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

State the working principle of potentiometer, explain with the help of circuit diagram how the emf of two primary cells are compared by using the potentiometer.

Accepted Answer

The potentiometer is basically a long piece of uniform wire, sometimes a few meters in length across which a standard cell is connected. A current $I$ flows through the wire which can be varied by a variable resistance (rheostat, $R$ ) in the circuit. Since the wire is uniform, the potential difference between $A$ and any point at a distance $l$ from $A$ is given by,

$E (l) = ϕ l$ where $ϕ$ is the potential drop per unit length.

For comparing the emf $E_{1}$ and $E_{2}$ of two primary cells the potentiometer is connected as shown in the below circuit:

Here, the points marked $1$ , $2$ , $3$ form a two way key. Consider first a position of the key where $1$ and $3$ are connected so that the galvanometer is connected to $E_{1}$ . The jockey is moved along the wire till at a point $N_{1}$ , at a distance $l_{1}$ from $A$ , there is no deflection in the galvanometer. We can apply Kirchhoff's loop rule to the closed loop $A N_{1} G 31 A$ and get,

$ϕ l_{1} + 0 - E_{1} = 0$

Similarly, if another emf $E_{2}$ is balanced against $l_{2} (A N_{2})$

$ϕ l_{2} + 0 - E_{2} = 0$

From these equations, we get, $\frac{E_{1}}{E_{2}} = \frac{l_{1}}{l_{2}}$ .

Thus, using this equation, we can compare the emfs of any two primary cells.

State the working principle of potentiometer, explain with the help of circuit diagram how the emf of two primary cells are compared by using the potentiometer.

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