What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Why can magnetic monopoles not be isolated?

Magnetic field lines form closed loops; cutting a magnet always produces new pole pairs.

What is the potential energy of a dipole in a magnetic field?

\(U=-\vec{m}\cdot\vec{B}=-mB\cos\theta\), minimum when aligned with the field.

The angle that the Earth's total magnetic field makes with the horizontal at a given place.

Write the relation between B, H and M.

\(\vec{B}=\mu_0(\vec{H}+\vec{M})\), linking field, magnetising field and magnetisation.

How does a diamagnetic material behave in a field?

It is weakly repelled and tends to move to regions of weaker field; \(\chi\) is small and negative.

State Curie's law for paramagnets.

Susceptibility is inversely proportional to absolute temperature, \(\chi=C/T\).

What characterises a ferromagnetic material?

Large positive susceptibility, domain structure, and strong attraction; it shows hysteresis.

What happens above the Curie temperature?

A ferromagnet loses its spontaneous magnetisation and becomes paramagnetic.

What does the area of a hysteresis loop represent?

The energy dissipated as heat per unit volume per cycle of magnetisation.

Which material suits a permanent magnet?

One with high retentivity and high coercivity, such as steel or alnico.

Magnetism and Matter

Class 12 Physics · Physics Part I · Chapter 5

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Summary

A bar magnet behaves as a magnetic dipole with two poles that cannot be isolated; its field resembles that of a solenoid, and it has a magnetic moment \(\vec{m}\). In a uniform field it experiences a torque \(\vec{\tau}=\vec{m}\times\vec{B}\) and has potential energy \(U=-\vec{m}\cdot\vec{B}\), oscillating like a torsional pendulum. The Earth itself acts as a giant magnet; its field at a place is described by declination, dip (inclination) and the horizontal component. Magnetic properties of materials are captured by the magnetising field \(\vec{H}\), magnetisation \(\vec{M}\), and permeability, with \(\vec{B}=\mu_0(\vec{H}+\vec{M})\) and susceptibility \(\chi=M/H\). Materials are classified as diamagnetic (weakly repelled, \(\chi\) small and negative), paramagnetic (weakly attracted, small positive \(\chi\), obeying Curie's law \(\chi\propto 1/T\)), and ferromagnetic (strongly attracted, large \(\chi\), showing domains and hysteresis). Ferromagnets lose their magnetism above the Curie temperature, becoming paramagnetic. Hysteresis, the lag of magnetisation behind the field, and the area of the loop indicate energy loss, guiding the choice of materials for permanent magnets and electromagnets.

Bar magnet as a dipoleTorque and potential energyEarth's magnetism and its elementsMagnetisation, H and susceptibilityDia-, para- and ferromagnetismHysteresis and magnetic materials

Key terms

Magnetic dipole moment

Strength and orientation of a magnet, \(\vec{m}\); torque \(\vec{\tau}=\vec{m}\times\vec{B}\).

Magnetic declination

Angle between geographic and magnetic meridians at a place.

Angle of dip

Angle the Earth's field makes with the horizontal at a place.

Magnetic susceptibility

Ratio \(\chi=M/H\) measuring how readily a material magnetises.

Curie's law

For paramagnets, magnetisation \(\chi\propto 1/T\); susceptibility falls as temperature rises.

Hysteresis

Lagging of magnetisation behind the magnetising field in ferromagnets.

Extra questions & answers

Try one from this chapter

Why can isolated magnetic monopoles not be obtained by cutting a magnet?

Flashcards & interactive practice