JEE Mains Physics · Teaching notes
Magnetism and Matter — JEE Mains Physics
Magnetism and Matter has 46 past-year questions from 2021 to 2026, and 10 of them ask for a number rather than an option. Half of them are about magnetic materials, and those are mostly statements to judge, each settled by one exact fact about susceptibility, temperature or the hysteresis loop. The largest group of calculations turns a magnet or a coil in a uniform field and needs only the torque and the energy, read with the right angle. The earth's field has not been asked since 2023. Marks are lost on the sign of the energy, on reading a coil's plane as its moment, and on calling a strongly attracted material paramagnetic.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Bar Magnets and Magnetic Dipoles
14 PYQsA bar magnet, a current loop and an orbiting electron are all magnetic dipoles: each has a moment M, makes a field that falls as 1/r³, and in a uniform field feels a torque MB sin θ with potential energy −MB cos θ.
The Earth's Field: Dip and Oscillating Magnets
9 PYQsThe earth's field B splits into a horizontal part B cos δ and a vertical part B sin δ, where δ is the angle of dip; the horizontal part is the one that cancels a magnet's field at a neutral point and sets the period of a magnet swinging in a horizontal plane.
Magnetic Properties of Materials and Hysteresis
23 PYQsA material placed in a field H becomes magnetised, M = χH, so the field inside rises to μ₀(H + M); the sign and size of χ sort materials into diamagnetic, paramagnetic and ferromagnetic, and a ferromagnet's hysteresis loop decides what it is used for.
Formula & revision sheet
5 formulas · 2 reference tables · 17 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
5 formulas · 2 reference tables · 17 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Watch out for (6)
- Bending keeps the pole strength, not the moment→ Magnetic dipole moment and the field of a bar magnet
- The equatorial field points opposite to the moment→ Magnetic dipole moment and the field of a bar magnet
- Using the axial formula at an equatorial point→ Magnetic dipole moment and the field of a bar magnet
- Dropping the minus sign in the energy→ Torque and potential energy of a dipole in a uniform field
- Reading the plane of a coil as its moment→ Torque and potential energy of a dipole in a uniform field
- Work is a difference of cosines, not of angles→ Torque and potential energy of a dipole in a uniform field
Formulas (2)
Watch out for (4)
- Swapping sine and cosine→ Components of the earth's field and the angle of dip
- Geographic meridian in place of magnetic→ Components of the earth's field and the angle of dip
- Using the total field for the period→ Horizontal component: neutral points and oscillating magnets
- Oscillations per minute are a frequency→ Horizontal component: neutral points and oscillating magnets
Reference tables (2)
Diamagnetic, paramagnetic and ferromagnetic materials7 rows
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Susceptibility χ | Small, negative: −1 ≤ χ < 0 | Small, positive | Very large, positive |
| Relative permeability μᵣ | Slightly less than 1 | Slightly more than 1 | Much greater than 1 |
| Atomic moments | None; the field induces a moment opposite to itself | Permanent, randomly oriented without a field | Permanent, aligned within domains |
| In a non-uniform field | Moves from strong to weak field | Moves from weak to strong field, weakly attracted | Strongly attracted |
| Effect of temperature | χ does not depend on temperature | χ = C/T (Curie's law) | Paramagnetic above the Curie temperature |
| M against H | Straight line, negative slope | Straight line, small positive slope | Curved, saturates, shows hysteresis |
| Examples | Bismuth, copper, water, superconductors | Aluminium, sodium, oxygen | Iron, cobalt, nickel |
Hysteresis and choosing a magnetic material3 rows
| Use | Material | Retentivity | Coercivity | Hysteresis loop |
|---|---|---|---|---|
| Electromagnet core | Soft iron | Low | Low | Narrow, small area |
| Transformer core | Soft iron, laminated | Low | Low | Narrow, so little energy is lost each cycle |
| Permanent magnet | Steel, alnico, cobalt steel | High | High | Wide, large area |
Watch out for (7)
- Fraction and percentage→ Magnetic intensity, susceptibility and permeability
- Relative permeability and susceptibility differ by one→ Magnetic intensity, susceptibility and permeability
- Calling a strongly attracted material paramagnetic→ Diamagnetic, paramagnetic and ferromagnetic materials
- Getting the direction of drift backwards→ Diamagnetic, paramagnetic and ferromagnetic materials
- Letting diamagnetism depend on temperature→ Diamagnetic, paramagnetic and ferromagnetic materials
- Swapping retentivity and coercivity→ Hysteresis and choosing a magnetic material
- Wanting a large retentivity in an electromagnet→ Hysteresis and choosing a magnetic material
PYQ weightage by concept
7 concepts · 46 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
7 concepts · 46 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Torque and potential energy of a dipole in a uniform field | 8 | 17% |
| Magnetic dipole moment and the field of a bar magnet | 6 | 13% |
| Concept | PYQs | Share |
|---|---|---|
| Components of the earth's field and the angle of dip | 5 | 11% |
| Horizontal component: neutral points and oscillating magnets | 4 | 9% |
| Concept | PYQs | Share |
|---|---|---|
| Diamagnetic, paramagnetic and ferromagnetic materials | 10 | 22% |
| Magnetic intensity, susceptibility and permeability | 9 | 20% |
| Hysteresis and choosing a magnetic material | 4 | 9% |
Test yourself on Magnetism and Matter
15 past JEE Mains questions from this chapter, timed at 36 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.