Lecture # 02

Electric ChargeElectric charge is the physical property of

matter that causes it to experience a force when placed in an electromagnetic field.

There are two types of electric charges – positive and negative.

It is designated by symbol q

Electrons in the outer band of an atom can be easily displaced by the application of external force.

When an electron is forced out of an atom, the atom it leaves behind has more protons than electrons. This atom now has a positive charge.

Atoms or molecules of a material can also have an excess of electrons, giving the material a negative charge.

A positive or negative charge is caused by an absence or excess of electrons. The number of protons remains constant.

Elements of an Atom:All matter is made up atoms. Atoms have a

nucleus with electrons in motion around it. The nucleus is composed of protons and neutrons.

Electrons have a negative charge (-). Protons have a positive charge (+). Neutrons are neutral.

ElectronsThe smallest amount of electrical charge

having the quality called negative polarity.Electrons orbit the center of atoms.

Electrons (- charge) are attracted to protons (+ charge), this holds the atom together

Some materials have strong attraction and refuse to loss electrons, these are called insulators (air, glass, rubber, most plastics)

Some materials have weak attractions and allow electrons to be lost, these are called conductors (copper, silver, gold, aluminum)

Electrons can be made to move from one atom to another, this is called a current of electricity.

Surplus of electrons is called a negative charge (-). A shortage of electrons is called a positive charge (+).

A battery provides a surplus of electrons by chemical reaction.

By connecting a conductor from the positive terminal to negative terminal electrons will flow.

ProtonsThe proton is a basic particle with positive

polarity.

Protons are located in the nucleus of atoms along with neutrons, particles which have neutral polarity.

VoltageThe force required to make electricity

flow through a conductor is called a difference in potential, electromotive force (emf), or voltage.

It is designated by symbol VThe unit of measurement for voltage is the

volt which is also designated by the letter V.

VoltageA battery has a positive terminal (+) and a negative

terminal (-). The difference in charge between each terminal is the potential energy the battery can provide.

Water Analogy

Voltage Sources:

CurrentThe flow of free electrons in a material from

one atom to the next atom in the same direction is referred to as current and is designated by the symbol I.

Current

Uniform flow of electrons through a circuit is called current.

WE WILL USE CONVENTIONAL FLOW NOTATION ON ALL SCHEMATICS

CurrentWhen a charge is forced to move because of a

potential difference (voltage) current is produced.

In conductors - free electrons can be forced to move with relative ease, since they require little work to be moved.

So current is charge in motion.The more electrons in motion the greater the

current.

Amperes

Current indicates the intensity of the electricity in motion. The symbol for current is I (for intensity) and is measured in amperes.

The definition of current is: I = Q/T Where I is current in amperes, Q is charge

in coulombs, and T is time in seconds.

1 ampere = 1 coulomb per second

ResistanceOpposition to the flow of current is termed

resistance.

The fact that a wire can become hot from the flow of current is evidence of resistance.

Conductors have very little resistance.

Insulators have large amounts of resistance.

It is designated by symbol R

Resistance

All materials have a resistance that is dependent on cross-sectional area, material type and temperature.

A resistor dissipates power in the form of heat

Various resistors types

Ohms The practical unit of resistance is the ohm

designated by the Greek letter omega: Ω A resistor is an electronic component

designed specifically to provide resistance.

Resistor Color Code

The End