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Magnetism and Matter
Physics · Class 12 · Part 1NCERT pp. 136–1492 sets × 25 questions
Key facts and formulas for Magnetism and Matter, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Key facts for this chapter
| Idea | Rule or fact |
|---|
| Torque and energy of dipole | τ = m B sin θ; U = −m B cos θ; work = m B (cos θ₁ − cos θ₂) |
| Short magnet fields | Axial: (μ₀/4π) 2m/d³; equatorial: (μ₀/4π) m/d³, half of axial |
| Field lines and Gauss law | Closed loops; total flux through a closed surface is zero; no monopoles |
| Material types | Dia: χ small negative; Para: small positive; Ferro: χ ≫ 1; superconductor: χ = −1 |
| Relations | μr = 1 + χ; B = μ₀(1 + χ)H; M = m/V; solenoid H = nI |
| Curie law | Paramagnet χ ∝ 1/T; diamagnet nearly temperature independent; above Curie temperature a ferromagnet is paramagnetic |
| Earth's field | H = B_E cos δ, Z = B_E sin δ, tan δ = Z/H; declination is the angle between meridians |
| Special places | Equator: δ = 0, H maximum; pole: δ = 90°, H = 0; apparent dip tan δ' = tan δ / cos α |
| Hysteresis and materials | Loop area = energy loss per volume per cycle; electromagnet core: soft iron; permanent magnet: high retentivity and coercivity |
| Cut and bent magnets | Cutting across halves the moment; semicircle bend gives 2M/π; Bohr magneton ≈ 9.27×10⁻²⁴ A m² |
Three sample questions
Q1. Statements:
I. A bar magnet cut into two pieces gives two weaker magnets, each with an N and an S pole.
II. Isolated N poles can be obtained by cutting a magnet at its middle.
Which is correct?
- II only
- I only
- Both I and II
- Neither I nor II
Hint and solution
Hint: Think about what you actually get when you snap a magnet. Has anyone ever found a lone north pole?
- Cut magnet → two complete magnets — Breaking a bar magnet gives two smaller magnets, each with its own N and S pole. So I is true.
- Magnetic monopoles never found — Unlike electric charges, a lone north or south pole has never been observed. So II is false.
- Only I is correct — I is true and II is false, so the correct choice is I only. Option B.
Answer: B. Isolated poles (monopoles) have never been found. Cutting gives complete weaker magnets.
Q2. Which statement about magnetic field lines is NOT correct?
- They are closed loops
- Their tangent gives the direction of B
- Two lines never cross
- They start at the north pole and end at the south pole
Hint and solution
Hint: Field lines of a magnet continue inside the magnet. Do they really stop at the south pole?
- Magnetic lines are closed loops — Outside a magnet lines go from N to S, and inside they return from S to N.
- Check each statement — Closed loops, tangent gives B, and no crossing are all true properties.
- Lines do not end at south pole — Saying they start at N and end at S ignores the part inside the magnet. This is the wrong statement. Option D.
Answer: D. Outside the magnet they run N to S, but inside they continue S to N, so each line is a closed loop.
Q3. A magnet of moment 3 A m² makes 30° with a uniform field of 0.4 T. The torque on it is
- 0.6 N m
- 1.2 N m
- 1.04 N m
- 0.3 N m
Hint and solution
Hint: Use torque = m B sin θ with θ the angle between m and B. What is sin 30°?
- τ = m B sin θ — Torque on a magnetic dipole in a uniform field is m B sin θ.
- τ = 3 × 0.4 × sin 30° — Put m = 3 A m², B = 0.4 T and sin 30° = 0.5.
- 3 × 0.4 = 1.2; 1.2 × 0.5 = 0.6 N m — Multiply: 3 times 0.4 is 1.2, and half of it is 0.6 newton metre. Option A.
Answer: A. τ = m B sin θ = 3 × 0.4 × 0.5 = 0.6 N m.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.