Physics · Class 12 Science
Ch 5Magnetism and Matter — Class 12 Physics, concept-first.
Magnetism is not a rare phenomenon — it is everywhere. From the largest galaxies to the smallest atoms, magnetic fields are present. Even the Earth itself is a giant magnet, and its magnetism existed long before humans evolved.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Magnetic Poles
Imagine you have a bar magnet — the kind you might have stuck on your refrigerator. If you bring two of them close, something interesting happens. Sometimes they snap together with a satisfying click.
Most relevant Q&A
- (a) What happens if a bar magnet is cut into two pieces: (i) transverse to its length, (ii) along its length? (b) A magnetised needle in a u…Preview
- Figure 5.4 shows a small magnetised needle P placed at a point O. The arrow shows the direction of its magnetic moment. The other arrows sho…Preview
- A short bar magnet placed with its axis at $30^\circ$ with a uniform external magnetic field of $0.25\ \text{T}$ experiences a torque of mag…Free
- A short bar magnet of magnetic moment $m = 0.32\ \text{J T}^{-1}$ is placed in a uniform magnetic field of $0.15\ \text{T}$. If the bar is f…Free
- A closely wound solenoid of $800$ turns and area of cross section $2.5 \times 10^{-4}\ \text{m}^2$ carries a current of $3.0\ \text{A}$. Exp…Free
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
Magnetism is not a rare phenomenon — it is everywhere. From the largest galaxies to the smallest atoms, magnetic fields are present.
The Bar Magnet
A bar magnet is the simplest example of a magnetic dipole. Its behavior can be understood by analogy with an electric dipole, but with magnetic poles instead of electric charges.
The Magnetic Field Lines
Magnetic field lines are a way to visualize the magnetic field in space. They are not real physical lines, but a conceptual tool that helps us understand the direction and strength of the magnetic fie…
Bar Magnet as an Equivalent Solenoid
A current-carrying solenoid behaves like a magnetic dipole. The magnetic field lines of a bar magnet and a finite solenoid are strikingly similar, especially at large distances.
The Dipole in a Uniform Magnetic Field
When a magnetic dipole (like a compass needle) of magnetic moment is placed in a uniform magnetic field , it experiences a torque. This torque tends to rotate the dipole to align it with the field.
The Electrostatic Analog
The key insight is that the magnetic field of a bar magnet at large distances behaves exactly like the electric field of an electric dipole.
Magnetism and Gauss's Law
Gauss's Law for magnetism is a fundamental statement about the nature of magnetic fields. Unlike electric fields, which can start and end on charges, magnetic field lines are continuous and form close…
+−Worked Examplesi2 questions
- Example 5.3Many of the diagrams given in Fig. 5.6 show magnetic field lines (thick lines in the figure) wrongly. Point out what is wrong with them. Som…Free
- Example 5.4(a) Magnetic field lines show the direction (at every point) along which a small magnetised needle aligns (at the point). Do the magnetic fi…Preview
Magnetisation and Magnetic Intensity
In any bulk material, the magnetic moments of individual atoms (due to circulating electrons) add up as vectors.
Magnetic Properties of Materials
The response of a material to an external magnetic field is determined by its magnetic susceptibility () and relative permeability ().
Diamagnetism
Diamagnetism is a fundamental magnetic property of matter. A diamagnetic substance, when placed in an external magnetic field, develops a net magnetic moment in the direction opposite to the applied f…
Paramagnetism
Paramagnetism is a weak form of magnetism. A paramagnetic substance, when placed in an external magnetic field, becomes weakly magnetised in the same direction as the field.
Ferromagnetism
Ferromagnetic materials are the most familiar magnetic substances—think of iron, cobalt, nickel, and their alloys.
Summary
- Magnetic dipole moment of a bar magnet: , where is pole strength and is the separation between poles. - Magnetic field due to a bar magnet at an axial point: ; at an equatorial point: .
Points to Ponder
1. The practical use of magnets for direction-finding existed for about two thousand years before physicists, around 1800 AD, finally explained magnetism using moving charges and currents.
