Magnetism and Matter
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Bar Magnet and Dipole Behaviour
Part of: Bar Magnets and Dipole Behaviour
A bar magnet's field lines loop continuously, leaving the north pole outside and returning through the magnet. In a uniform field the magnet feels a torque toward alignment, with lowest energy when parallel, exactly like an electric dipole, and a current solenoid behaves as an equivalent bar magnet.
Why it matters. Compasses, motors and MRI magnets all exploit dipole alignment. Treating field lines as open curves, or missing the solenoid equivalence, breaks direction predictions.
Formulas
- Torque: T = m B sin(theta)
- Energy: U = -m B cos(theta)
- Solenoid moment: m = N I A
Key words
- Magnetic dipole moment
- : Pole strength times separation, or NIA for a coil, pointing south to north.
- Magnetic field lines
- : Continuous closed curves showing field direction at every point.
- Stable equilibrium
- : Aligned dipole position of lowest energy that restores after small turns.
Try it in two minutes
- Sketch loops leaving north and entering south outside.
- Compute torque as moment times field times sine of angle.
- Compare aligned and anti-aligned energies.
Try at home. Float a magnet on cork in water and watch it settle north-south.
Gauss's Law and Earth's Magnetism
Part of: Bar Magnets and Dipole Behaviour
Because isolated poles do not exist, the net magnetic flux through any closed surface is zero. Earth's field is described by declination, the horizontal shift from true north, and inclination, the dip below horizontal, with the horizontal component equal to total field times cosine of dip.
Why it matters. Navigation and compass corrections depend on these angles. Expecting monopoles, or mixing dip with declination, gives wrong headings.
Formulas
- Gauss for magnetism: net flux through closed surface = 0
- Horizontal component: BH = B cos(delta)
- Vertical component: BV = B sin(delta)
Key words
- Magnetic monopole
- : A hypothetical lone pole, never observed in nature.
- Declination
- : Angle between true north and magnetic north at a place.
- Inclination
- : Dip of Earth's field below the horizontal at a place.
- Horizontal component
- : Part of Earth's field along the ground, equal to B cos of dip.
Try it in two minutes
- Count field lines entering and leaving any closed surface.
- Mark true north, magnetic north and the angle between.
- Split Earth's field into horizontal and vertical parts with cosine and sine.
Try at home. Look up your city's declination and note which way a compass points off true north.
Magnetic Materials and Classification
Part of: Magnetic Materials and Hysteresis
Magnetisation measures dipole moment per volume, susceptibility chi links it to the magnetising field, and relative permeability is 1 plus chi. Diamagnets have small negative chi and are weakly repelled, paramagnets small positive chi and are weakly attracted, while ferromagnets have huge chi from aligned domains.
Why it matters. Choosing core materials, shields and sensors depends on these classes. Calling every attraction ferromagnetism, or missing the sign of chi, mislabels materials.
Formulas
- Magnetisation link: M = chi H
- Relative permeability: mu_r = 1 + chi
- Flux density: B = mu0 (H + M)
Key words
- Susceptibility
- : Ratio of magnetisation to magnetising field, marking material response.
- Diamagnetism
- : Weak repulsion with negative susceptibility, present in all matter.
- Paramagnetism
- : Weak attraction with small positive susceptibility from unpaired electrons.
- Ferromagnetism
- : Strong magnetism from aligned domains, lost above Curie temperature.
- Curie temperature
- : Point above which a ferromagnet turns paramagnetic.
Try it in two minutes
- Check the sign and size of susceptibility.
- Predict weak repulsion, weak attraction or strong attraction.
- Heat past Curie point and watch ferromagnetism vanish.
Try at home. Sort iron, aluminium and bismuth by expected response to a strong magnet.
Hysteresis and Permanent Magnets
Part of: Magnetic Materials and Hysteresis
Cycling a ferromagnet traces a hysteresis loop: retentivity is the magnetisation left at zero field and coercivity the reverse field needed to wipe it. The loop area measures energy lost as heat each cycle. Permanent magnets need both values high; transformer cores and electromagnets need narrow loops with low loss.
Why it matters. Motors, hard disks and transformers are all chosen from loop shapes. Picking a wide-loop material for a transformer wastes power as heat every cycle.
Formulas
- Loop area = energy lost per unit volume per cycle
- Permanent magnet: high retentivity and high coercivity
- Core material: narrow loop with low hysteresis loss
Key words
- Retentivity
- : Magnetisation remaining when the magnetising field returns to zero.
- Coercivity
- : Reverse field needed to erase the remaining magnetisation.
- Hysteresis loss
- : Energy turned to heat each magnetisation cycle, given by loop area.
Try it in two minutes
- Read retentivity off the loop's vertical intercept.
- Read coercivity off the horizontal intercept.
- Compare loop areas to rank energy loss.
Try at home. Explain why a steel nail keeps magnetism but an iron nail loses it fast.
What your child will practise
- Bar Magnet and Dipole Behaviour: Ocean Quick Fire (Quick Fire)
- Gauss's Law and Earth's Magnetism: Desert Puzzle (Clue Puzzle)
- Gauss's Law and Earth's Magnetism: Desert Quest (Flip and Match)
- Hysteresis and Permanent Magnets: Candy Quick Fire (Quick Fire)
- Magnetic Materials and Classification: Ocean Puzzle (Clue Puzzle)
- Quiz: Bar Magnet and Dipole Behaviour (8 questions)
- Quiz: Gauss's Law and Earth's Magnetism (8 questions)
- Quiz: Hysteresis and Permanent Magnets (8 questions)
- Quiz: Magnetic Materials and Classification (8 questions)