flow of water, the greater the current; flows of water are often called currents.

In the electrical terms, current is the flow of electrons, which are one of the basic building blocks of atoms. While electrons are not the only basic particles that have charge, and are not the only particle that can bear a current; they are the most common within the context of electronic digital computers. Were one interested in electro-chemistry he or she might be more interested in the flow of positively charged ions.

All particles have one of three basic electronic charges: positive, negative, or neutral. Within an atom, the proton has the positive charge, the electron has the negative charge, and the neutron has no charge. In normal life, we do not see the interior of atoms, so our experience with charges relates to electrons and ions. A neutral atom is one that has the same number of protons as it has electrons. However, electrons can be quite mobile, so that an atom may gain or lose electrons and, as a result, have too many electrons (becoming a negative ion) or too few electrons (becoming a positive ion). For the purposes of this course, we watch only the electrons and ignore the ions.

An electric charge, usually denoted by the symbol Q, is usually associated with a large number of electrons that are in excess of the number of positive ions available to balance them. The only way that an excess of electrons can be created is to move the electrons from one region to another – robbing one region of electrons in order to give them to another. This is exactly what a battery does – it is an electron “pump” that moves electrons from the positive terminal to the negative terminal. Absent any “pumping”, the electrons in the negative terminal would return to the positive region, which is deficient in electrons, and cause everything to become neutral. But the pumping action of the battery prevents that. Should one provide a conductive pathway between the positive and negative terminals of a battery, the electrons will flow along that pathway, forming an electronic current.

Materials are often classified by their abilities to conduct electricity. Here are two common types of materials.

Conductor A conductor is an substance, such as copper or silver, through which
electrons can flow fairly easily.

Insulator An insulator is a substance, such as glass or wood, that offers
significant resistance to the flow of electrons. In many of our
circuit diagrams we assume that insulators do not transmit electricity
at all, although they all do with some resistance.

The voltage is amount of pressure in the voltage pump. It is quite similar to water pressure in that it is the pressure on the electrons that causes them to move through a conductor. Consider again our flashlight example.

The battery provides a pressure on the electrons to cause them to flow through the circuit. When the switch is open, the flow is blocked and the electrons do not move. When the switch is closed, the electrons move in response to this pressure (voltage) and flow through the light bulb. The light bulb offers a specific resistance to these electrons, as a result of which it heats up and glows.

As mentioned above, different materials offer various abilities to transmit electric currents. Those materials that easily conduct electrons we call conductors; those that do not we call insulators. Insulators oppose the flow of electrons to a much greater degree than conductors.

We have a term that measures the degree to which a material opposes the flow of electrons; this is called resistance, denoted by R in most work. Conductors have low resistance (often approaching 0), while insulators have high resistance. In resistors, the opposition to the flow of electrons generates heat – this is the energy lost by the electrons as they flow through the resistor. In a light bulb, this heat causes the filament to become red hot and emit light.