Wheatstone and meter bridge: Class 12 practice
A Wheatstone bridge is balanced (no galvanometer current) when P/Q = R/S. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Ratios
The idea behind wheatstone and meter bridge
A Wheatstone bridge is balanced (no galvanometer current) when P/Q = R/S. In a meter bridge with balance length l cm, R/S = l/(100 − l).
Step-by-step method
- Label P, Q, R, S around the bridge.
- Balance: P/Q = R/S.
- Meter bridge: X/R = l/(100 − l).
Worked example 1 (Basic)
A Wheatstone bridge has P = 20 Ω, Q = 8 Ω, R = 3 Ω. Find S for balance.
- P/Q = R/S
- S = QR/P = 1.2 Ω
- Answer: 1.2 Ω
Worked example 2 (Standard)
A Wheatstone bridge has P = 14 Ω, Q = 6 Ω, R = 2 Ω. Find S for balance.
- P/Q = R/S
- S = QR/P = 0.857 Ω
- Answer: 0.857 Ω
Worked example 3 (Challenge)
In a meter bridge, balance is at 68 cm with the unknown X in the left gap and 10 Ω in the right. Find X.
- X/10 = 68/32
- X = 21.25 Ω
- Answer: 21.25 Ω
Common mistakes and misconceptions
- Pairing the wrong arms
- Using l/100 instead of l/(100 − l)
Practice questions
- (Basic) A Wheatstone bridge has P = 3 Ω, Q = 12 Ω, R = 1 Ω. Find S for balance.
- (Basic) A Wheatstone bridge has P = 16 Ω, Q = 8 Ω, R = 15 Ω. Find S for balance.
- (Standard) A Wheatstone bridge has P = 13 Ω, Q = 5 Ω, R = 10 Ω. Find S for balance.
- (Standard) A Wheatstone bridge has P = 14 Ω, Q = 17 Ω, R = 3 Ω. Find S for balance.
- (Challenge) In a meter bridge, balance is at 69 cm with the unknown X in the left gap and 10 Ω in the right. Find X.
- (Challenge) In a meter bridge, balance is at 23 cm with the unknown X in the left gap and 15 Ω in the right. Find X.
Answer key with full working
- Q1: P/Q = R/S → S = QR/P = 4 Ω → Answer: 4 Ω
- Q2: P/Q = R/S → S = QR/P = 7.5 Ω → Answer: 7.5 Ω
- Q3: P/Q = R/S → S = QR/P = 3.846 Ω → Answer: 3.846 Ω
- Q4: P/Q = R/S → S = QR/P = 3.643 Ω → Answer: 3.643 Ω
- Q5: X/10 = 69/31 → X = 22.258 Ω → Answer: 22.258 Ω
- Q6: X/15 = 23/77 → X = 4.481 Ω → Answer: 4.481 Ω
Syllabus note
This skill is listed under "Current Electricity" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is wheatstone and meter bridge in?
"Current Electricity", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.