NEB Class 12 Physics Magnetic properties of materials Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Magnetic properties of materials Note Handwritten in PDF
Magnetic Field in Materials
When a material is placed in a magnetic field:
- Magnetic field intensity (H): Applied external field
- Magnetization (M): Magnetic moment per unit volume induced in material
- Magnetic flux density (B): Total field = B = ΞΌβ(H + M) = ΞΌβΞΌ_r H
Magnetic Susceptibility and Relative Permeability
- Magnetic susceptibility (Ο): Ο = M/H (how easily a material becomes magnetized)
- Relative permeability (ΞΌ_r): ΞΌ_r = B/ΞΌβH (how much better material conducts flux than vacuum)
- Relation: ΞΌ_r = 1 + Ο
Three Types of Magnetic Materials
1. Diamagnetic Materials:
- Ο is small and negative (Ο β β10β»β΅)
- ΞΌ_r slightly less than 1
- Weakly repelled by magnets
- No permanent dipoles magnetism induced opposite to field
- Examples: Bismuth, Copper, Gold, Water, Wood
- Explanation: Orbital motion of electrons creates opposing field
2. Paramagnetic Materials:
- Ο is small and positive (Ο β 10β»Β³ to 10β»β΅)
- ΞΌ_r slightly greater than 1
- Weakly attracted by magnets
- Have unpaired electrons β permanent dipoles β align with field
- Examples: Aluminium, Platinum, Oxygen, Sodium
- Magnetism lost when field removed
3. Ferromagnetic Materials:
- Ο is large and positive (Ο = 10Β² to 10βΆ)
- ΞΌ_r >> 1 (can be thousands)
- Strongly attracted by magnets
- Retain magnetism even after field removed (permanent magnets)
- Examples: Iron, Nickel, Cobalt, Alnico, Neodymium
- Explanation: Domain theory large regions (domains) where all dipoles are aligned
Domain Theory of Ferromagnetism
In ferromagnetic materials, there are microscopic regions called magnetic domains where all atomic magnetic dipoles are aligned in the same direction.
- In an unmagnetized piece of iron: domains point in random directions β net magnetism = 0
- In external field: domains aligned with field grow, others shrink β net magnetism appears
- If field is strong enough: all domains align β saturation magnetization
- When field is removed: domains don’t fully return to random β residual magnetism (retentivity)
Hysteresis
When a ferromagnetic material is subjected to a cycle of increasing and decreasing magnetic field (alternating current in an electromagnet, for example), the B-H curve forms a closed loop the hysteresis loop.
Key terms from the hysteresis loop:
- Retentivity (B_r): The residual magnetic flux density when H is reduced to zero (value of B when H = 0)
- Coercivity (H_c): The reverse field required to reduce B to zero (width of loop)
- Saturation magnetization (B_s): Maximum B when all domains are aligned
Hysteresis loss: Energy dissipated as heat per cycle = proportional to area of hysteresis loop
Choosing materials based on hysteresis:
| Purpose | Need | Material |
|---|---|---|
| Transformer core | Small loop area (low loss), high ΞΌ_r | Soft iron, Silicon steel |
| Permanent magnets | Large B_r, large H_c | Steel, Alnico, Neodymium |
| Electromagnets | High ΞΌ_r, low retentivity | Soft iron |
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Frequently Asked Questions
What is the difference between diamagnetic paramagnetic and ferromagnetic materials?
Diamagnetic (bismuth, copper): Ο small negative, weakly repelled, no permanent dipoles. Paramagnetic (aluminium, oxygen): Ο small positive, weakly attracted, unpaired electrons align with field. Ferromagnetic (iron, nickel, cobalt): Ο very large positive, strongly attracted, magnetic domains align retain magnetism after field removed.
What is hysteresis in Class 12 Physics?
Hysteresis is the lagging of magnetisation behind the applied magnetising field. When alternating field is applied to ferromagnet, the B-H curve forms a closed loop called hysteresis loop. Area of loop represents energy lost as heat per cycle. Materials with small loop area are used for transformer cores to minimise energy loss.
What is retentivity and coercivity in hysteresis loop?
Retentivity (B_r): residual magnetism remaining when external field is reduced to zero. Coercivity (H_c): reverse field needed to reduce magnetisation back to zero. High retentivity + high coercivity β good permanent magnet (steel, neodymium). Low retentivity + low coercivity β good transformer core material (soft iron, silicon steel).
What is magnetic susceptibility and relative permeability Class 12?
Magnetic susceptibility Ο = M/H measures how easily a material magnetises. Relative permeability ΞΌ_r = B/ΞΌβH measures how much better material conducts magnetic flux compared to vacuum. Relationship: ΞΌ_r = 1 + Ο. Diamagnets: Ο negative, ΞΌ_r < 1. Paramagnets: Ο slightly positive. Ferromagnets: Ο very large, ΞΌ_r can reach thousands.
Why is soft iron used for transformer cores and not steel?
Soft iron has a narrow hysteresis loop very small loop area means small energy loss per cycle as heat. Transformer core is magnetised and demagnetised thousands of times per second (at mains frequency). Steel has a wide loop causing large energy loss. Soft iron also has high permeability for efficient magnetic flux conduction.
What is domain theory of ferromagnetism Class 12?
Ferromagnetic materials have microscopic regions called magnetic domains where all atomic dipoles are aligned in the same direction. In unmagnetised iron, domains point randomly net magnetism is zero. In external field, aligned domains grow and others shrink net magnetism appears. At saturation all domains are aligned in field direction.
What is the difference between hard and soft magnetic materials?
Soft magnetic materials (soft iron): small hysteresis loop, low coercivity, low retentivity easy to magnetise and demagnetise. Used in transformer cores, electromagnets, motors. Hard magnetic materials (steel, alnico, neodymium): large hysteresis loop, high coercivity, high retentivity β difficult to demagnetise. Used for permanent magnets.
Which magnetic properties topics are most important for Exam?
Most important: comparison of diamagnetic, paramagnetic, ferromagnetic with examples (4 marks), hysteresis loop with retentivity and coercivity (4 marks), domain theory of ferromagnetism (2 marks), soft versus hard magnetic materials (2 marks). Hysteresis loop diagram and explanation appears in most NEB board papers.
