In a meter bridge experiment, for measuring unknown resistance 'S', the null point is obtained at a distance 30 cm from the left side as shown at point D. If R is 5.6 k Ω, then the value of unknown resistance 'S' will be __________ Ω.

Solution

Physics sir · 4 steps

S sits on the left gap, so it pairs with the left 30 cm of wire; R pairs with the other 70 cm.

1. Balance condition. At the null point no current flows through the galvanometer. The resistance in each gap is then proportional to the length of wire on its own side of D.

   S/R = l/(100 - l)

2. Match the sides. S is in the left gap, so it goes with l = 30 cm. R is in the right gap, so it goes with 100 - 30 = 70 cm.
3. Put in numbers. S = 5600 Ω × 30/70 = 5600 × 3/7 = 2400 Ω.
4. So. The unknown resistance is S = 2400 Ω.

The common slip: 2.4. Leaving the answer in kilohms gives 2.4. The blank asks for ohms, so convert 5.6 kΩ to 5600 Ω first.

The idea to keep. In a metre bridge, each resistance goes with the wire length on its own side of the null point. Check which gap holds the unknown before you write the ratio.

Answer from the NTA final answer key. Solution by PrepWiser.