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SPH 111

Handout I

MAGNETISM

1
...
10 Magnetic Properties of Materials and Their Uses
We begin the discussion of magnetic materials by defining the different terms used, ltis

the
properties of various materials because It leads us
necessary
to
investigate
magnetic
to decide whether thiey are suitable lor permanent magnets such as are used in
loudspeakers or temporary magnets as are used in transformers (as cores)
...


1
...
The magnetic field is represented by the field vector H characterizing the

magnet

current carrying conductoras the

of a

force due to this field
...
The other basic magnetic field vector B, is called the

magnetic field induction or the magnetic Mux density
...
The tangent to a line of induction at any point gives the direction of B at that

point
...
Where the lines are close together B is large and

is
proportional to magnitude
where they are far apart then B Is small
...
Then we say that the

v

magnetic field is present at the point, and B is the veclor whose magnitude is given by the
relation

F-gvBsinG
Where F is in a direction perpendicular to both v and B and 6 is the angle between v and
B
...

defined
may
as
current length and at right angles to the

referred to tesla
...

The product qv is a current so any assembly of moving charges or current will

experience a sideways force

when in

a

magnetic

field

In general B is proportional to H and the constant of proportionality
permeabillty) depends on the medium of the space where H is present

Chatterbox tin

(called the

Thus B uH
utoH, is
Ina vacuum H, and is calledthe permeability
of
free
space
...
e
...


Q:What is the
What

relative

are the

permeability of vacuumn

units of

Magnetic flux o

u

The product of intensity B normal to an area and the cross-sectional area A through

which
Bpases is called the flux
...
Thus we can write
-Bacos
whereG is the angle between the lines and the area
...
The orientation of the magnetic moment due to a coll

is

specilied by the vector Hlying along the axis of the coil and pointing in the direction

related to tne current Dy the rignt hand rule
...

the
perpendicular to the
tun,
and
area
A
The
direction
of
vector
is
plane of the coil in the same sense given by the right hand rule, that is

NAI

And is measured in Am*
A
ina
an
acceleration and provided there is damping ultimately comes to rest with its axis parallel
field
the
the
The direction
of the
a
is the
into
the
vector of either a small plane coil or a small magnet tends to turn when the small coil or
magnet is placed at that point in space
...
12: Magnetization

magnet

to the

uniform magnetic field experiences acouple which gives it angular
and so magnetic moment vectorand field vector align
...

This
may
hapPen
the presence
absence of a magnetic field
...

a1
...


Consider a toroid of length L, as shown:

fotn cote

fux density
...


Thus we have
B= B, + B

wire, and B, is the fux density due
is
where
B,
the
flux
density
due
to
the
current
Iin
the
to the magnetization of the material
...
The

magnetization
flux density Ba is produced by many small circulating curents inside the
magnetic material, due to the circulating and spinning electrons in the atoms
...
This current is called Surface
or Magnetization current
...
The
flux density B,
current produces
actual
Let n be the turns per unit length (n - NL), where L
a

B, =Hnl =

=4,

L
2r

Chatterbox tn

2 7 Then

marviktintor

Surtace current Im produces a flux density Bam which is given by the equation below

Therefore, the total flux density B is given by
B= B,+ B
...
For the whole toroid, the

where A

is given by

nLAl

magnetic moment

N

Magnetic moment per unit volume, le
...
=4,nl
...
=#_nl+4_n
...
What are the units for H
2
...
Also find out the value for Ho

1
...
(H+M)

H+M=1+M
H
H

And

The ratio MHis called the magnetic susceptibility
...
of the material
...
e
...
, is subjected to an increasing field, the
intensity of magnetization M increases until it reaches a maximum value, as shown below
...
e
...
But B can still continue to increase with H
...
15
Types
of Magnetic
Various
materials
respondMaterials
differently to changes in the field H
...
Thus magnetization, M will be in the opposite directon to H and

Chatterbox tin

5

marvikiinto

therefore HM is negative, and hence susceptibility x is negative
...
000015
...
But if a
field is applied, each atomic moment will try to align in the direction of the field even

though themal motion will prevent complete alignment
...
Hence susceptiblity x of a paramagnet

Substance is very small and positive
...
00O1
...
Thus a ferromagnetic material aligns itself with an applied

field
...
Feromagnetic materlals are further
Brouped into two:

Soft Magnetic Materialss
These magnetise easily and demagnetise when the applied field is removed
...


These
are hard to magnetise but retain their magnetism when the applied field is removed
...

1
...

0a
abthe applied field His reduced to zero and the field B follows the path ab
...

bcthe applied field
is done with cd and da
...
When the applied field His reduced

to zero, there is still some magnetic field remaining in the specimen as indicated at b
...
At the point b, the reverse applied Hls applied to
the specimen
...


Eventually, the flux density is reduced to zero, when the opposing field Hhas a value H
...
This is a measure of the difficulty of
breaking up the alignment of the domains
...
Instead it forms a closed loop
B
is defined
the
as the
behind the applied field Hwhen the specimen is taken through a magnetic cycle
...
Hysteresis

lagging of

magnetic induction

Home WorkI

a) Give an account of the domain theory of magnetisation

(6) How does the theory explain the process of magnetization and demagnetization?

a) What is meant by magnetic hysteresis? Sketch a typical hysteresis curve and explain
...
5A
...


Assume an empty core
...
82
...

A cast iron ring has a mean diameter of O
...


I is

unitormly wound with 2000 ums caTying a curent of 2
...
What is the relative permeability of the
iron?Ans=z000
...
0A
...
With the aid of a search coil and charge measuring
instrument, the magnetic field is found to be 1
...
Calculate
(a) The magnetic intensity
(b) The magnetization

c) The magnetic susceptibility
surface current
(d)
The equivalent
(e) The relative permeability

Chatterbox in



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