Magnetism and Matter
21.0 Important Points
21.0 Important Points
- Magnetic length $ = \frac{5}{6}\times $ geometric length of magnet.
- About $90\%$ of magnetic moment is due to spin motion of electrons and remaining $10\%$ of magnetic moment is due to the orbital motion of electrons.
- When a magnet having magnetic moment $M$ is cut into two equal parts
(i) Parallel to its length $$M' = \frac{m}{2}\times 1 = \frac{M}{2}$$
(ii) Perpendicular to its length $$M' = \frac{m}{2}\times \frac{1}{2} = \frac{M}{2}$$
When a magnet of length $I,$ pole strength m and of magnetic moment $M$ is turned into a semicircular arc then it new magnetic moment $$M = m\times 2R = m\times 2\times \frac{1}{{\left( {\pi R = I} \right)}}$$ $$ = \frac{{2M}}{{\pi \left( {M = m\times 1} \right)}}$$
A thin magnet of moment $M$ is turned into an arc of $90°.$ Then new magnetic moment $$M = 2\sqrt {\frac{{2m}}{\pi }} $$
- A thin magnet of moment M is turned at mid point 90°.Then new magnet moment $$M = \frac{M}{{\sqrt 2 }}$$
- A thin magnet of moment $M$ is turned into an arc of $60°.$ Then new magnetic moment $$M = \frac{{3M}}{\pi }$$
- A thin magnet of moment $M$ is bent at mid point at angle $60°$. Then new magnetic moment. $$M = \frac{M}{2}$$
- Original magnet $MOS$ is bent at $O,$ the mid point at $60°$. All sides are equal.
- The mutual interaction force between two small magnets of moments $M_1$ and $M_2$ is given by $$F = \frac{{K\;6{M_1}{M_2}}}{{{d^4}}}$$
Here, $d$ denotes the separation between magnets.
- Cause of diamagnetism is orbital motion and cause of paramagnetism is spin motion of electrons. Cause of ferromagnetism lies in formation of domains.
- The perpendicular bisector of magnetic axis is known as neutral axis of magnet. Magnetism at neutral axis is zero and at poles is maximum.
- For steel coercivity is large. However, retentivity is comparatively smaller in case of steel. So, steel is used to make permanent magnets.
- For soft iron, coercivity is very small and area of hysteresis loop is small. So, soft iron is an ideal material for making electromagnets.
