1.
Do
magnets ever lose their magnetism?
Yes, it
is possible for a permanent magnet to lose its magnetism.
There are three common ways for this to occur:
1) Via heat: Ferromagnet materials will lose their magnetism
if heated above a point known as the Curie temperature. At
this point, the energy being put into the magnet from the
heat will permanently disrupt the magnetic domain structure
of the material, turning it into a paramagnetic material [a
similar effect occurs in materials called hard ferrites,
which exhibit a form of magnetism called ferrimagnetism; the
analogous temperature for these materials is known as the
Neel point]. You would have to re-magnetize the magnet
again, either in a solenoid or with another permanent
magnet, in order to restore the magnetism. If you heat a
magnet up a little bit, it will lose some of its magnetism,
but on returning to room temperature [depending on how high
it was heated, and on the shape of the magnet itself], full
magnetism can be restored.
2) Via a demagnetizing magnetic field: permanent magnets
exhibit a characteristic called coercivity, which is the
ability of a material to withstand being demagnetized by an
applied magnetic field. Modern permanent magnet materials
such as Sm-Co and Nd-Fe-B have high coercivities; older
materials such as Alnico or ceramic [hard ferrite] materials
have lower coercivities. With a strong enough magnetic field
of opposite polarity, it is therefore possible to
demagnetize the magnet [whether this comes from another
permanent magnet, or a solenoid]. Interestingly, an opposing
magnetic field is sometimes applied to a magnet in order to
'knock it down', or to lower its overall magnetic output, so
that it can be used appropriately in an application.
3) Via shock: this really only applies to older materials
such as magnetic steels and Alnico materials; the mechanism
that creates coercivity means that they are susceptible to
being demagnetized if enough energy is transmitted through
the material via a shock, such as being dropped or hit with
a hammer. Modern materials do not suffer this type of
problem.
2. Can magnetic lines of force intersect?
No,
magnetic lines of force cannot intersect with each other. As
the direction of the magnetic field at any point on the
magnetic lines of force is the tangent to the line at that
point. Hence, if we suppose that two magnetic lines of force
intersect with each other at a point then there are two
tangents at that point showing two different directions of
the magnetic field at the same point that implies there are
two magnetic intensities at the point which is not possible.
Hence, two magnetic lines of force never intersect with each
other.
3. What happens to magnetic property of a
bar magnet, if it is heated?
If a bar
magnet is heated,
due to the thermal or heat energy, the tiny or small
molecular magnet regains the kinetic energy and orient
themselves in any directions. As the temperature further
increases to its melting point, the orientation of the
molecular magnets are completely changed and they are
arranged in random or haphazard fashion resulting in the
lost of its magnetic property i.e.
ferromagnet materials will lose their magnetism if heated
above a point known as the Curie temperature. At this point,
the energy being put into the magnet from the heat will
permanently disrupt the magnetic domain structure of the
material, turning it into a paramagnetic material [a similar
effect occurs in materials called hard ferrites, which
exhibit a form of magnetism called ferrimagnetism; the
analogous temperature for these materials is known as the
Neel point]. You would have to re-magnetize the magnet
again, either in a solenoid or with another permanent
magnet, in order to restore the magnetism. If you heat a
magnet up a little bit, it will lose some of its magnetism,
but on returning to room temperature [depending on how high
it was heated, and on the shape of the magnet itself], full
magnetism can be restored.
4. What is the reason behind the
attraction of a magnet with the magnetic substances?
The
magnetic substances contains a large number of magnetic
domains which are oriented randomly in all directions so
there is no magnetic effect. When the magnet is brought
nearer to the magnetic substance then the magnetic field of
the magnet causes alignment of domains of magnetic substance
along the magnetic field so that the magnetic substance
becomes temporary magnet. During this process, the south &
north pole of temporary magnet lies close to north & south
pole of permanent magnet respectively.
As
unlike poles attract each other.
Hence, this is the reason behind the attraction of a magnet
with the magnetic substances.
5. Define neutral point. When the north
pole of a bar magnet points south where will the neutral point
lies?
When two
magnetic fields act against each other, there exist certain
regions or points where they totally cancel each others effect.
When certain magnetic materials are placed at such region or
points, they do not feel any force and so they don't have any
particular orientation. Such points are called as neutral point.
Hence, neutral point is defined as that point in the magnetic
field where the field due to a magnet is completely neutralized
by the horizontal component of the earth's magnetic field.
When the
north pole of a bar magnet points south then the cancellation of
magnetic forces can occur only at certain points along the
equatorial line where the fields are in opposite directions but
equal in magnitude. That means the neutral point lies on the
equatorial line of the magnet where the fields are in opposite
directions with each other and equal in magnitude.
6. Why the magnetic needle cannot show
proper direction when kept at neutral point?
At
neutral point, the field due to the magnet is completely
neutralized by the horizontal component of the earth's
magnetic field. Hence, there is no net force to influence a
magnetic needle kept at that point. So, the magnetic needle
cannot show proper direction when kept at the neutral point.
7. Classify ferro, dia and paramagnetic
material on the basis of susceptibility.
