Chapter 17: Magnetic Properties of Materials

17.1 7.1 Magnetic Field Lines and Magnetic Flux

• Magnetic field lines: Imaginary curves showing the direction of the magnetic field.
  • Outside magnet → from North pole to South pole.
  • Inside magnet → from South pole to North pole.
• Properties:
  • Never intersect.
  • Denser lines → stronger field.
  • Continuous closed loops.

Magnetic flux (Φ):

Φ=B⋅A⋅cosθ

where B = magnetic field, A = area, θ = angle between field & area normal.

17.2 Flux Density, Relative Permeability, Susceptibility

17.3 Hysteresis

  When a ferromagnetic material is magnetized and demagnetized repeatedly, a hysteresis loop is formed between B vs H.

  Key terms:

• Retentivity → ability of material to retain magnetization.
• Coercivity → magnetizing field required to reduce B=0.
• Area of loop → energy loss per cycle.

  Applications:

• Permanent magnets → high retentivity & coercivity.
• Transformer cores → low hysteresis loss (small loop).

17.4 Types of Magnetic Materials

Short Questions and Answers in Short

Q1. Define magnetic flux.
👉 Total magnetic field passing through a surface.

Q2. What is the relation between susceptibility and relative permeability?
👉 μr​=1+χ.

Q3. Give one example each of diamagnetic, paramagnetic, and ferromagnetic materials.
👉 Diamagnetic: Copper; Paramagnetic: Aluminium; Ferromagnetic: Iron.

Q4. What is hysteresis?
👉 The lag of magnetization behind the magnetizing force when a ferromagnetic material is magnetized and demagnetized.

Q5. Which type of materials are used for making permanent magnets?
👉 Ferromagnetic materials with high retentivity and coercivity.

Q6. Which type of materials are used in transformer cores? Why?
👉 Ferromagnetic materials with low hysteresis loss, e.g., soft iron.

Q7. Why are diamagnetic materials repelled by a magnet?
👉 Because they develop an induced magnetic moment opposite to the applied field.

Q8. State one property of ferromagnetic materials.
👉 They retain magnetism even after the magnetizing field is removed (retentivity).

✅ Formula Sheet (Quick Revision)

Numerical Problems

Example 1: Magnetic flux through a tilted surface

Example 2: Flux density in a magnetic material

Example 3: Magnetization, permeability and B

Example 4: Hysteresis power loss

Example 5: Identify material type from χ