MAGNETIC FIELD
MAGNETIC FIELD
DEFINITION
OF MAGNET AND MAGNETIC MATERIALS.
Magnets are substances that are able to attract and hold items.
Lodestone is the only known natural magnet which was discovered by the Chinese
2,000 years ago.
A
magnet is defined as an object which is capable of producing magnetic field and
attracting unlike poles and repelling like poles.
MAGNETIC PROPERTIES.
Following
are the basic properties of magnet:
1.
When a magnet is dipped in iron filings, we can observe that the iron filings
cling to the end of the magnet as the attraction is maximum at the ends of the
magnet. These ends are known as poles of the magnets.
2.
Magnetic poles always exist in pairs.
3.
Whenever a magnet is suspended freely in mid-air, it always points towards
north-south direction. Pole pointing towards geographic north is known as North
pole and the pole pointing towards geographic south is known as South pole.
4.
Like poles repel while unlike poles attract.
5.
The magnetic force between the two magnets is greater when the distance between
these magnets are lesser.
Magnets and non-magnetic materials
Magnetic materials are those that are strongly attracted by
magnets while non-magnetic ones are those that are not affected by magnets.
Iron, steel, cobalt and nickel are magnetic substances, while wood, glass and
copper are examples of non-magnetic substances. Substances that are
repelled by magnets are said to be diamagnetic whereas those which are strongly
attracted i.e. iron, nickel, cobalt are called ferromagnetic materials.
The materials that are so lightly attracted such that the magnet seems to have
no effect on them are called paramagnetic materials (mostly
non-magnetic materials). Ferrites are a mixture of iron
oxide and barium oxide are the most newly developed
magnetic materials. Ceramic magnets or magnadur magnets are made from ferrites
and are very strong.
TYPES OF MAGNETS
There are three types of magnets and they are as follows:
1.
Permanent magnet
2.
Temporary magnet
3. Electromagnets
Permanent Magnet
Permanent magnets are those magnets that are commonly used. They
are known as permanent magnets because they do not lose their magnetic property
once they are magnetized.
Following are the ways to demagnetize the permanent magnets:
Exposing magnets to extreme temperatures.
The magnetic attraction between the magnet’s atoms gets loosen
when they are hammered.
Stroking one magnet with the other in an inappropriate manner will
reduce the magnetic strength.
There are four types of permanent magnets:
i. Ceramic or ferrite ii. Alnico iii. Samarium Cobalt (SmCo) iv.Neodymium Iron Boron (NIB)
Temporary Magnet
Temporary magnets can be magnetized in the presence of a magnetic field.
When the magnetic field is removed, these materials lose their magnetic
property. Iron nails and paperclips are examples of the temporary magnet.
Electromagnets
Electromagnets consist of a coil of wire wrapped around the metal
core made from iron. When this material is exposed to an electric current, the
magnetic field is generated making the material behave like a magnet. The
strength of the magnetic field can be controlled by controlling the electric
current.
DIFFERENCE
BETWEEN ELECTROMAGNET AND PERMANENT MAGNET
As
the name suggests permanent magnet magnetic field is permanent and
electromagnets magnetic field depends upon the flow of the electrical current. The electromagnet constitutes a coil made of wire which acts
as a magnet when current is passed through it. Usually a ferromagnetic material
like steel is wrapped by an electromagnet to enhance its magnetic field.
|
Difference
Between Electromagnet and Permanent Magnet |
|
|
Electromagnet |
Permanent
Magnet |
|
The
magnetic properties are displayed when current is passed through it |
Magnetic
properties exist when the material is magnetized |
|
The
strength is adjusted depending upon the amount of flow of current |
The
strength depends upon the nature of the material used in its creation |
|
Removal of
magnetic properties is temporary |
Once
magnetic properties is lost, it becomes useless |
|
It requires
a continuous supply of electricity to maintain its magnetic field. |
It doesn’t
require a continuous supply of electricity to maintain its magnetic field |
|
It is
usually made of soft materials |
It is
usually made of hard materials |
|
The poles
of this kind of magnet can be altered with the flow of current |
The poles
of this kind of magnet cannot be changed. |
MAGNETIZATION AND DEMAGNETIZATION
Making
magnetization
The following are methods used to make magnets.
a) Magnetic induction – this is a process by which magnets
are made by placing ferromagnetic materials in a magnetic field. Materials like
iron lose their magnetism easily and are said to be soft while others like
steel gain magnetism slowly but retain it longer and are therefore said to be
hard and are used to make permanent magnets.
b) Magnetizing by stroking – the object to be magnetized is
placed on a bench then a bar magnet is dragged along the length of the bar from
one end to the other.
This is repeated several times and the object becomes magnetized.
This method is known as single-stroke method.
c) Magnetizing using an electric current – this is the use of magnetic effect of an electric current through a solenoid (insulated wire of many turns).
Demagnetizing is the process of removing magnetic properties of a
magnet .
The following methods are which a magnet can lose its magnetism;
a) Hammering them hard with their poles facing E-W direction
b) Heating them strongly
c) Placing a magnet inside a solenoid and passing an a.c. current
through it for a short time.
