Intro to Basic Electricity

Lesson C

The Resistor

The resistor is a device that is common to practically every circuit known to man. Resistors are designed to have specific values of resistance. Resistance opposes the flow of electrons. A resistor is a device that limits the amount of current flowing through a circuit for a particular applied voltage. That is to say... only so much current will flow with a certain amount of voltage and resistance. For example...

The basic unit of resistance is the ohm. 1 Ohm of resistance is defined as ... the resistance of a circuit in which a 1-amp current flows when 1 volt is applied.

The Inductor

One of the most fascinating things about electricity is how it can be used to create a magnetic field. Electromagnets were used in telegraph relays for years before the advent of radio. Coils are still used today in relays, solenoids, transformers, and inductors. Basically, current flows through a coil of wires and the energy from the current flow produces a magnetic field. This ability to store electrical energy in a magnetic field is called inductance.

The inductance of a coil would change depending on the number of turns of wire, the length of the coil, and the diameter of the coil. There is a basic unit for inductance ... the Henry (abbreviated "H").

One application of the inductor is to limit the flow of alternating current while allowing direct current to pass freely.

The Capacitor

Another way that electrical energy can be stored is in an electric field. A good example of an electric field is static electricity and static buildup. Many of you that live in the North where it gets very dry in the cold winter months have experienced the sensation of static electricity when shaking hands with one another, or touching a door knob, refridgerator handle, or light switch. This is electrical energy that has been built up and stored in you (and your clothing). Another example is seen in lightning. The clouds build up a static charge that is exhibited in great bolts of static electricity leaping from cloud to cloud, or cloud to ground. The capacitor is merely a device that will store certain amounts of static electricity ... creating what is known as an electric field. Capacitance is the ability to store an electric field. The basic unit of capacitance is the Farad (abbreviated "F").

In a capacitor, there are two electric plates separated by an insulating material (plastic, glass, air...). These two plates (or in some cases... series of plates - large capacitors often have many plates hooked up together to increase the capacitance) are hooked up to two leads that allow the current to flow in and out of the capacitor. As the current flows, electrons build up on one plate. At the same time, electrons flow out of the other plate. Eventually, the capacitor is completely "charged up" and no more current will flow. There is a positive charge on one plate and a negative charge on the other plate. No more current will flow, because the voltage is not able to charge the plates up any higher.

Now if the current was only a direct current, a capacitor would eventually become charged up and no more current would flow through the capacitor. BUT, if the current was an alternating current, and the current switched directions backwards, then the capacitor would start to "uncharge" as the current began to flow the opposite direction.

One application of capacitors in electronic circuits is to block the flow of direct current while allowing alternating current to pass.

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