JEE Main 2025 · 28 January · evening · Numerical answer · +4 / −1
The value of current I in the electrical circuit as given below, when potential at A is equal to the potential at B, will be _______ A.
Answer and solution
Answer 2
Solution
Physics sir · 4 steps
If A and B are at the same potential, the bridge is balanced, so the 30 Ω resistor carries nothing.
- Balanced bridge. The 30 Ω resistor joins A and B. With no potential difference across it, no current flows through it. This is a balanced Wheatstone bridge, so the ratio of resistances on the left equals the ratio on the right.
10/R = 20/40 ⇒ R = 20 Ω
- Remove 30 Ω. Since it carries no current, drop the 30 Ω resistor. The top branch is 10 + 20 = 30 Ω and the bottom branch is 20 + 40 = 60 Ω.
- Equivalent resistance. The two branches are in parallel: R_(eq) = (30 × 60)/(30 + 60) = 20 Ω.
- So. I = (40 V)/(20 Ω) = 2 A, so the answer is 2.
The idea to keep. Equal potential at the two middle junctions means a balanced Wheatstone bridge. Use the ratio condition to find the unknown, then delete the middle resistor before you combine the rest.
Answer from the NTA final answer key. Solution by PrepWiser.