Q.171 Physics ⭐ 360° Option Elimination Active

NEET 2020 Physics Question 171 Solution

Topic: Magnetism and Matter · Official NTA Booklet Code E1

Question

An iron rod of susceptibility 599 is subjected to a magnetising field of 1200 A m⁻¹. The permeability of the material of the rod is: (μ₀ = 4π×10⁻⁷ T m A⁻¹)

✓

Verified Against NTA Final Answer Key

Option (1) : 2.4pi x 10^-4 T m A^-1

📚 Magnetism and Matter Easy

📋 Given Parameters & Problem Setup

Magnetic Susceptibility (chi) 599

Magnetising Field (H) 1200 A m^-1 (surplus data)

Permeability of Free Space (mu0) 4pi × 10^-7 T m A^-1

⚡

The 30-Second Exam Hall Shortcut: ⚡ "mu_r = 1 + chi = 1 + 599 = 600. Permeability mu = mu_r mu0 = 600 × 4pi × 10^-7 = 2400pi × 10^-7 = 2.4pi × 10^-4 T m A^-1!"

⏱️ 20 seconds

Relative permeability is mu_r = 1 + chi = 1 + 599 = 600. Permeability mu = mu_r · mu0 = 600 × 4pi × 10^-7 = 2400pi × 10^-7 = 2.4pi × 10^-4 T m A^-1. Notice that magnetising field H = 1200 A/m is surplus data! Direct 5-second calculation gives Option (1).

1 Formula:

\mu = \mu_0 (1 + \chi_m)

2 Calculate relative permeability:

\mu_r = 1 + 599 = 600

3 Substitute

\mu_0 = 4\pi \times 10^{-7}

\mu = 600 \times 4\pi \times 10^{-7} = 2400\pi \times 10^{-7}

4 Format:

2400\pi \times 10^{-7} = 2.4\pi \times 10^{-4}\text{ T m A}^{-1}

5 Select Option (1).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Physics · Magnetism and Matter (Magnetic Properties of Materials: Susceptibility and Permeability)

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Relationship Between Susceptibility and Relative Permeability

Magnetic susceptibility $\chi_m$ indicates how easily a substance magnetizes in an applied magnetic field. Relative magnetic permeability $\mu_r$ is related to susceptibility by: $\mu_r = 1 + \chi_m$ .

Step 2

Calculate Relative Permeability

Given $\chi_m = 599$ : $\mu_r = 1 + 599 = 600$ .

Step 3

Calculate Absolute Permeability of the Material

Absolute permeability $\mu$ is given by: $\mu = \mu_r \mu_0$ Substitute $\mu_r = 600$ and $\mu_0 = 4\pi \times 10^{-7}\text{ T m A}^{-1}$ $\mu = 600 \times (4\pi \times 10^{-7}) = 2400\pi \times 10^{-7} = 2.4\pi \times 10^{-4}\text{ T m A}^{-1}$ .

Step 4

Identify Extraneous Information

The problem statement specifies $H = 1200\text{ A m}^{-1}$ . Because permeability is an intrinsic material property in the linear regime defined by $\mu = \mu_0(1+\chi)$ , $H$ is superfluous data. Matches Option 1.

⭐

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 A

✓ 100% Correct

Verdict & Scientific Reason:

Accurate value; mu = 600 × 4pi × 10^-7 = 2.4pi × 10^-4 T m A^-1.

💡

Why this is airtight: None; clean calculation of mu = mu0(1 + chi).

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; result of attempting an unnecessary division by H = 1200 A m^-1.

⚠️

Student Fallacy (Why candidates pick this trap): Misusing the extraneous H value.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; power-of-ten exponent arithmetic blunder (10^-5 instead of 10^-4).

⚠️

Student Fallacy (Why candidates pick this trap): Evaluating 2400 × 10^-7 as 2.4 × 10^-5.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; forgot to multiply by the factor of 1000 from relative permeability.

⚠️

Student Fallacy (Why candidates pick this trap): Neglecting the 600 multiplier.

⚠️

NTA Trap Alert: Distractor Data Distraction

NEET often introduces extra variables like H = 1200 A/m to tempt students into convoluted B = mu H calculations. Permeability mu depends only on chi and mu0!

🛡️ How to avoid the -1 mark: Focus strictly on what is asked: Permeability = mu_r · mu0 = (1 + chi) · mu0.

📖

Core NEET Syllabus Standard

Physics · Magnetism and Matter Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Permeability Calculation from Magnetic Susceptibility for Ferromagnetic Iron

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Physics Question 171 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "mu_r = 1 + chi = 1 + 599 = 600. Permeability mu = mu_r mu0 = 600 × 4pi × 10^-7 = 2400pi × 10^-7 = 2.4pi × 10^-4 T m A^-1!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Relationship Between Susceptibility and Relative Permeability

Calculate Relative Permeability

Calculate Absolute Permeability of the Material

Identify Extraneous Information

360° Option Elimination Engine Key Feature

NTA Trap Alert: Distractor Data Distraction

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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