An Overview of Ncert Books Class 12 Physics Chapter 5 Free Download
FAQs on Ncert Books Class 12 Physics Chapter 5 Free Download
1. What are the most important topics in Class 12 Physics Chapter 5, Magnetism and Matter, for the 2025-26 board exams?
For the CBSE 2025-26 board exams, the most important topics from Magnetism and Matter are:
- Properties of magnetic field lines.
- The bar magnet as an equivalent solenoid, including the derivation for its axial field.
- Torque on a magnetic dipole (bar magnet) in a uniform magnetic field.
- Gauss's Law for Magnetism and its significance.
- Classification of materials: Diamagnetic, Paramagnetic, and Ferromagnetic substances, with examples and properties like magnetic susceptibility and permeability.
- Hysteresis curve and its implications for selecting materials for electromagnets and permanent magnets.
This chapter is part of Unit III, which, combined with Unit IV, carries a significant weightage of around 17 marks.
2. What types of questions, in terms of marks, can be expected from Magnetism and Matter?
Based on the CBSE board pattern, you can expect a mix of questions from this chapter:
- 1-mark questions: Typically objective or very short answer questions based on definitions (e.g., magnetic moment, angle of dip) or identifying properties of magnetic materials.
- 2 or 3-mark questions: These often involve short derivations (like torque on a dipole), explaining concepts (like Gauss's law), or comparing the properties of dia-, para-, and ferromagnetic materials.
- 5-mark questions: A long-answer question could involve the detailed derivation for the magnetic field of a bar magnet as an equivalent solenoid, often combined with a small numerical problem.
3. Why is Gauss's Law for magnetism significant for board exam questions?
Gauss's Law for magnetism is conceptually very important. It states that the net magnetic flux through any closed surface is always zero (∮B·dA = 0). Its significance, which is a frequently asked conceptual question, is that it implies magnetic monopoles do not exist. Magnetic field lines always form continuous closed loops, meaning there is no isolated north or south pole. This is a fundamental difference from electric fields, where isolated positive and negative charges exist.
4. How can I score full marks in questions asking to differentiate between diamagnetic, paramagnetic, and ferromagnetic materials?
To score full marks, present your answer in a table format comparing the materials on at least three to four key properties:
- Behaviour in an external magnetic field: Diamagnetic materials are feebly repelled, paramagnetic are feebly attracted, and ferromagnetic are strongly attracted.
- Magnetic Susceptibility (χ): Small and negative for dia, small and positive for para, and very large and positive for ferro.
- Relative Permeability (μr): Slightly less than 1 for dia, slightly greater than 1 for para, and much greater than 1 for ferro.
- Example: Provide at least one example for each, such as Bismuth (dia), Aluminium (para), and Iron (ferro).
5. What is the logic behind choosing soft iron for electromagnets and steel for permanent magnets? This is a common HOTS question.
This question tests your understanding of the hysteresis curve. Soft iron is used for electromagnets because it has high permeability and low retentivity. This means it can be strongly magnetised by a current but loses its magnetism almost completely when the current is switched off. Steel is used for permanent magnets because it has high retentivity and high coercivity. This allows it to retain its magnetism strongly and resist being demagnetised by stray fields or temperature changes.
6. Which derivations from Magnetism and Matter are most frequently asked in exams?
The two most critical derivations from this chapter for the CBSE board exam are:
- The expression for the magnetic field intensity at a point on the axial line of a bar magnet, treating it as an equivalent solenoid. This is a potential 5-mark question.
- The expression for the torque experienced by a magnetic dipole (bar magnet) when placed in a uniform magnetic field. This is often asked as a 2 or 3-mark question.
Practising these derivations step-by-step is essential for scoring well.
7. What is a common misconception about the Earth's magnetic elements that could lead to losing marks?
A common mistake is confusing the angle of declination with the angle of dip (or inclination). It's crucial to be clear:
- Declination is the angle between the geographic meridian and the magnetic meridian at a place. It tells us the direction of true north versus where a compass points.
- Dip is the angle that the Earth’s total magnetic field vector makes with the horizontal in the magnetic meridian. It is zero at the magnetic equator and 90 degrees at the magnetic poles.
Using the wrong term in a definition or problem will lead to a loss of marks.
