Units of Resistance

Units of Resistance

To measure current, the unit of measure used is ampere (A). To measure voltage, volt (V) is used. These units are necessary in order to compare different currents and different voltages. In the same manner, a unit of measure is needed to compare the resistance of different conductors. As we have learned earlier that the basic unit of resistance is ohm, equal to that resistance which will allow exactly one ampere of current to flow when one volt of emf is applied across the resistance. When one volt causes one ampere of current flow, the resistance is one ohm. Units of resistance are converted in the same manner as unit of current or voltage.

FACTORS AFFECTING RESISTANCE

Factors Affecting Resistance

As previously discussed, every material offers some resistance for opposition to the flow of current. Some conductors such as copper, silver, and aluminum offer very little resistance to current flow. Insulators such as glass, wood, and paper, offers high resistance to the flow of current.

RESISTANCE

Resistance

You have found earlier that voltage is an electromotive force that forces the electrons to move from one point to another or from one circuit to another and the amount or the number of free electrons that are moving in a particular time is called current, another fundamental quantity in electronics that affects the behavior of electrons is the resistance.

Connecting Cells and Batteries

Connecting Cells and Batteries

To increase voltage and/or current cells and batteries can be connected together. They can be connected in series, parallel, or in series-parallel.

ELECTROMOTIVE FORCE (EMF)

Electromotive Force

In a piece of wire, the electrons move in random without direction. To produce a current flow, all electrons must move in one direction. To produce motion in one direction, energy must be provided to the electrons in the conductor or wire.

SCIENTIFIC NOTATION

Scientific Notation

It is common to encounter very small and very large numbers in electronics. Scientific notation expresses a very large or small number as a numeral from 1 to 9 to a power of ten. For example 5000 in scientific notation is 5x10³.

TWO CONVENTONS OF CURRENT FLOW

Two Conventions of Current Flow

There are two ways of looking at the current flow, the electron current flow, and the conventional current flow.

Electron current flow is the flow of current from low to high potential, or from negative to positive. Benjamin Franklin, an American scientist, studied electricity and was one of the first to actually describe current flow. According to him, the positive charge flows in conductor from high to low potentials or from positive to negative as shown in figure 3. This theory of current flow is called as the conventional current flow. It makes no difference which principle is used to describe electricity for all the effects expected are the same.

ELECTRIC CURRENT FLOW

Electric Current Flow

An electric current is the amount or number of electrons flowing through a conductor or a circuit from an area of negative charge to an area of positive charge. Electric current is represented by letter I when used in a circuit. Letter “I” means “intensity of current flow”. The basic unit used to measure the intensity of current flow is called ampere. It is named after Andre Marie Ampere. Ampere represents the amount of current in the conductor when one coulomb of charge moves past a point in one second. The relationship between amperes and coulomb per second can be expressed as:

ELECTRICAL CHARGE

Electrical Charge

You have learned that electrons travel around the nucleus of an atom and are held in orbits by the attraction of the positive charge in the nucleus. When you force electron out of its orbit, then the electron’s action becomes what is known as electricity.