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This MCQ module is based on: Magnetic Materials
TOPIC 22 OF 38
Magnetic Materials
🎓 Class 12
Physics
CBSE
Theory
Ch 5 – Magnetism and Matter
⏱ ~14 min
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Magnetic Materials
5.6 Magnetic Properties of Materials
Different substances respond differently to an applied magnetic field. Based on the sign and magnitude of χ, materials fall into three classes - diamagnetic, paramagnetic, and ferromagnetic.
5.6.1 Diamagnetism
A diamagnetic material is one in which the induced magnetisation \(\vec M\) opposes the applied field \(\vec H\), so χ is small and negative.
Microscopic origin: in a diamagnetic atom, the electrons in pairs leave a zero net magnetic moment. When an external field is applied, the orbital motion of each electron is perturbed (Lenz-like reaction at the atomic scale), producing a small induced moment opposing the field. Examples: bismuth, copper, water, lead, gold, hydrogen.
A perfect diamagnet (χ = −1) excludes magnetic field lines completely. Superconductors are perfect diamagnets - the famous Meissner effect.
5.6.2 Paramagnetism
A paramagnetic material has atoms with a net non-zero magnetic moment (e.g. odd electron, unpaired spin). Without a field these moments point randomly. An applied H aligns them slightly, giving small positive χ.
Examples: aluminium, calcium, oxygen (gas/liquid), platinum, sodium.
Curie's law: For a paramagnetic substance at temperature T,
\[\chi = \dfrac{C}{T}\]
where C is the Curie constant. The susceptibility falls as the temperature rises - thermal motion randomises the dipoles.5.6.3 Ferromagnetism
In a ferromagnetic material the atomic dipoles are not just paramagnetic - they spontaneously align with their neighbours due to a strong quantum-mechanical exchange interaction. Each region of aligned dipoles is called a domain.
In an unmagnetised piece of iron, domains point in all directions and cancel out. Applying an external field grows the domains favourably aligned with H, until at saturation the entire sample is one giant domain.
Curie temperature TC: Above a critical temperature, thermal motion overcomes the exchange interaction and the material becomes paramagnetic.
\[\chi(T > T_C) = \dfrac{C'}{T - T_C}\]
For iron TC ≈ 1043 K, cobalt 1394 K, nickel 631 K, gadolinium 293 K.5.6.4 Hysteresis loop
Plot B (or M) versus H for a ferromagnetic sample, increasing then decreasing H. The curve does not retrace itself - the sample "remembers" its history. The closed curve is called the hysteresis loop.
5.6.5 Soft vs hard ferromagnets
| Property | Soft ferromagnet (e.g. soft iron, mu-metal) | Hard ferromagnet (e.g. AlNiCo, NdFeB) |
|---|---|---|
| Retentivity Br | Low | High |
| Coercivity Hc | Low (~10 A/m) | High (10⁵-10⁶ A/m) |
| Loop area (energy loss/cycle) | Small ⇒ low losses | Large |
| Initial μr | Very high | Moderate |
| Use | Transformer cores, electromagnet cores | Permanent magnets (loudspeakers, motors) |
5.6.6 Comparison: dia, para, ferro
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Sign of χ | χ < 0 | χ > 0 (small) | χ ≫ 0 (and nonlinear) |
| Range of χ | ~ −10⁻⁵ | ~ 10⁻⁵ to 10⁻³ | ~ 10² to 10⁵ |
| μr | < 1 | > 1 (slightly) | ≫ 1 |
| χ vs T | Independent of T | χ ∝ 1/T (Curie's law) | χ ∝ 1/(T − TC) above TC |
| Behaviour in non-uniform B | Repelled (toward weaker field) | Attracted (toward stronger field) | Strongly attracted |
| Examples | Bi, Cu, H₂O, Au, Pb | Al, Pt, O₂, Na | Fe, Co, Ni, Gd, Alnico, NdFeB |
Worked Example 5.7 - Curie's law
Example 5.7 L3 Apply
A paramagnetic salt has χ = 4.0 × 10⁻⁴ at 300 K. Estimate χ at 200 K, assuming Curie's law.
χ ∝ 1/T ⇒ χ₂/χ₁ = T₁/T₂ ⇒ χ₂ = (300/200)(4.0 × 10⁻⁴) = 6.0 × 10⁻⁴.
Lower temperature ⇒ less thermal randomisation ⇒ stronger response.
Worked Example 5.8 - Loop-area energy loss
Example 5.8 L4 Analyse
The hysteresis loop of a transformer core encloses an area of 200 J/m³ per cycle. The core has volume 5 × 10⁻³ m³ and is driven at 50 Hz. Find the average power dissipated as heat in the core.
Energy lost per cycle = (loop area) × (volume) = 200 × 5 × 10⁻³ = 1 J.
Power = energy × frequency = 1 × 50 = 50 W.
Soft-iron cores with small loop area minimise this loss; that is why transformer steels are designed for narrow loops.
Interactive: Hysteresis Loop Animator L3 Apply
Step the field H around a full cycle and watch B trace the loop. Slider sets coercivity-to-retentivity character.
