Download Question bank of magnetism - Kendriya Vidyalaya SAC, Vastrapur

KENDRIYA VIDYALYA SANGATHAN, AHMEDABAD REGION
PHYSICS WORKSHOP (II) JULY 2016
QUESTION BANK-PHYSICS
CHAPTER 5 MAGNETISM
SECTION A (1 MARK QUESTIONS)
1. Write the name of a physical quantity having unit J/T. State, whether it is a scalar or a vector
2. A short bar magnet placed with its axis making an angle Ө with uniform magnetic field experience
a torque. What is the magnetic moment of the magnet?
3. If the horizontal and vertical component of earth’s magnetic field are equal at a place. Find the
angle of dip.
4. Which is the material used in making the core of a moving coil galvanometer?
5. A magnetic needle free to rotate in vertical plane orients itself vertically at a certain place on the
earth. What are the values of (i) Horizontal component of the earth’s magnetic field (ii) angle of
dip at this place?
SECTION B (2 MARK QUESTIONS)
1. An electron beam passes through a region of crossed electric and magnetic field of strength E
and B respectively. For what values of the electron speed the beam will remain undeflected?
2. Write any three characteristics of Ferro magnetic substance should possesses if it is to be used
to make a permanent magnet.. Give an example of such a material.
3. If χ stands for the magnetic susceptibility of a given material. Identify the class of materials for
which (i) -1≥ χ<0 (ii) 0< χ<a (A small positive number).
Write the range of relative magnetic permeability of these materials.
4. The susceptibility of a magnetic material is 2.6 X10-5.Identify the type of the magnetic material
and states its two properties.
5. State Curie’s law in magnetism. Name the material which does not obey this law.
SECTION C (3 MARK QUESTIONS)
1. Define magnetic dipole moment. Derive an expression for the magnetic moment of a dipole
placed in uniform magnetic field.
2.The horizontal component of earth’s magnetic field at a given place is 0.4x10-4 Wb/m2 and andle of
dip is 30°.Calculate the value of (i)Vertical component (ii)Total intensity of earth’s magnetic field.
3. Define intensity of magnetisation and Magnetic susceptibility. Write their expression also.
4. Define magnetic elements of the earth. Define them.
5.A bar magnet of magnetic moment 1.5 JT-1 lies aligned with the direction of a uniform magnetic
field of 0.22 T
(a) What is the amount of work required by an external torque to turn the magnet so as to align its
magnetic moment? (i)Normal to the field direction (ii) Opposite to the field direction.
(b) What is the torque on the magnet in case (i) and (ii)?
SECTION D (5 MARK QUESTIONS)
1. Define paramagnetic, diamagnetic and ferromagnetic substances. Write three properties of each.
2. An electron is revolving around the nucleus in a circular orbit of radius r with speed v. Derive an
expression for its orbital angular momentum. Hence define Bohr’s magneton.
VALUE BASED QUESTION
1. Mrs Lata found that her son could not hear properly. The specialist prescribed hearing aid for her
son. Hearing aids consist of electromagnets in the loudspeakers used in the device..
(i) What two values does Ms Nita exhibit towards her son and students? Mention any two .
(caring attitude, sensitive towards society, concern for others
(ii) What is an electromagnet? In what way its hysteresis curve is different from that used for
permanent magnets?
Ans. Electromagnet- temporary magnet. Hysteresis curve has small are, small coercivity, small
retentivity.
2. Ms Sumathy wife of Mr Varadan complained about the non availability of gas cylinders and
explained to him to look out for alternate method for cooking Mr Varadan bought an induction
stove to over come the fuel problem. The next day Ms Sumathy used her copper bottom cooker
and kept it on the induction stove. But even after using it for half an hour she found that the
cooker was not hot and food not cooked. AS she was not aware of the method to use the
induction stove, she asked her elder daughter Dhanya, studying first year engineering about it.
She told her that some vessels cannot be used on this stove. She took the instruction manual and
explained to her mother, that the stove works on magnetic induction and copper being a
diamagnetic material will not respond to it.
(I).What values did Mr Varadan and Dhanya exhibit towards Ms Sumathy?
(II).Give few example of diamagnetic material and explain how their susceptibility varies with
temperature?
3. Rohit’s mother put many clothes for washing in the washing machine. She could not start the
washing machine as an indicator was showing that the lid was not closed. On seeing his mother
struggling, Rohit tried his best to close the lid but soon he realised that the closing mechanism
was not a manual system but a magnetic system. He went to the shop and purchased a magnetic
closer. He replaced the older closer by the new one. The machine started working. The mother
was happy that her son helped her to save three hundred rupees.
(a) What are the values being displaced by Rohit?
(b)A magnetised needle in a uniform magnetic field experiences a torque but no net force. An iron
nail near a bar magnet, however experiences a force of attraction in addition to a torque. Why?
HOTS
1. From molecular point of view discuss the temperature dependence susceptibility for
diamagnetism, paramagnetism and ferromagnetism.
2. The following figure shows the variation of intensity of magnetisation versus applied the magnetic
field intensity H, for two magnetic materials A and B.
(a) Identify the materials A and B.
(b) For the material A, plot the variation of intensity of magnetisation verses temperature.
B
I
A
H
3. A proton has spin and magnetic moment just like an electron. Why then its effect is neglected in
the magnetism of the material?
SOLUTION OF QUESTION BANK
CHAPTER 5 MAGNETISM
SECTION A (1 MARK QUESTIONS)
1. Magnetic dipole moment, vector
2. M=Ƭ/ (B Siin Ө)
3. Tan δ BV/BH =1, δ =450
4. Soft iron core, because it has high initial value of magnetic permeability and low hysteresis loss.
5. δ =00 & δ =900
SECTION B (2 MARK QUESTIONS)
1. An electron beam passes through undeflected if electric and magnetic force on electron is equal
and opposite
Where q E =q v B so v =E/B
2. Characteristics of permanent magnet
(i) High permeability
(ii) High retentivity
(iii) High coercivity
Example:-- steel, alnico
3. (i) Diamagnetic material (ii) paramagnetic material
µr =1+ χ
(i)range of relative magnetic permeability for diamagnetic is 0≤µr <1
(ii)range of relative magnetic permeability 1<µr <1+a
4. Paramagnetic material.
(i) They have tendencies to move from weak magnetic field to the stronger magnetic field.
(ii) When placed in external magnetic field, the field lines get concentrated inside the material and
the field inside is enhanced.
5. The magnetic susceptibility of the paramagnetic material is inversely proportional to the absolute
temperature. Diamagnetic substance does not obey this law.
SECTION C (3 MARK QUESTIONS)
1.
1. Dipole moment M: - Product of the pole strength of one
of the pole & effective length of a magnet is called dipole
moment M vector. It is a vector quantity directed from South
Pole to North Pole.
5.5 Torque on a Bar magnet in a magnetic field: Consider a magnet ns placed in a magnetic field of intensity H
vector (uniform) making angle  with H vector. Then it will
experience two equal but opposite forces F = m H parallel to
the field. This two equal & opposite force will setup a couple.
Torque of the couple  = Force  perpendicular distance In SAN , sin  
SA
SA

