JEE Main · 2020 · 07 Jan 2020 · Shift 2 · Current Electricity

The balancing length for a cell is 560 cm in a potentiometer experiment. When an external resistance of

10Ω

is connected in parallel to the cell, the balancing length changes by 60 cm. If the internal resistance of the cell is

\frac{N}{10} Ω

, where N is an integer then value of N is ______.

Answer: 12

Solution

In a potentiometer the balancing length is proportional to the cell's terminal voltage, so the internal resistance follows

r = (\frac{ℓ _{1} - ℓ _{2}}{ℓ _{2}}) R

. With the length dropping from

560

cm to

500

cm when a

10 Ω

resistor is placed across the cell,

r = \frac{560 - 500}{500} \times 10 = 1.2 Ω

. Writing this as

\frac{N}{10}

gives

N = 12

JEE Main · 2020 · 07 Jan 2020 · Shift 2 · Integer answer