Magnetism
Category : Banking
Magnetism
·
A bar magnet consists of two equal and opposite magnetic poles separated by a small distance. Poles are not exactly at the ends. The shortest distance between two poles is called effective length (\[{{L}_{e}}\]) and is less than its geometric length (\[{{L}_{g}}\]). For bar magnet \[{{L}_{g}}=2l\]and\[{{L}_{e}}=(5/6){{L}_{g}}\]
(i) Attractive property: When a magnet is dipped into iron fillings it is found that the concentration of iron filings, i.e., attracting power of the magnet is maximum at two points near the ends and minimum at the centre. The places where its attracting power is maximum are called poles.
(ii) Directive property: When a magnet is suspended its length becomes parallel to N-S direction. The pole pointing north is called the north-pole while the other pointing in the geographical south is called the south pole of the magnet. The line joining the two poles of a magnet is called magnetic axis and the vertical plane passing through the axis of a freely suspended or pivoted magnet is called magnetic meridian.
(iii) Poles of a magnet always exist in pairs: In a magnet the two poles are found to be equal in strength and opposite in nature. If a magnet is broken into number of pieces, each piece becomes a magnet with two equal and opposite poles. This shows that monopole do not exist.
(iv) Repulsive property: A pole of a magnet attracts the opposite pole while repels similar pole.
A magnet gets demagnetised, i.e., loses its power of attraction if it is heated, hammered or alternating current is passed through a wire wound over it.
The permanent artificial magnets are made of some metals and alloys like Carbon-steel, Alnico, Platinum-cobalt, Alcomax, Ticonal etc. The permanent magnets are made of ferromagnetic substances with large coercivity and retentivity. The temporary artificial magnets like electromagnets are prepared by passing current through coil wound on soft iron core. These cannot retain its strength for a long time. These are made from soft iron, non-metal and alloy. Electromagnets are stronger than permanent magnet.
(i) Electric motors
(ii) Doorbells
(iii) In scrapyards to separate iron from other metals
If two magnetic poles of strengths \[{{m}_{1}}\] and \[{{m}_{2}}\] are kept at a distance r apart then force of attraction or repulsion between the two poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them
i.e., \[F\propto \frac{{{m}_{1}}{{m}_{2}}}{{{r}^{2}}}\] or, \[F=\frac{{{\mu }_{0}}}{4\pi }\frac{{{m}_{1}}{{m}_{2}}}{{{r}^{2}}}\]
where \[F=\frac{{{\mu }_{0}}}{4\pi }={{10}^{-7}}Wb{{A}^{-1}}{{m}^{-1}}={{10}^{-7}}\]henry/m \[{{\mu }_{0}}\] is permeability of free space or absolute permeability
The space around a magnet (or a current carrying conductor) in which its magnetic effect can be experienced is called the magnetic field. If a magnet is cut into two equal parts along the length then pole strength is reduced to half and length remains unchanged. New magnetic dipole moment \[M'=m'(2l)=\frac{m}{2}\times 2l=\frac{M}{2}\]. If a magnet is cut into two equal parts transverse to the length then pole strength remains unchanged and length is reduced to half.
New magnetic dipole moment \[M'=m\left( \frac{2l}{2} \right)=\frac{M}{2}\]
Magnetic line of force is an imaginary curve tangent to which at a point gives the direction of magnetic field at that point or the magnetic field line is the imaginary path along which an isolated north pole will tend to move if it is free to do so. Magnetic lines of force do not intersect each other. Because if they do, there will be two directions of magnetic field which is not possible.
The surface integral of magnetic field \[\overrightarrow{B}\] over a closed surface S is always zero.
Mathematically, \[\oint\limits_{S}{\overrightarrow{B}.\overrightarrow{d}a=0}\]
The branch of Physics which deals with the study of earth's magnetic field is called terrestrial magnetism. William Gilbert suggested that earth itself behaves like a huge magnet.
(a) A freely suspended magnet always comes to rest in N-S direction.
(b) A piece of soft iron buried in N-S direction inside the earth acquires magnetism.
The physical quantities which determine the intensity of earth’s total magnetic field completely both in magnitude and direction are called magnetic elements.
(i) Component in horizontal plane called horizontal component\[{{B}_{H}}\].
(ii) Component in vertical plane called vertical Component\[{{B}_{V}}\].
\[{{B}_{V}}=B\,\sin \theta \] \[{{B}_{H}}=B\,\cos \theta \]
so, \[\frac{{{B}_{V}}}{{{B}_{H}}}=\tan \theta \] and \[B=\sqrt{{{B}_{{{H}^{2}}}}+{{B}_{{{V}^{2}}}}}\]
It is defined as the magnetic dipole moment developed per unit volume when a magnetic material is subjected to magnetising field. Intensity of magnetisation, \[I=\frac{Magnetic\,\,dipole\,\,moment}{Volume}=\frac{M}{V}\]
The magnetic susceptibility of a magnetic substance is defined as the ratio of the intensity of magnetisation to magnetic intensity. \[{{\chi }_{m}}=\frac{I}{H}\] The value of\[{{\chi }_{m}}\] depends on nature of material and temperature.
The magnetic permeability of a magnetic substance is defined as the ratio of the magnetic induction to the magnetic intensity, \[\mu =\frac{B}{H}\]
Hysteresis
The lagging of intensity of magnetisation (I) or magnetic induction (B) behind the magnetising field (H) during the process of magnetisation and demagnetisation of a ferromagnetic material is called hysteresis.
(i) The area of hysteresis loop for soft iron is much smaller than for steel, so energy loss per unit volume per cycle for soft iron is smaller than steel.
(ii) The retentivity of soft iron is greater than that of steel.
(iii) The coercivity of steel is much larger than that of soft iron.
(iv) The magnetisation and demagnetisation is easier in soft iron than steel.
(v) Soft iron acquires saturation magnetisation for quite low value of magnetising field than in case of steel or soft iron is much strongly magnetised than steel.
You need to login to perform this action.
You will be redirected in
3 sec
