Class-12 Physics Chapter-5 MVVI SHORT Questions
Magnetism & Matter (चुम्बकत्व एवं द्रव्य):-
1. Write the four properties of diamagnetic substance and
paramagnetic substance.
Ans:- Properties of diamagnetic substance:-
(i) It may be solid, liquid or gaseous. (ii) It feebly repelled by a magnetic
field. (iii) Relative permeability less than 1. (iv) The susceptibility is small
and negative.
Ans:- Properties of paramagnetic substance.
(i) It may be solid, liquid or gaseous. (ii) It feebly attracted by a magnetic
field. (iii) It has permanent dipole moment. (iv) Relative permeability
greater than 1.
2. What is magnetic flux density? Write its SI unit and dimensions.
Write the SI unit of magnetic induction/magnetic flux density.
Ans:- Magnetic flux density:- The magnetic flux passes through per unit
area is called magnetic flux density. Its value is also equal to intensity of
magnetic field. It is denoted by B. It is a vector quantity.
Its SI unit is weber/(metre)² =Tesla
Its dimensional formula is = (ML0T-2A-1)
Ans:- The SI unit of magnetic induction is Tesla (T) and 1T = 1 N A-1m-1.
3. What do you mean by permeability?
Write SI unit of magnetic permeability.
Ans:- Permeability represents degree of conductivity of magnetic lines of
force than air. It is denoted by μ.
Iron is good conductor of magnetic lines of force than air.
μ = μ0μr, where μ0 = permeability for free space.
The SI unit of magnetic permeability is TmA-1.
PHYSICS, By:- Mani Sir @ Education Baba
4. What do you mean by magnetic elements of the earth?
At a place the horizontal component of Earth's magnetic field is
times its vertical component. What is the value of dip at this
place?
Ans:- The quantities which completely describe the magnitude and
direction of the magnetic field of earth at a place are called magnetic
element of earth. There are three magnetic elements of the earth-
(i) inclination
(ii) declination
(iii) Horizontal component of earth's magnetic field. BH
Ans:- In the figure, dip is given by
= 300 Bv
5. What do you mean by ferromagnetism?
What do you mean by magnetic hysteresis and coercive force?
Ans:- A substance which acquires strong magnetism in the external
magnetic field and retains magnetization even after removal of field is
called ferromagnet and the property is called ferromagnetism. It arises due
to strong coupling of neighboring domains in the substance. Domains in
presence of external field get aligned to greater extert in the direction of
field, and retain the alignment even after the field is removed. These have
high and positive magnetic susceptibility and show hysteresis. Iron, cobalt,
nickel, alnico, etc. are its examples.
Ans:- When magnetic substance undergoes a magnetic cycle, field
increasing graph and field decreasing graph
do not coincide. This lack of coincidence is
called magnetic hysteresis.
The field applied in reverse direction
to diminish the residual magnetism is called
coersive force.
PHYSICS, By:- Mani Sir @ Education Baba
6. Why steel is used in making permanent magnet, while soft iron is
used in making electromagnet?
Explain the process of demagnetisation.
Ans:- Magnetic susceptibility of soft iron is greater than steel where as
magnetic retentivity of steel is greater than retentivity of soft iron. Due to
this reason soft iron is used in making electromagnet while steel is used in
making permanent magnet.
Ans:- According to domain theory there are seven small domains in
magnetic materials. When rod is magnetised, align along the direction of
field. When rod is heated to very high temperature their alignment is
destroyed so they are disturbed. As a result net magnetic moment
becomes zero and rod is demagnetized.
7. What do you mean by apparent dip angle at any place?
Write a relationship among horizontal intensity, vertical intensity
and dip of the magnetic field of the Earth.
Ans:- Apparent angle:- The angle between the magnetic meridian and the
geographical meridian at a place is called apparent dip angle. When the dip
circle does not lies on magnetic meridian.
BH
Ans:- In the figure, the magnetic induction BE
has two components BH and BV. The angle of
dip is . These are related as
Bv
8. Define magnetic susceptibility and retentivity.
What do you mean by magnetic meridian?
Ans:- Magnetic Susceptibility is property of a magnetic substance by which
we know that upto which extent a substance can acquire magnetism. Soft
iron acquires more magnetism than steel. It is generally denoted by K.
Magnetic susceptibility = K=I/H.
PHYSICS, By:- Mani Sir @ Education Baba
So, it is the ratio of intensity of magnetization (I) to the magnetic field.
Magnetic retentivity is property of a magnetic
substance by which we know that upto which duration a magnetic
substance retain its magnetism.
Ans:- A vertical plane passing through the magnetic axis is called magnetic
meridian.
9. What do you mean by intensity of magnetization?
Write its SI unit. What do you mean by neutral point?
Ans:- When a magnetic substance is kept in a magnetic force field then it
acquires magnetic moment. The magnetic moment acquired per unit
volume is called intensity of magnetisation.
I=
Its S.I. unit is ampere/meter.
Ans:- The point where net magnetic field is zero is called a neutral point.
The magnetic force field in which the magnetic lines
of force are drawn is made up of two fields. First is horizontal component
(BH) of earth's magnetic force field and the other is magnetic force field of
magnet. So we say that those points where BH and magnetic force field of
magnet balances each other are called neutral points.
10. According to modern theory of magnetism, what are the causes of
magnetism?
Define Magnetic potential.
Ans:- Modern theory of magnetism:- The atomic model of magnet is
concerned with atomic structure. In each atom there is a positive charged
nucleus around which electrons revolve in different orbits. The orbital
motion of electron is equivalent to an elementary current.
Due to orbital motion of electron magnetism is produced & there is
magnetic moment. Besides orbital motion there is spin liked to electron;
magnetic moment is also exist due to this spin. So net magnetic moment is
resultant of the two magnetic moments.
PHYSICS, By:- Mani Sir @ Education Baba
Ans:- Magnetic potential:- The amount of work-done to carry the unit
north pole from infinity to any point in the magnetic field is called
magnetic potential at that point.
It is denoted by V. Its SI unit is volt. It is a scalar quantity.
V=
When m = pole strength, r = distance.
PHYSICS, By:- Mani Sir @ Education Baba
