Q.10 Physics ⭐ 360° Option Elimination Active

Vault ↗

NEET 2023 Physics Question 10 Solution

Topic: Current Electricity · Official NTA Booklet Code G5

Question

If the galvanometer G does not show any deflection in the circuit shown, the value of R is given by:

✓

Verified Against NTA Final Answer Key

Option (4) : 100 Ω

📚 Current Electricity Easy

📋 Given Parameters & Problem Setup

Primary Battery 10 V

Series Primary Resistor 400 Ω

Secondary Null-Balance Battery 2 V

Condition Galvanometer G shows zero deflection (i_g = 0)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Voltage Divider Ratio Mental Shortcut

⏱️ Save ~45 seconds

Zero deflection means voltage across R is exactly 2 V! The 10 V battery splits into 8 V across 400 Ω and 2 V across R. Since 2 V is 1/4th of 8 V, R = 400/4 = 100 Ω in 10 seconds!

1 Null deflection (i_g = 0) means no current enters the galvanometer branch.

2 Therefore, the voltage across resistor R must exactly equal the secondary battery: V_R = 2 V.

3 Since total supply is 10 V, the remaining voltage across the 400 Ω resistor is 10 - 2 = 8 V.

4 By simple proportion of series resistors: R / 400 = V_R / V_400 = 2 / 8 = 1 / 4.

5 R = 400 / 4 = 100 Ω. Instantly pick Option (4)!

🛡️

Why You Can 100% Trust This Shortcut: Fundamental DC Circuit Theory · Kirchhoff's Laws & Voltage Division (Series voltage divider ratio: V₁/V₂ = R₁/R₂).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Significance of Null Deflection in Galvanometer Branch

When the galvanometer G shows zero deflection, the current flowing through its branch is zero (i_g = 0). By Kirchhoff's Current Law at the junction, the secondary branch is completely isolated in terms of current flow:

i_g = 0 \implies V_R = 2\text{ V}

Step 2

Current in the Primary Series Loop

All current from the 10 V source circulates exclusively through the series combination of 400 Ω and R:

I = \frac{10}{400 + R}

Step 3

Voltage Balance across Resistor R

The potential drop across resistor R is given by Ohm's law and must balance the 2 V battery:

V_R = I \times R = \left(\frac{10}{400 + R}\right) R = 2\text{ V}

Step 4

Algebraic Solution for R

Solve the linear equation for resistance R:

10 R = 2(400 + R) \implies 10 R = 800 + 2 R \implies 8 R = 800 \implies R = 100\ \Omega

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

If R were 400 Ω, V_R would be 5 V (half of 10 V), creating a 3 V unbalance across the 2 V cell.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate assumed symmetrical resistance division (R = 400 Ω).

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

If R were 200 Ω, V_R = (200/600) × 10 = 3.33 V ≠ 2 V.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate incorrectly multiplied 100 Ω by 2.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

If R were 50 Ω, V_R = (50/450) × 10 = 1.11 V ≠ 2 V.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate made an arithmetic division slip, dividing 100 Ω by 2.

4 (4)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate value producing exact 2 V drop across R to achieve galvanometer null deflection.

💡

Why this is airtight: None. Candidate correctly applied the potentiometer balance condition.

⚠️

NTA Trap Alert: Applying Wheatstone Bridge Formula in an Unrelated Two-Battery Loop

Some students attempt to treat this circuit as a Wheatstone bridge and search for four ratio arms. This is simply a potentiometer-style null balance circuit.

🛡️ How to avoid the -1 mark: Whenever a galvanometer has a battery in series and shows zero deflection, set the node voltage equal to that battery voltage!

📖

Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Current Electricity Standard Benchmark

Core Scientific Principle Tested:

"At null balance (i = 0), the branch draws no current and the terminal potential difference across the parallel resistor equals the secondary EMF."

Syllabus Focus: Kirchhoff's Laws and Null Deflection Circuit Analysis

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2023 CBT Test →

← Previous (Q.9) All Physics Questions Grid Next (Q.11) →

🔗 Related Questions — Current Electricity

All 21 Current Electricity PYQs →

NEET 2026 Q36

Current Variation with Temperature for Series and Parallel Dual-Coefficient Resistors

View Solution →

NEET 2025 Q6

Spotted a mistake in this solution? Report an error (support@mycompetitiveexams.com)

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2023 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2023 Physics Question 10 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Voltage Divider Ratio Mental Shortcut

Step-by-Step Rigorous Workflow & Mathematical Proof

Significance of Null Deflection in Galvanometer Branch

Current in the Primary Series Loop

Voltage Balance across Resistor R

Algebraic Solution for R

360° Option Elimination Engine Key Feature

NTA Trap Alert: Applying Wheatstone Bridge Formula in an Unrelated Two-Battery Loop

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Current Electricity

Current Variation with Temperature for Series and Parallel Dual-Coefficient Resistors

Wheatstone Bridge Reduction & Battery Branch Current

Series and Parallel Combination of Resistor Pieces Cut from Single Wire

Current Through Cross-Connecting Wire CD in Resistor Network

Solve 4 NEET Questions a Day — One per Subject