Actual Meaning of Battery and EMF

    Battery and EMF :- 

What is battery? 

What is EMF?

Let's have some feeling of these two!

A battery is a device which maintains a potential difference between its two terminals A and B.

  

Some " internal mechanism" exerts forces on the charges of the battery material (chemical of the battery). This force drives the postive charges of the battery towards A and the negative charges towards B. The force on a positive charge q is shown as Fb.  As positive charges accumulate on A and negative charges on B, a potential difference develops and grows between A and B. An electric field E is developed from A to B and exerts a force Fe= qE on a charge q.The direction of this force is opposite to that of Fb. In steady state, Fb=Fe and so no further accumulation of charge takes place.

If a charge q is taken from the terminal B to A , the work done by the battery force Fb is W=Fb(d) where d is the distance between A and B. This work per unit charge is e= W/q = Fb(d)/q.

This quantity e is called EMF . The full form of EMF is electromotive force.

The name is misleading in the sense that emf is not a force,it is work done per unit charge by the battery force.

If nothing is connected externally between terminals A and B,then 

    Fb=Fe=qE

or Fb(d)=qEd=qV

where V is the potential difference between the terminals A and B. Thus,

         e=Fb(d)/q =V

Therefore,the EMF of a battery equals the potential difference between its terminals when the terminals are not connected externally.

** It is very important to remember that the battery force Fb is non-electrostatic in nature. The potential difference V is generated by the electrostatic force of postive and negative charges on the two terminals A and B.

Now suppose the terminals of a battery are connected by a conducting wire as shown in the figure below:

As the terminal A is at higher potential than B,there is an electric field E in the wire.The free electrons in the wire move opposite to E and enter the battery at the terminal A.Some electrons are withdrawn from B which enter the wire.

Thus, the potential difference between the terminals A and B tends to decrease.Due to this,the electric field also decreases inside the battery.But the force Fb remains unchanged.Thus,there is now a net force on the charges from B to A. The positive charges start to move towards A to neutralize the incoming electrons from the wire and negative charges rush towards B.

This again develops potential difference V between A and B. Thus,the battery force maintains the potential difference between A and B at the constant value. That's how a battery maintains the potential difference between its two terminals even when connected externally with wires containing resistors. 

***  How is battery charged? 

Ans: In the charging process process of the battery, positive charges enter the postive terminals of the battery and and negative charges enter the negative terminal. The battery gains electrons at its negative terminal and positive charges at its postive terminal. 

# Keep in mind that all the electric phenomenon involves only the motion of electrons and not protons or nucleus. We actually think of the motion of positive charges in wires or battery as the motion of electron in the opposite direction.

      Thank you for giving your time to read!

Author:  Scientificirfan

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