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Cell,emf, terminal potential difference,internal resistance of the cell

 Physics 

Basic terms»»»

Basic Bhysics

Cell » Cell is a device which provide necessary potential difference to an electric circuit to maintain a continuous flow of current in it .
or
it is a source of energy that maintains a continuous flow of charge in a circuit Electric cell change chemical energy into electrical energy.

EMF ( electromotive force) of a cell »
EMF of a cell may be defined as the potential difference between its terminal i.e when circuit is open.
Or
The work done  by cell in moving unit positive charge through the whole circuit (include the cell ) is called EMF of the cell .
EMF = work/charge
Emf=w/q

SI unit – volt ( J/s)

Terminal potential difference:-
The potential drop across the terminals of a cell when current is being drawn from it is called terminal potential difference (closed circuit).

It is denoted by – 'V'
SI unit – volt

Internal resistance of a cell:-
The resistance offered by the electrolyte of cell to the flow of current between the electrode is called internal resistance of a cell
It is denoted by – ' r'
SI unit – ohm



★Internal resistance of a cell depends upon following factors :-
» Nature of electrolyte.
» Directly proportional to concentration of the electrolyte.
» Directly proportional to the separation distance between the two electrolytes.
» Inversely proportional to the area of the plate dipped into the electrolyte.
»It is increase with the decrease in  the temperature of electrolyte.




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  Potentiometer Questions from potentiometer Q1. Why is a potentiometer preferred over a voltmeter for measuring the emf of a cell? Ans. Potentiometer work on null method, It does not draw any current from the cell thus there there is no potential drop due to the internal resistance of the cell. It measures the potential difference in an open circuit which is equal to the actual EMF of the cell. It is very accurate device. On the other hand, voltmeter draw small current from the cell for its operation. So it measures terminal potential difference in a closed circuit which is less than EMF of the cell Q2. The sensitivity of a potentiometer increases with increase in length of its wire. Explain why? Ans. When the length of the wire increases the fall of potential per unit length of the wire become smaller (i.e potential gradient K= V/L ). Hence even a small potential difference can be measured. Q3. Sometime the balance point in the potentiometer may not be obtained on the wire ...