**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 kiloampere1 milliampere1 microampere |
1 kA1 mA1 µA |
1000 A1/1000 A1/1,000,000 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).

**Resistancecircuit****symbols**

- 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