Download Chapter 5 MAGNETISM AND MATTER

Chapter 5
MAGNETISM AND MATTER
Magnetism and Matter
* Magnetism is the proper of moving electric charge.
Qn.
Difference between electricity and magnetism?
In electricity isolated electric charge exists. In magnetism isolated magnetic poles do not exist.
* Is magnetic pole required for producing magnetic field?
No. magnetic field is produced by a current carrying conductor.
* Properties of a magnet (a) Attraction and repulsion (b) Magnetic length is less than its Geometric
length (c) Directive property d) Isolated magnetic poles do not exist.
* Magnetic potential energy - work done to turn a magnet in magnetic field.
w   t d q   mB Sinq d q  mBCosq
U  m.B
Case (i)
q =0 U=-mB, minimum - stable state
(ii)
q =90 U=0
(iii)
q =180, U= mB, maximum - unstable state
* Gauss’s Theorem in magnetism - The surface integral of magnetic field over a closed surface is Zero.
 Bds  0
* Gauss’s Theorem establishes that isolated magnetic poles do not exist.
Gauss’s Theorem in elecrostatistics

s
E .d s 
q
0
For an electric dipole net charge q=0 (Since it has equal and opposite charges)
So

E.ds  0
for dipole
Similarly

B.ds  0 says a dipole has North (N) and South (S) poles.
for dipole
* What are the magnetic properties of a material Intensity of magnetisation(M). The extent to which
a material is magnetised.
Magnetic intensity (H) - Ability of a magnetising field to magnetise a material.
Magnetic susceptibility (Xm)- Ability of a material to become a magnet.
Magnetic Permiability ( m ) - Ability of a material to transmit magnetic field lines.
* Relation between magnetic susceptibility and magnetic permiability, mr  1  Xm
* Air is not responding to external magnetic field - Because susceptibility of air is zero.
* Classification of magnetic materials - Faraday Classified
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Properties
1. Intensity of magnetisation
Diamagnetic
Feebly magnetised in
the direction opposite
to the magnetising field
2. Magnetic susceptibility Small and -ve
3. Magnetic Permiability
<1
4. Dipolemoment
Zero
5. Examples
Bi, Cu, Pb, Au, Ag, Hg
Paramagnetic
Feebly magnetised
in the direction of
magnetising field.
Small and +ve
>1
Non Zero
Al, Cr, Ca, Na, Mg,
Ferro magnetic
Strongly magnetic in the
direction of magnetising
field.
High and +ve
Very high
Non Zero
Fe, Co, Ni, Steel
* All the materials exhibit damagnetic property
Note: Magnetising field
* Behaviour of dia, para and Ferromagnetic material in a magnetic field. - Magnetic field used for
induced magnetism
N
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Dia
S
N
S
N
S
Ferro
Para
Core of a solenoid is used as Bismath - write your openion. Bismath is diamagnetic and field
will be less inside the solenoid but it is greater outside the solenoid.
Magnetic Hysterisis : The laging of induced magnetic
field behind the magnetising field.
Define Retentivity - The residual induced magnetism in a material
even when the magnetising field is reduced to Zero.
Coercivity - The reverse magnetising field is required to damagnetise a material.
Hysterisis loss - Energy loss per unit volume when a magnetic material undergoes a cycle of
magnetisation. It is given by area under the hysterisis curve greater the area greater in the hysterisis
loss.
Core of dynamo, Transformer made of soft iron not steel - Hysterisis loss is minimum.
One of the reason for the magnetic field of earth - electric current produced by molten metallic
fluids, mainly iron and nickel, with in its interior called Dynamo effect.
Magnetic elements of earth - The quantities which describe the magnetic field of earth.
(1) Declination (2) Inclination (dip) (3) Horizontal intensity.
Dipangle is Zero at equator and 90 at poles.
At the poles of earth magnetic instruments do not work - Horizontal intensity of earth’s magnetic
field in zero.
Dip needle - Compass needle, which is used to find the direction at the pole.
Horizontal intensity of Earth’s magnetic field (HE) = 0.38x10-4T.
Earth’s core contan Iron yet Geologists do not regard this as source of earths magnetism - In the
molten state Iron is not ferromagnetic.
Why soft iron is preferred morethan steel for making electromagnet.
It has high permiability succeptibility low coercivity.
Explanation of Hysterisis curve of Ferromagnetic (Fe, Co, Ni)
* It has large induced magnetism for small value of H.
M or B
* It has large mr and xm (slope of the graph indicates.
M
* It has low coercivity.
* Low hysterisis loss.
H
H
Note : Soft iron has low retentivity, but graph only indicates large induced magnetism for small magnetising
field.
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