The
property of the magnetic substance due to which
magnetization of the magnetic substance occurs to different
extents is called susceptibility which is defined as the
ratio of the intensity of magnetization to the strength of
the magnetising field. It is denoted by
and given as
1. The
magnetic materials which have large positive susceptibility
are called ferromagnetic material.
2.
Paramagnetic materials have small positive susceptibility.
3.
Dia
magnetic materials have small negative susceptibility.
8. When does a ferromagnetic substance becomes paramagnetic and
why?
A
ferromagnetic materials are those materials that are
strongly attracted by magnets and magnetic field. Its
susceptibility is positive and large and it has very high
permeability. Due to the fall in susceptibility above curie
temperature all its atomic magnets are randomly oriented so,
these materials becomes paramagnetic materials above curie's
temperature.
9. Does
diamagnetism depends upon the temperature or not?
No,
diamagnetism is independent of temperature. In diamagnetic
material, the induced magnetic moments of the atomic magnets
are always opposite to the applied magnetizing field.
So, the magnetic moments are independent of the temperature.
Hence, the diamagnetism is independent of the temperature.
10. On which
principle does the deflection magnetometer works?
The
deflection magnetometer works on the principle of tangent
law which states that "if a freely suspended magnet is in
equilibrium under the action of two mutually perpendicular
magnetic fields F & H (say) making angle f with H then the
relation between F & H is given as F = H tan f ".
11. While
performing an oscillation magnetometer experiment, we use a
brass bar to remove the torsion in the thread. Can we replace it
by a similar iron bar? Explain.
While
performing an oscillation magnetometer experiment, we use a
brass bar to remove the torsion in the thread. It is because
brass is as alloy, non-magnetic substance & does not align
in any particular direction. No, we can't replace it by a
similar iron bar as the iron bar is magnetic material & if
it is used then it is feebly magnetized due to the earth's
magnetic field. So, iron aligns itself along the direction
of earth's field.
Hence, we cannot replace a brass bar by a similar iron bar
to remove torsion in the thread.
12. Can the earth's magnetic field be vertical at a place? What
will happen to a freely suspended magnet at such place?
Yes, the
earth's magnetic field can be vertical at a place i.e. at
poles. At poles, the value of angle of dip is 900.
Using tangent law, the magnet placed at poles will make
angle of 900
i. e. the magnet will be vertical at such place.
13. The
Horizontal and Vertical components of the earth's magnetic field
are equal at a place. What is the value of angle of dip?
The
Horizontal component H and Vertical component V of the earth's
magnetic field are equal at a place i.e. V = H. The angle of dip
at a place is given by the relation:
Hence, the
angle of dip where the horizontal & vertical components of the
earth's magnetic field are equal
is 450.
14. What is
meant by the statement that the declination at a place is 130E
?
The
statement that the declination at a place is 130
E means that the magnetic meridian is 130 east of
the geographical meridian & the compass needle points 130
east of the north as the angle between the magnetic
meridian & the geographic meridian is called the
declination.
15. Why does
an iron nail experience a force of attraction when brought near
a magnet?
The
magnetic substances like iron contains a large number of
magnetic domains which are oriented randomly in all
directions so there is no magnetic effect. When the magnet
is brought nearer to an iron then the magnetic field of the
magnet causes alignment of domains of iron along the
magnetic field so that the iron becomes temporary magnet
whose North pole or South pole are close with South pole or
North pole of a permanent magnet .
As
unlike poles attract each other and there develops a
Coulomb's force of attraction between them. Hence, an iron
nail experience a force of attraction when brought near a
magnet.
16. Why do
both poles of a magnet attract an unmagnetized piece of iron?
An
unmagnetized piece of iron contains many domains arranged
randomly whose effect of each of these domains cancel one
another. So, the iron piece is not magnet and does not
follow the property of a magnet. There is no repulsion but
attraction between iron and each pole of a magnet. As we
know that the middle part attraction is poor but attractive
property of magnet at poles is maximum. So, both poles of a
magnet attract an unmagnetized piece of iron.
17. What
happens if a bar magnet is cut into two pieces perpendicular to
its length and along its length?
If a bar
magnet is cut into two pieces perpendicular to its length
and along its length then a single bar magnet is
transfigured into two bar magnets having magnetic moment
less than the original bar magnet.
18. Why is repulsion considered as sure
test of magnetism?
There is
always attraction between the unlike poles but there is
repulsion between like poles. Hence, the attraction occurs
between unlike poles & between magnet and magnetic
substances and there is cent percent for the repulsion of
like poles. Hence, repulsion is
considered as sure test of magnetism.
19. Is the magnetic field near a magnet uniform or non-uniform?
The
magnetic field near a magnet is non-uniform because uniform
magnetic field is the space in which the lines of force are
parallel to each other but in magnet, lines of force are
supposed to emerge from North pole & enter into magnet at
south pole which are not parallel. Hence, the magnetic field
near a magnet is non-uniform.
20. Does a
field of a bar magnet exert a torque on the magnet itself?
No, a field of
a bar magnet does not exert a torque on the magnet itself. A
magnet when place in the external uniform magnetic field
experience a torque but a field of a bar magnet is
non-uniform. So, a field of a bar magnet does not exert a
torque on the magnet itself.
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