Exercises
+−Show 7 questionsHide questions7 questions
- 5.1A short bar magnet placed with its axis at $30^\circ$ with a uniform external magnetic field of $0.25\ \text{T}$ experiences a torque of mag…Free
- 5.2A short bar magnet of magnetic moment $m = 0.32\ \text{J T}^{-1}$ is placed in a uniform magnetic field of $0.15\ \text{T}$. If the bar is f…Free
- 5.3A closely wound solenoid of $800$ turns and area of cross section $2.5 \times 10^{-4}\ \text{m}^2$ carries a current of $3.0\ \text{A}$. Exp…Free
- 5.4If the solenoid in Exercise 5.5 is free to turn about the vertical direction and a uniform horizontal magnetic field of $0.25\ \text{T}$ is…Preview
- 5.5A bar magnet of magnetic moment $1.5\ \text{J T}^{-1}$ lies aligned with the direction of a uniform magnetic field of $0.22\ \text{T}$. (a)…Preview
- 5.6A closely wound solenoid of $2000$ turns and area of cross-section $1.6 \times 10^{-4}\ \text{m}^2$, carrying a current of $4.0\ \text{A}$,…Preview
- 5.7A short bar magnet has a magnetic moment of $0.48\ \text{J T}^{-1}$. Give the direction and magnitude of the magnetic field produced by the…Preview
Exemplar Problems
Higher-order thinking / exemplar-style practice problems.
+−Show 21 questionsHide questions21 questions
- Q1A toroid of $n$ turns, mean radius $R$ and cross-sectional radius $a$ carries current $I$. It is placed on a horizontal table taken as $x$-$…Free
- Q2In a permanent magnet at room temperature (a) magnetic moment of each molecule is zero. (b) the individual molecules have non-zero magnetic…Free
- Q3Consider the two idealized systems: (i) a parallel plate capacitor with large plates and small separation and (ii) a long solenoid of length…Free
- Q4A paramagnetic sample shows a net magnetisation of $8\ \text{Am}^{-1}$ when placed in an external magnetic field of $0.6\ \text{T}$ at a tem…Preview
- Q5S is the surface of a lump of magnetic material. (a) Lines of B are necessarily continuous across S. (b) Some lines of B must be discontinuo…Preview
- Q6The primary origin(s) of magnetism lies in (a) atomic currents. (b) Pauli exclusion principle. (c) polar nature of molecules. (d) intrinsic…Preview
- Q7A long solenoid has 1000 turns per metre and carries a current of 1 A. It has a soft iron core of $\mu_r = 1000$. The core is heated beyond…Preview
- Q8Essential difference between electrostatic shielding by a conducting shell and magnetostatic shielding is due to (a) electrostatic field lin…Preview
- Q9A proton has spin and magnetic moment just like an electron. Why then its effect is neglected in magnetism of materials?Preview
- Q10A permanent magnet in the shape of a thin cylinder of length $10\ \text{cm}$ has $M = 10^6\ \text{A/m}$. Calculate the magnetisation current…Preview
- Q11Explain quantitatively the order of magnitude difference between the diamagnetic susceptibility of $\text{N}_2$ ($\sim 5 \times 10^{-9}$) (a…Preview
- Q12From molecular view point, discuss the temperature dependence of susceptibility for diamagnetism, paramagnetism and ferromagnetism.Preview
- Q13A ball of superconducting material is dipped in liquid nitrogen and placed near a bar magnet. (i) In which direction will it move? (ii) What…Preview
- Q14Verify the Gauss's law for magnetic field of a point dipole of dipole moment $m$ at the origin for the surface which is a sphere of radius $…Preview
- Q15Three identical bar magnets are rivetted together at their common centre and lie in the same plane, their axes equally inclined at 60 degree…Preview
- Q16Suppose we want to verify the analogy between electrostatic and magnetostatic by an explicit experiment. Consider the motion of (i) electric…Preview
- Q17A bar magnet of magnetic moment $m$ and moment of inertia $I$ (about centre, perpendicular to length) is cut into two equal pieces, perpendi…Preview
- Q18Use (i) the Ampere's law for $H$ and (ii) continuity of lines of $B$, to conclude that inside a bar magnet, (a) lines of $H$ run from the N…Preview
- Q19Verify the Ampere's law for magnetic field of a point dipole of dipole moment $\mathbf{m} = m\hat{k}$. Take C as the closed curve running cl…Preview
- Q20What are the dimensions of $\chi$, the magnetic susceptibility? Consider an H-atom. Guess an expression for $\chi$, upto a constant by const…Preview
- Q21There are two current carrying planar coils made each from identical wires of length L. $C_1$ is circular (radius $R$) and $C_2$ is square (…Preview
Sample & Board Papers
Sample papers and previous-year board questions for this subject.