KEEPERS
Caring for magnets
a) Magnets should be stored in pairs with unlike poles adjacent to
each other attached to pieces of soft iron called keepers.
b) Magnets should not be hammered especially with
their poles facing E-W direction.
c) Magnets should not be heated strongly or dropped roughly on
hard surfaces.
d) Magnets should not be placed near alternating currents.
e) Magnets should be kept dry and clean since rust can make them
lose their magnetism.
LINES OF FORCE
A line of force gives the direction of the magnetic field at each
point along it.
Magnetic lines of force are imaginary lines along which a
free-North pole would tend to move if place in the field. A line of force may
also be considered as a line such that the tangent to it at any point gives the
direction of the field at that point.
Using iron fillings to show field lines
Magnetic Field Patterns.
Plotting field patterns
Their closeness is a measure of the strength of the magnetic field
or of the force that would be exerted by the bar magnet.
Examples of field patterns.
The points marked ‘X’ are called neutral points
where there is no magnetic field at such points.
Watches (non-digital), electron beams in cathode ray tubes and TV
sets are shielded from external magnetic fields by placing a soft-iron cylinder
around the neck of the tube or watch.
THE EARTH’S MAGNETIC FIELD
The magnetic elements of a place are the Angle of Declination, the
Angle of Dip and the Earth’s magnetic field in that place. These three completely
determine the magnetic field of the earth at each place.
ANGLES OF INCLINATION AND DECLINATION.
At any place in the earth’s surface, the Magnetic North is not
usually in the same direction as the Geographic North. It is therefore
necessary to differentiate between two planes, the magnetic meridian and
geographic meridian.
The Magnetic Meridian at any place is the vertical plane passing
through the magnetic axis of a free suspended magnet at rest under the
influence of Earth’s magnetic field.
The Geographical Meridian is the vertical plane containing the
geographic north and south pole of the Earth.
Angle of
declination or variation is the angle between the geographic and magnetic
meridians at a place.
Example
If the angle of declination in a place is 10o,
calculate the true geographic bearing if the compass needle reads N40o
E.
Solution
Angle of true bearing = angle of declination + angle of magnetic
meridian(at the same direction) = 40o
+ 10o = N50oE
Angle of Dip
or Inclination (I)
A freely suspended magnet will not only come to rest pointing
approximately in a North-South direction, but will also be found to tilt (or
dip) downwards at some angle to the horizontal.
The angle of
dip (I) is the angle between the direction of the earth’s resultant magnetic
field and the horizontal.
The angle of dip at any place may be measured with an instrument
called the dip circle.
The angle of dip also
varies all over the earth’s surface from 0o at the magnetic equator to 90o at
the magnetic poles of the earth.
Angle of dip (Ф)= tan
Note, H
=B cos Ф and V= B sin Ф
MAGNETIC FORCE ON A CHARGE MOVING IN A MAGNETIC FIELD
A magnetic field exerts a force on a charge moving in the field.
The magnetic force on a charged particle moving across a magnetic field is
given by F= qVxB = qVB sin ᴓ. Where q is the charge in Coulombs, V is the
average velocity of the charge in metre per seconds, B is the flux density or
magnetic induction whose unit is the Telsa (T) [ 1 Telsa = 1 weber per square
metre] and F is the force in Newtons.
The force is greatest when the charged particle moves
perpendicular to B (ᴓ = 90o). The force is zero if the particle
moves parallel to the field lines (ᴓ=0o). The direction of the
magnetic force is perpendicular to the magnetic field B and to the velocity, V,
of the particle.
Example
Find the magnetic force experienced by an electron of charge
1.6x10-19C projected into a magnetic field of flux density 10T, with
a velocity of 3x107 ms-1, in a direction:
I. Parallel to the
field II. At right angle to the
field III. At 30o
to the field
Solution
q =1.6x10-19C ; B = 10T; V= 3x107 ms-1
I. Parallel to the field
When V and B are parallel i.e ᴓ=0; since sin ᴓ= 0, Hence F=qVB sin
ᴓ =0
II. At right angle to the field i.e when ᴓ =90o
F = qVBsin ᴓ = 1.6x10-19C X 10T
X 3x107 ms-1 x sin
90 = 4.8 x 10-11N
III. At 30o to the field
F = qVBsin ᴓ = 1.6x10-19C X 10T
X 3x107 ms-1 x sin
30 = 2.4 x 10-11N
Assignment
1. A mariner’s compass
gives a bearing 15o West of North at a place where the declination
is 19o West of North. What is the true bearing of the place if the
declination has been 19o East of North, what would have been the
true bearing?
Solution
Angle of true bearing = angle of declination + angle of magnetic
meridian(at the same direction) = 19o
+ 15o = W34oN
Angle of true bearing = angle of declination + angle of magnetic
meridian(at opposite direction) = 19o
- 15o = E4oN
2. A charge of 1.6x10-19C
enter a magnetic field of flux density 2.0T, with a velocity of 2.5x107
ms-1, at an angle of 30o with the field. Calculate the
magnetic of the force exerted on the charge of the field
Solution
q =1.6x10-19C ; B = 2.0T; V= 2.5x107 ms-1
; ᴓ =30o
F = qVBsin ᴓ = 1.6x10-19C X 2.0T
X 2.5x107 ms-1 x
sin 30 = 4.0 x 10-12N
Uses of magnets
1. Used in making other magnets
2. Used in making loud speakers
3. Used in making moving coil meters
4. Used in making telephone speakers.
5. used in magnetic compass
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