Magnetism is the property of certain type of material because
of which they are able to attract some other kind of materials like iron and
copper. There are some natural magnetic materials that can do that but to use
the property practically magnets were made artificially.
Bar magnets are
popular artificial magnets and they are in bar shape with north and south poles
at the edges. The distance between two poles is called length of the magnet and
it is bit less than the geometrical length of the magnet.
There will be a field around every pole up to where it can
influence the surroundings and the field is called magnetic field. Magnetic
line of force is the imaginary path followed by a unit north pole and they
start from North Pole and move towards South Pole of the magnet. They are
closed loops. They travel from South Pole to North Pole inside the magnet. Two
lines of force never intersect with each other. The tangent drawn at any point
of the line of force gives the direction of the magnetic field at any given
point.
If they intersect with each other, at the point of
intersection, we can draw two tangents and hence it is supposed to have two
directions at that point. It is not practically possible for a field to possess
two directions at a given point simultaneously and hence two lines of force
never intersect with each other.
If the strength of the magnetic field is high, we can find
more number of magnetic lines of force around it. If the magnetic field is
uniform, magnetic lines of force around that field will be parallel to each
other.
Number of magnetic lines of force in a given area is called
magnetic flux and magnetic flux per unit area is called magnetic field
induction or magnetic field strength. It is measured with a unit weber per
meter square.
Torque experienced by a bar magnet in a uniform magnetic
field
Every bar magnet do have two poles called north and south
poles. Their pole strength is same in magnitude but opposite in nature. Each
pole when placed in a magnetic field experience a force in a specified
direction. Thus the magnet has a pair of forces acting on its poles in opposite
direction at its different ends. This produce a turning effect in the magnet
and it is called torque. We can find the torque as the product of any one force
and the perpendicular distance between them.
We can draw a diagram to represent the force and torque as
shown in the diagram below.
Let us assume each pole of the bar magnet has pole strength m
and let it is placed in a uniform magnetic field of field strength B. Hence
each pole experience a force and the two forces are acting on two different
edges of in opposite direction. They together turn the bar magnet in the
anti-clock wise direction and hence there is a torque acting on it.
We can find the value of perpendicular direction basing on
the angle made by the magnet with the field as shown in the diagram below.
If the angle between the field and the bar magnet is zero,
then the toque acting is zero and if they are perpendicular to each other,
magnetic fields torque is maximum.
Torque can be defined as the magnetic moment of a bar magnet
when it is placed in a uniform magnetic field of field strength one unit and it
is perpendicular to the field.
Force between two poles
It is experimentally observed that similar poles experience a
force of repulsion and opposite poles experience a force of attraction. The
magnitude of force of attraction or repulsion is directly proportional to the
product of the pole strengths and is inversely proportional to the square of
the distance of the separation. This force depends on the media between the two
poles and two study that, we introduce a term permeability. It is the measure
of nature of the medium that how good or bad the medium is in passing the
magnetic field through it.
Pole strength is measured with a unit ampere-meter. Pole
strength of a pole is said to be one unit when two identical poles separated by
unit distance of separation in vacuum experience a force in the order of
negative 7 orders.
If the medium is different between the two poles, the force
between them changes and we shall use the permeability of that medium. We can
define relative permeability as the ratio of permeability of the medium to the
permeability of the vacuum.
Magnetic field induction varies on axial and equatorial line.
The line passing through both the poles and also along the center of the magnet
is called axial line. The line passing through the center of the magnet and
perpendicular to the axial line is called equatorial line.
It can be proved that the magnetic induction on axial line is
double the induction value on the equatorial line at the same distance.