Activity 5.4 - Iron, water and a strong magnetL4 Analyse
- Suspend a small iron screw and a small piece of bismuth (or just a glass of water) by thin threads.
- Bring a strong neodymium magnet near each one in turn.
Predict: which will be attracted strongly, weakly, or repelled?
The iron screw is strongly attracted (ferromagnet). Bismuth is weakly repelled (diamagnet). Water is weakly repelled too (very small effect, requires a strong magnet to see). Aluminium foil would be slightly attracted (paramagnet).
Competency-Based Questions L1-L6
A transformer core operating at 50 Hz dissipates power proportional to the area of its hysteresis loop. Engineers can choose between soft iron, silicon steel and a hard NdFeB ferrite.
1. Which material is most appropriate for the transformer core? L3 Apply
(b) Silicon steel has a narrow loop (small area) ⇒ small energy loss per cycle ⇒ efficient.
2. State Curie's law and explain why χ falls with temperature. L2 Understand
χ = C/T. Higher temperature ⇒ greater random thermal motion of atomic dipoles ⇒ harder for an applied H to align them ⇒ smaller M and χ.
3. Define retentivity and coercivity. L1 Remember
Retentivity Br: residual B in the sample when the applied H is reduced to zero. Coercivity Hc: reverse H required to bring B back to zero.
4. Above its Curie temperature, why does iron behave as a paramagnet? L4 Analyse
Below TC, exchange forces overcome thermal randomisation and produce spontaneous alignment in domains. Above TC, kBT exceeds the exchange energy ⇒ alignment fails; only weak partial alignment under an external field remains - the paramagnetic regime described by χ ∝ 1/(T - TC).
5. Design a permanent-magnet motor for a small electric scooter. List two material requirements and explain why. L6 Create
(i) High retentivity and high coercivity (hard magnet, e.g. NdFeB) so the field stays strong even when the rotor's reverse fields try to demagnetise it. (ii) High Curie temperature (NdFeB ~ 580 K, but operating temperatures of motors approach 150 °C; SmCo with TC ~ 1100 K is preferred for high-temperature applications) so the magnet doesn't lose strength when hot.
Assertion-Reason Pairs L4 Analyse
Options: (A) Both true, R correct explanation. (B) Both true, R not the explanation. (C) A true, R false. (D) A false, R true.
Assertion: A diamagnetic substance moves toward weaker regions of a non-uniform field.
Reason: Its induced magnetisation opposes the field, leading to a force from strong to weak field regions.
(A). Reason directly explains the assertion.
Assertion: Soft iron is preferred over steel for the core of an electromagnet.
Reason: Soft iron has high retentivity and low coercivity.
(C). Soft iron has low retentivity (so it loses magnetisation when current is switched off) and high permeability. Hence reason is incorrect.
Assertion: The susceptibility of a ferromagnet drops sharply at the Curie temperature.
Reason: Above TC, thermal energy disrupts long-range domain alignment.
(A). Both true; reason explains assertion.
Frequently Asked Questions - Magnetic Materials
What is the main concept covered in Magnetic Materials?
In NCERT Class 12 Physics Chapter 5 (Magnetism and Matter), "Magnetic Materials" covers the core principles and equations students need for board exam success. The MyAiSchool lesson explains the topic with definitions, derivations, worked examples, and interactive simulations. Key formulas and dimensional analysis are included to build conceptual depth and problem-solving skills aligned with the CBSE 2025-26 syllabus.
How is Magnetic Materials useful in real-life applications?
Real-life applications of "Magnetic Materials" from NCERT Class 12 Physics Chapter 5 include electronics, communication systems, medical imaging, solar energy, semiconductor devices, and modern technology. The MyAiSchool lesson links every concept to a tangible example so students see physics as a problem-solving framework for the physical world, not as abstract formulas.
What are the key formulas in Magnetic Materials?
Key formulas in "Magnetic Materials" (NCERT Class 12 Physics Chapter 5 Magnetism and Matter) are derived step-by-step in the MyAiSchool lesson. Students should memorize the final formula AND understand its derivation for full board marks. Each formula is listed with its dimensional formula, SI unit, applicability range, and common pitfalls. The Summary section at the end of each part includes a quick-reference formula card.
How does this part connect to other parts of Chapter 5?
NCERT Class 12 Physics Chapter 5 (Magnetism and Matter) is structured so each part builds on the previous one. "Magnetic Materials" connects directly to neighbouring parts via shared definitions, units, and methodology. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Magnetic Materials?
CBSE board questions from "Magnetic Materials" typically include: (1) 1-mark MCQs on definitions and formulas, (2) 2-mark short-answer derivations or applications, (3) 3-mark numerical problems with units, (4) 5-mark long-answer derivations followed by application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Magnetic Materials" lesson allows students to adjust input parameters (sliders or selectors) and see physical quantities update in real time. To use it effectively: (1) try extreme values to understand limiting cases, (2) compare with the analytical formula, (3) check unit consistency, (4) test special configurations from worked examples. The simulation reinforces conceptual intuition that pure formula manipulation cannot.
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