Q.6 Physics ⭐ 360° Option Elimination Active
NEET 2025 Physics Question 6 Solution
Topic: Current Electricity · Official NTA Booklet Code 45
Question
The current passing through the battery in the given circuit, is:
✓
Verified Against NTA Final Answer Key
Option (2) : 0.5 A
📚 Current Electricity Medium
📋 Given Parameters & Problem Setup
Bridge Resistors R_AB = 5 Ω, R_BC = 2.5 Ω, R_AD = 6 Ω, R_FE = 3 Ω, R_ED = 1.5 Ω
Short Circuits (Zero-Resistance Wires) Line AF (V_A = V_F) and Line CD (V_C = V_D)
External Series Resistors 1.5 Ω + 5.5 Ω = 7 Ω connected to positive, 1/3 Ω connected to negative
Battery EMF 5 V
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⏱️ Save ~60 seconds The 30-Second Exam Hall Shortcut: ⚡ Balanced Wheatstone Spotting Rule
R_AB/R_BD = 5/2.5 = 2 and R_AE/R_ED = 3/1.5 = 2 ⟹ 6 Ω Resistor Carries ZERO Current ⟹ R_eq = 10 Ω ⟹ I = 0.5 A!
1 Notice wires AF and CD collapse nodes: Node F = Node A, and Node C = Node D.
2 Between terminal B and terminal E, four resistors form a Wheatstone bridge with central 6 Ω resistor connecting A and D.
3 Check ratio: 5 / 2.5 = 2 and 3 / 1.5 = 2. Bridge is perfectly BALANCED! Discard the 6 Ω resistor.
4 Upper branch = 5 + 3 = 8 Ω; Lower branch = 2.5 + 1.5 = 4 Ω.
5 Bridge resistance R_BE = (8 × 4) / (8 + 4) = 32/12 = 8/3 Ω.
6 Add external series resistors: R_total = 8/3 + 1/3 + 7 = 3 + 7 = 10 Ω.
7 Current from 5 V battery: I = V / R_total = 5 / 10 = 0.5 A.
8 Select Option (2) in under 30 seconds!
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Why You Can 100% Trust This Shortcut: Balanced Wheatstone Bridge Condition (NCERT Physics · Current Electricity: R1/R2 = R3/R4).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Node Potential Identification
In the circuit diagram, line AF is a plain conducting wire, which implies V_A = V_F. Similarly, line CD is a plain conducting wire, meaning V_C = V_D. Redrawing the internal mesh between nodes B and E:
- • Resistor R_FE (3 Ω) is effectively connected between node A and node E.
- • Resistor R_BC (2.5 Ω) is effectively connected between node B and node D.
- • Resistor of 6 Ω is connected diagonally across node A and node D.
Step 2
Wheatstone Bridge Balance Condition
Examine the ratios of the arms connected between terminals B and E:
- • Because the ratios are equal, potential at node A equals potential at node D (V_A = V_D).
- • No current flows through the 6 Ω bridge resistor; it can be removed from the circuit.
Step 3
Equivalent Resistance & Total Current
With the 6 Ω resistor removed, the upper series arm (5 + 3 = 8 Ω) is in parallel with the lower series arm (2.5 + 1.5 = 4 Ω):
- • The external circuit contains series resistors: (1.5 + 5.5) = 7 Ω and (1/3) Ω.
- • Total resistance: R_total = R_BE + 7 + 1/3 = 8/3 + 1/3 + 7 = 3 + 7 = 10 Ω.
- • Total current through battery: I = V / R_total = 5 V / 10 Ω = 0.5 A.
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 A
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Obtained if internal bridge resistance is miscalculated as zero.
⚠️
Student Fallacy (Why candidates pick this trap): Assuming short circuit across terminals.
2 B
✓ 100% Correct Verdict & Scientific Reason:
100% correct. Balanced Wheatstone bridge gives R_BE = 8/3 Ω, total R = 10 Ω, yielding I = 5/10 = 0.5 A.
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Why this is airtight: None; elegant network reduction.
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Dividing battery voltage by internal branch only.
⚠️
Student Fallacy (Why candidates pick this trap): Ignoring the series resistors in the external branch.
4 D
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Calculation mistake in parallel combination.
⚠️
Student Fallacy (Why candidates pick this trap): Adding branch resistances in parallel incorrectly.
⚠️
NTA Trap Alert: Hidden Wheatstone Bridge & Wire Short Traps
Students often fail to recognize that wire AF and wire CD collapse nodes into common potentials, making the circuit look like an unsolvable 5-mesh network instead of a classic balanced Wheatstone bridge.
🛡️ How to avoid the -1 mark: Always label identical potential nodes on bare wires first.
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Core NEET Syllabus Standard
NEET Physics Core Syllabus · Current Electricity Standard Benchmark
Core Scientific Principle Tested:
"When a Wheatstone bridge is balanced, no current flows through the galvanometer/central arm."
Syllabus Focus: Kirchhoff’s Rules & Wheatstone Bridge
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Collapse bare wires, check arm ratios: 5/2.5 = 3/1.5 ⟹ 6 Ω is DEAD!
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