Name: Meter Bridge
Uploaded: 2019-07-19
Duration: 7 min 2 s
Description: Measurement, by comparison, is always accurate. The video talks about a metre bridge device that helps us find the unknown resistance by comparing it with a...

Question

Write the underlying principle of potentiometer. Draw the current diagram of the experimental set-up used for determining the internal resistance of a cell by potentiometer. Write the necessary formula used.

Accepted Answer

Principle of potentiometer: If constant current is flowing through a wire of uniform area of cross-section at constant temperature, the potential drop across- any portion of wire is directly proportional to the length of that portion

$V \propto l$

Determination of internal resistance of potentiometer.

(i) Initially key $K$ is closed and a potential difference is applied across the wire $A B$ . Now rheostat ( $R h$ ) is so adjusted that on touching the jockey $J$ at ends $A$ and $B$ of potentiometer wire, the deflection in the galvanometer is on both sides. Suppose that in this position the potential gradient on the wire is $k$ .

(ii) Now key $K_{1}$ is kept open and the position of null deflection is obtained by sliding and pressing the jockey on the wire. Let this position be $P_{1}$ and $A P_{1} = l_{1}$ . In this situation the cell is in open circuit, therefore the terminal potential difference will be equal to the emf of cell, i.e.,

$E m f (ε) = k l_{1} . . . (i)$

(iii) Now a suitable resistance $R$ is taken in the resistance box and key $K_{1}$ is closed. Again, the position of null point is obtained on the wire by using jockey $J$ . Let this position on wire be $P_{2}$ and $A P_{2} = l_{2}$ . In this situation the cell is in closed circuit, therefore the terminal potential difference( $V$ ) of cell will be equal to the potential difference across external resistance $R$ , i.e.,

$V = k l_{2} . . . . . . . . . . . . . (i i)$

Dividing equation (i) by (ii), we get

$\frac{ε}{V} = \frac{l_{1}}{l_{2}}$

Therefore, internal resistance of cell,

$r = (\frac{ε}{V} - 1) R = (\frac{l_{1}}{l_{2}} - 1) R$

From this formula $r$ may be calculated.

Write the underlying principle of potentiometer. Draw the current diagram of the experimental set-up used for determining the internal resistance of a cell by potentiometer. Write the necessary formula used.

Important Questions on Current Electricity

Learn and Prepare for any exam you want !