## Current

• Electricity is the flow of free electrons in a conductor from one atom to the next atom in the same general direction.
• This flow of electrons is referred to as current and is designated by the symbol “I”.
• Electrons move through a conductor at different rates and electric current has different values.
• Current is determined by the number of electrons that pass through a cross-section of a conductor in one second.

• We must remember that atoms are very small. It takes about 1,000,000,000,000,000,000,000,000 atoms to fill one cubiccentimeter of a copper conductor.
• This number can be simplified using mathematical exponents. Instead of writing 24 zeros after the number 1, write 1024.
• Trying to measure even small values of current would result in unimaginably large numbers.
• For this reason current is measured in amperes which are abbreviated “amps”.
• The symbol for amps is the letter “A”.
• A current of one amp means that in one second about 6.24 x 1018 electrons move through a cross-section of conductor.
• These numbers are given for information only and you do not need to be concerned with them.
• It is important, however, that the concept of current flow be understood.

## Units of measurement for current

Thefollowingchartreflectsspecialprefixesthatareusedwhendealingwithverysmallorlargevaluesofcurrent:

 Prefix Symbol Decimal 1 kiloampere 1 milliampere 1 microampere 1 kA 1 mA 1 µA 1000 A 1/1000 A 1/1,000,000 A

## Direction of current flow

• Some authorities distinguish between electron flow andcurrent flow.
• Conventional current flow theory ignores the flow of electrons and states that current flows from positive to negative.

## Voltage

• Electricity can be compared with water flowing through a pipe.
• A force is required to get water to flow through a pipe. This force comes from either a water pump or gravity.

• Voltage is the force that is applied to a conductor that causes electric current to flow.
• Electrons are negative and are attracted by positive charges.
• They will always be attracted from a source having an excess of electrons, thus having a negative charge, to a source having a deficiency of electrons which has a positive charge.
• The force required to make electricity flow through a conductor is called a difference in potential, electromotive force (emf), or more simply referred to as voltage.
• Voltage is designated by the letter “E”, or the letter “V”. The unit of measurement for voltage is volts which are designated by the letter “V”.

## Voltage sources

An electrical voltage can be generated in various ways.

• A battery uses an electrochemical process.
• A car’s alternator and a power plant generator utilize a magnetic induction process.
• All voltage sources share the characteristic of an excess of electrons at one terminal and a shortage at the other terminal.
• This results in a difference of potential between the two terminals.

## Voltage circuit symbol

• The terminals of a battery are indicated symbolically on an electrical drawing by two lines.
• The longer line indicates the positive terminal.
• The shorter line indicates the negative terminal.

## Units of measurement for voltage

The following chart reflects special prefixes that are used when dealing with very small or large values of voltage:

 Prefix Symbol Decimal 1 kilovolt 1 millivolt 1 microvolt 1 kV 1 mV 1 µV 1000 V 1/1000 V 1/1,000,000 V

## Resistance

• A third factor that plays a role in an electrical circuit is resistance.
• All material impedes the flow of electrical current to some extent.
• The amount of resistance depends upon composition, length, cross-section and temperature of the resistive material.
• As a rule of thumb, resistance of a conductor increases with an increase of length or a decrease of cross-section.
• Resistance is designated by the symbol “R”.
• The unit of measurement for resistance is ohms (Ω).

## Resistancecircuitsymbols

• Resistance is usually indicated symbolically on an electrical drawing by one of two ways.
• An unfilled rectangle is commonly used.
• A zigzag line may also be used.

• Resistance can be in the form of various components.
• A resistor may be placed in the circuit, or the circuit might contain other devices that have resistance.

## Units of measurement for resistance

The following chart reflects special prefixes that are commonly used when dealing with values of resistance:

 Prefix Symbol Decimal 1kiloohm 1megaohm 1kΩ 1MΩ 1000Ω 1,000,000Ω

AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble