Charge, Current , Power , Potential and Potential Difference

Charge and Current
  • The law of conservation of energy states that the charge can neither created nor destroyed, it can only transferable from one place to another place. 
  • Therefore the algebraic sum of all charge in an electrical circuit is equal to zero. 
  • The electrical charge is mobile therefore it can be transferred to one place to another. 
  • The positive charge moves in one direction whereas the negative charge moves in the other direction. 
  • The motion of electrical charge creates electrical current in a closed circuit. 
  • The Benjamin Franklin invented that the conventional direction of electrical current is opposite to negative charge or electron. 
  • There is a following relation between charge and current

                  i = dq / dt
                 q = ∫ i dt

Direct Current ( DC ) or Direct voltage
  • If the magnitude of current or voltage does not change with time, it is called as direct current. 
  • The magnitude of direct voltage and current remains constant with respect to time.

Alternating Current ( AC )  or Alternating voltage
  • If the magnitude of current or voltage changes sinusoidal with time, it is called as alternating current. 
  • The alternating supply may be single phase or three phase.


Voltage or Potential
  • It is defined as the work done in bringing a positive charge of one coulomb from infinity to that point against magnetic field. 
  • The potential is equal to work done per unit charge. 
  • The earth is taken as reference potential zero because the potential of earth remains constant in spite of it keeps and losing charge.

       One voltage = Work done / Charge
                           = Joule / Coulomb
EMF ( Electro – Motive Force )
  • It is force that requires moving electrons to produce an electrical current. It is better known as EMF or battery source. 
  • It is measured in voltage. 
  • The word force is used for electrical potential not for mechanical force.

  • It is source of energy that causes electrical current to flow in the given closed circuit.

Potential Difference
  • It is defined as potential difference of one voltage between two points if one joule of work is done for shifting a point charge from one point to other point. 
  • The potential difference between two points is considered by considering one point as reference potential. 
  • Let us consider that the potential of point is + 10 V as compared to potential of point B. Similarly the potential of point P is – 10 V as compared to potential of point Q.


  • It is defined as rate doing work. It is a scalar quantity. 
  • The unit of power is watt or joule per second in the honour of James watt.

       P = dW / dt
   dW = Rate of work
     dt = Rate of change of time
      P = dW / dt
         = ( dW / dq ) ( dq / dt )
         = VI
         = I2R  ( V = IR )
         = V2 / R ( I = V / R )
  • If the voltage and current are time varying quantity, the power is called as instantaneous power. 
  • Therefore the power is defined as the product of voltage across element and current passes through element. 
  • The power has positive sign if it is delivered to load it means that voltage and current both have positive sign. 
  • The power has negative sign if it is delivered from load it means that either voltage or current has negative sign not both.
  • Therefore

        Power absorbed = – Power supplied
Law of Conservation of Power
  • The algebraic sum of power in a circuit is equal to zero. Therefore

        ∑ P = 0