FUNDAMENTALS OF ELECTRICITY

Unit of charge:

Coulomb is used as the unit of charge.

Charge of an electron is,

e = -1.60217733 x 10-19 C

Electric current:

Directed flow of free electrons(charge)is called electric current.

Actual direction of current is from negative terminal to positive terminal.

Current flowing from positive terminal to negative terminal is called conventional current.

Types of electric current:

Electric current is classified into the following: -

1. Steady current
2. Varying current
3. Alternating current

Steady Current: When the magnitude of current does not change with time, it is called as steady current.

Varying current: When the magnitude of current changes with time, it is called as varying current.

Alternating current: When the magnitude of current changes continuously with time and direction changes periodically, it is called as alternating current.

Electric Potential:

The capacity of a charged body to do work is called as electric potential.
Electric potential, V = Work-done/Charge = W/Q
Unit of Electrical potential, V = Joule/Coulomb = Volt

Potential Difference:

The difference in the potentials of two charged bodies is called potential difference.

It is also called as voltage.

The current will flow in a circuit if a potential difference exists in the circuit.

Resistance:

The opposition of the flow of electric current is called its resistance.

Unit of resistance is Ohm and is denoted by Ω.

Factors upon which resistance depends:

The resistance of a conductor,

• Is directly proportional to its length(l).
• Is inversely proportional to its cross sectional area(a).
• Depends upon the nature of the material.
• Changes with temperature.

i.e.,

Rα l/a

R=pl/a

Where,

P=Resistivity

Resistivity or specific resistance:

It is defined as the amount of resistance offered by a material of length ‘l’ and a cross sectional area ‘a’.

It is denoted by p.

Effect of temperature on resistance:

1. The resistance of pure metal (copper, aluminum), conductors increases with increase in temperature. It has a positive temperature coefficient of resistance.
2. Resistance of electrolytes, insulators, semiconductors (silicon, germanium) decreases with increase in temperature. It has a negative temperature coefficient of resistance.
3. Resistance of alloys increases with increase in temperature but this increase is very small and irregular.

Ohm’s law:

It states that the ratio of potential difference (v) across the end of a conductor to the current (I) flowing between them is constant, provided the physical condition (temp) does not change.

V/I = Constant = R

Electric Power: Rate at which work is done in an electrical circuit is called electric power.

Electric power = Work done in electric circuit/Time

P = VIt/t = VI = I²R = V²/R

Unit of power is Joule/Sec or watt.

1KW = 1000 watts

1MW = 10⁶ watts = 10³ KW

Electrical Energy:

The total work done in an electrical circuit is called electrical energy.

Electrical energy = Electric power x time

= VIt

=I²Rt

=V²t/R

Its unit is Kwh (Kilowatt- hour)

1Kwh of electrical energy is called Board of time (BOT) unit.

Power Rating:

Ability of a resistor to dissipate heat without destructive temperature built up