Question

In potentiometer experiment, null point is obtained at a particular point for a cell on potentiometer wire 'x' cm long. If length of potentiometer wire is increased by few cm without changing the cell, the balancing length will (Driving source is not changed)

• A. decrease
• B. increase
• C. not change
• D. become zero

Hint:

The potential gradient falls when the wire becomes longer, so the balancing length increases.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In a potentiometer, the potential gradient is \(k=\dfrac{V}{L}\), where \(V\) is the driving voltage across the whole wire of length \(L\). The null point is at \(l=\dfrac{E}{k}\).

Step 2: Effect of lengthening the wire:
With the same driving source, the same voltage is spread over a longer wire (and the longer wire has more resistance, so the current drops). The potential gradient \(k\) becomes smaller.

Step 3: Effect on balancing length:
Since the cell emf \(E\) is the same, \(l=\dfrac Ek\) increases. Option B.

Step 4: Why the other options are wrong.
Decreasing or no change would need the gradient to rise or stay the same. A balancing length of zero is not possible for a real cell.

Final Answer:
The balancing length increases. \[ \boxed{\text{(B) }\text{increase}} \]