Q.21 Physics ⭐ 360° Option Elimination Active
NEET 2023 Physics Question 21 Solution
Topic: Magnetism and Matter · Official NTA Booklet Code G5
Question
The net magnetic flux through any closed surface is:
✓
Verified Against NTA Final Answer Key
Option (2) : Zero
📚 Magnetism and Matter Easy
📋 Given Parameters & Problem Setup
Surface Any arbitrary closed surface
Field Magnetic field B⃗
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⏱️ Save ~30 seconds The 30-Second Exam Hall Shortcut: ⚡ Gauss's Law for Magnetism Universal Invariant
Magnetic monopoles do not exist; every magnetic field line entering a closed surface must exit it, making net magnetic flux strictly ZERO!
1 Recall Maxwell's second equation (Gauss's law for magnetism): ∮ B⃗ · dA⃗ = 0.
2 This physical law reflects the experimental fact that isolated magnetic poles (monopoles) do not exist in nature.
3 Magnetic field lines are continuous closed loops with no starting or ending points.
4 Thus, incoming flux equals outgoing flux through any closed surface, giving a net flux of zero.
5 Instant Option (2) in 2 seconds!
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Why You Can 100% Trust This Shortcut: Fundamental Electromagnetism · Gauss's Law for Magnetism (Maxwell's divergence equation: ∇ · B = 0 → ∮ B · dA = 0).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Statement of Gauss's Law for Magnetism
Gauss's law for magnetism is one of the four fundamental Maxwell equations governing classical electrodynamics. It states that the net magnetic flux Φ_B through any closed surface S is identically zero:
Step 2
Physical Meaning: Non-existence of Magnetic Monopoles
In electrostatics, Gauss's law allows net flux because isolated positive and negative charges exist (∮ E · dA = q_enclosed / ε₀). In magnetism, magnetic poles always occur in equal and opposite dipoles (North and South). The net magnetic charge enclosed by any closed volume is always zero:
Step 3
Continuous Closed Loop Geometry
Magnetic field lines form continuous closed loops without beginning or end. Every field line that enters a closed surface must necessarily emerge from it, ensuring total incoming flux equals total outgoing flux.
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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 (1)
✗ Incorrect Verdict & Scientific Reason:
Net magnetic flux cannot be negative for a closed surface.
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Student Fallacy (Why candidates pick this trap): Candidate confused magnetic flux with electrostatic flux enclosing a net negative charge.
2 (2)
✓ 100% Correct Verdict & Scientific Reason:
Universal physical law: net magnetic flux through any closed Gaussian surface is identically zero.
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Why this is airtight: None. Candidate recalled Gauss's Law for magnetism.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
Net magnetic flux cannot be positive for a closed surface.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate confused magnetic flux with electrostatic flux enclosing a net positive charge.
4 (4)
✗ Incorrect Verdict & Scientific Reason:
Completely unphysical.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate guessed randomly.
⚠️
NTA Trap Alert: Confusing Electric Flux with Magnetic Flux
Electric flux through a closed surface depends on enclosed charge (∮ E · dA = q/ε₀, which can be positive, negative, or zero). Magnetic flux is ALWAYS zero because monopoles do not exist.
🛡️ How to avoid the -1 mark: Remember: Closed surface + Magnetic field = ZERO (always and unconditionally).
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Core NEET Syllabus Standard
National Eligibility cum Entrance Test (NEET UG) · Magnetism and Matter Standard Benchmark
Core Scientific Principle Tested:
"Magnetic field lines form closed continuous loops; isolated magnetic monopoles do not exist, making net flux zero (∮ B · dA = 0)."
Syllabus Focus: Gauss's Law for Magnetism and Magnetic Field Lines
Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗
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