Explain potentiometer and it's application
Answers
The Potentiometer is an electric instrument that used to measure the EMF (electro motive force) of a given cell, the internal resistance of a cell. And also it is used to compare EMFs of different cells. It can also use as a variable resistor in most of the applications. These potentiometers are used in huge quantities in the manufacture of electronics equipment that provides a way of adjusting electronic circuits so that the correct outputs are obtained. Although their most obvious use must be for volume controls on radios and other electronic equipment used for audio.
Potentiometer
Potentiometer
Why is Potentiometer chosen over Voltmeter to measure the potential (EMF) of a cell? When we use Voltmeter, current flows through the circuit and because of the internal resistance of the cell, always terminal potential will be less than the actual cell potential. In this circuit, when the potential difference is balanced (using a Galvanometer null detection), no current flows in the circuit, so the terminal potential will be equal to the actual cell potential. So we can understand that the Voltmeter measures the terminal potential of a cell, but this measures actual cell potential. The schematic symbols of this is shown below.
Schematic Symbols of a Potentiometer
Schematic Symbols of a Potentiometer
Construction and Working Principle
The potentiometer consists of a long resistive wire L made up of magnum or with constantan and a battery of known EMF V. This voltage is called as driver cell voltage. Connect the two ends of the resistive wire L to the battery terminals as shown below; let us assume this is a primary circuit arrangement. One terminal of another cell (whose EMF E is to be measured) is at one end of the primary circuit and another end of the cell terminal is connected to any point on the resistive wire through a galvanometer G. Now let us assume this arrangement is a secondary circuit. The arrangement of the potentiometer as shown below.
Construction of Potentiometer
Construction of Potentiometer
The basic working principle of this is based on the fact that the fall of the potential across any portion of the wire is directly proportional to the length of the wire, provided wire has uniform cross-sectional area and the constant current flowing through it.“When there is no potential difference between any two nodes there is electric current will flow”.
Now the potentiometer wire is actually a wire with high resistivity (ῥ) with uniform cross-sectional area A. Thus, throughout the wire, it has uniform resistance. Now this potentiometer terminal connected to the cell of high EMF V (neglecting its internal resistance) called driver cell or the voltage source. Let the current through the potentiometer is I and R is the total resistance of the potentiometer.
Then by Ohms law V=IR
We know that R= ῥL/A
Thus, V= I ῥL/A
As ῥ and A are always constant and current I is kept constant by a rheostat.
So L ῥ/A=K (constant)
Thus, V= KL. Now suppose a cell E of lower EMF than the driver cell is put in the circuit as shown above. Say it has EMF E. Now in the potentiometer wire say at length x the potentiometer has become E.
E= L ῥx/A=Kx
When this cell be put in the circuit as shown above figure with a jokey connected to the corresponding length (x), there will be no flow of current through the galvanometer because when the potential difference is equal to zero, no current will flow through it. So the galvanometer G show null detection. Then the length (x) is called the length of the null point. Now by knowing the constant K and the length x. We can find the unknown EMF.
E= L ῥx/A=Kx
Secondly, EMF of two cells may also be compared, let the first cell of EMF E1 given a null point at a length= L1 and the second cell of EMF E2 show a null point at length= L2
Then,
E1/E2= L1/L2