At rest conditions, the quantity of electricity taken by a body is called a charge. It is denoted by the letter Q that the unit of charge is the Coulomb.

One coulomb = charge of


The voltage is the capacity of a charged body to do some work and is known as the voltage or electric potential.

S.I. unit of voltage is Volt and is denoted by the letter "V".

Potential Difference

The difference in the electrical potentials between two charged bodies is known as the electrical potential difference. The S.I. Unit is Volt.


The capacitor or condenser is an effective device that stores electricity.


The capacitance of a condenser is the property at which it deposits a quantity of electricity between two conducting plates. It is denoted by the letter "C". The S.I. unit of the capacitance is farad and denoted by the letter "F".

Q = CV unit is coulomb.
C = Capacitance of the condenser in farad.
V = Surface potential in volt.


The flow of electrons in a closed circuit is known as electric current. the electric current in the S.I. unit is Ampere. Current denoted by the letter "A".


Resistance is the property of a material by which it opposes the flow of electric current through the material. The S.I. The unit of resistance is ohm and it is denoted by the letter ''R''. The resistance is classified into three categories from the point of measurement :

  1. Low resistance - one ohm or less
  2. Medium resistance - Above one ohm and up to 105 ohms.
  3. High resistance - 105 ohms and above.
The circuit current (I) = Voltage (V) / resistance (R) unit is Ampere.


When an electric current flows through a coil, a magnetic field or flux is created and the electrical energy is stored in the form of a magnetic field. The inductance is the property of a coil by which it opposes any change in current in it. The unit of inductance is Henry "H".

So, v = l.(di/dt)
Inductance 'L' = NΦ/I
Here, N = number of turns.
      Φ = Flux is wb.
      I = current in A.


Work is said to be done by a force when the point of application is displaced along the direction of force or across the direction of the force against the force of gravity. the S.i. unit of work is Newton-metre or Joule. In M.K.S system unit is Kg-m.


Power is the rate of doing work. The S.I. unit of power is Watt. 1 watt = (Joule/Second)

Power = Workdone/Time or, Energy/Time
1 KW = 1000 watt, 1MW = 1000000 watt.
1 horse power = 75kg-m/second = 735.5 watt.


It is the capacity of doing work. Energy = power x time In electrical energy the smaller unit is watt-second or joule and the bigger unit which is used for commercial purposes is kilo watt hour (kW-hr.) Again one Board of Trade unit (B.O.T) = 1 kWhr.

Mechanical energy is composed of two different forms:

(a) Potential energy

(b) Kinetic energy.

There are different types of energy we normally get. These are (a) electrical energy; (b) mechanical energy: (c) sound energy; (d) chemical energy. (e) atomic energy. (f) heat or thermal energy. (g) light energy.

Direct Current (d.c.)

D.C. is the current which is flowing in one direction.

The basic D.C Supply source is composed of:

  1. Two-Wire d.c: One wire is called the positive (+ve) wire and the other wire is called the negative (-ve) wire. This is suitable to feed lighting loads.
  2. Three Wired d.c: It is composed of one (+ve) wire, one (-ve) wire, and one number for the neutral wire. It is used to feed bigger lighting loads. (a few loads to feed bigger lighting loads (a few loads are supplied by (+ve) wire and the neutral and remaining lighting loads are supplied by the negative (-ve) wire and the neutral wire. Power loads (motor loads) are supplied by positive (+ve) wire and negative (-ve) wire. The neutral wire is obtained normally through a special type of electrical equipment known as a balancer set.

Alternating Current (a.c.)

A.C. is the current which changes its amplitude regularly and also the direction of flow with respect to time.

The basic A.C Supply source is composed of:

  1. Three Phase Four Wire (of which three numbers live conductors and one number neutral conductor): This type of supply is used to feed medium voltage motor loads and lighting loads.
  2. Three Phase Three Wire: It is used to feed motor loads only (medium voltage, that is the 415-volt motor.)
  3. Single Phase Supply: It is used to feed lighting loads & fractional horsepower motors (using one live conductor and one neutral conductor).

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