SN
2l
dipole moment M  2 m l

Hence
-
- - (1)
SA  2l sin 
  mH  2l sin 
Putting value in eq1
But

-

So
  MH sin 

  M H
Direction of the torque is perpendicular
to both M & B & can be given by the Fleming's right hand rule.
2.(i)Given H=0.4x10-4Wb/m2 ,Ө=30°
tan Ө= V/H
Vertical component=H Tan Ө= H=0.4x10-4xtan30= 0.23x10-4Wb/m2
(ii) Total intensityof earth’s magnetic field
Be=(H2+V2)1/2= ((0.4x10-4)2+(0.4x10-4/(3)1/2)2)1/2
Be=0.46x10-4Wb/m2
3. Intensity of magnetisation: - When a magnetic substance placed in a magnetic field it
becomes magnetized and acquires a magnetic moment pet unit volume of the substance is
known as intensity of magnetization I.
I 
M
V
 I
m  2l
A  2l

I

m
Amp/m.
A
Pole strength developed per unit area of the substance is known as intensity of
magnetization I.
Magnetic Susceptibility ( ) : -Intensity of magnetization ‘I’ is proportional to the intensity of
magnetizing field
‘H’ in which the substance is placed.
I

H
I = m H Where m is a constant called magnetic susceptibility.
If H = 1 & then  = I
Intensity of magnetization developed in a substance
when placed in uniform magnetic field of unit intensity is define as magnetic susceptibility.
It is ability of the substance to acquire magnetization when placed in magnetic field.
4. Magnetic element of the earth’s magnetic field: -The quantities, which
completely determine the magnetic field of the earth, are called magnetic elements of the
earth.1) Declination 2) Dip or inclination 3) Horizontal component of the earth
magnetic field
1) Angle of Declination: - Angel between the geographical meridian & magnetic meridian is
called Declination ‘’. It is about 17 from the direction of the magnetic meridian &
Geographical meridian, the declination can be measured. It is helpful in steering ship in
the right direction with the help of mariners compass
2) Angle of Dip or inclination: -If a magnetic needle is free to rotate in a vertical plane, it
does not remain horizontal but makes an angel with the horizon.
The needle must be in the magnetic meridian then the north pole of the needle is
depressed below the horizontal, while the South Pole of the needle is pointing above
the horizontal the direction of the needle give the direction of the total intensity of
earth magnetic field.
5. (a)Work done W= MB(cosӨ1-cosӨ2)
(i) Ө1=0° and Ө2=90°
W= 1.5X0.22 (1-0)= 0.33J
(ii) Ө1=0° and Ө2=180°
W=2X1.5X0.22=0.66J
(b) Torque=MBsinӨ
(i)Ө=90°, torque=1.5x0.22=0.33Nm
(ii) Ө=180° torque=0
SECTION D (5 MARK QUESTIONS)
1.
Diamagnetic substances: - Those substances which are repelled by a magnet are
called diamagnetic substances.
OR
Those substance which when placed in a magnetic field are feebly magnetized in a direction
opposite to the magnetizing field are called diamagnetic substances. Eg: - Copper, Zinc, Bismuth,
Silver, Gold, Lead, Glass, Marble, Water, Helium, Argon, Sodium Chloride, Inert Gasses.
Which is opposite to the external magnetic field. All substances show diamagnetic
behavior.
Properties: -1) A magnet feebly repels a diamagnetic substance.
2) In the second property, weak magnetization produced in the substance opposite to the
direction of the magnetic field.
3) Diamagnetic substances moves form the stronger magnetic field to weaker magnetic field.
4) M is small & negative so susceptibility  = M / H is small & negative.
Permeability <1
8)
5) A diamagnetic substance loses its magnetism as the external field is removed.
Paramagnetic Substances: - Paramagnetic substances are those substances, which are
feebly attracted by the magnets.
OR
Those substances which when placed in a magnetic field are feebly magnetized by the
magnetizing field are called paramagnetic substances Eg. Aluminum, Sodium, Platinum,
CuCl2, Mn, Cr, liquid O2.
Properties: -1) Paramagnetic substances are feebly attracted by the magnets.
2) When a paramagnetic substance is suspended in the uniform magnetic field then it
becomes parallel to the magnetic field & magnetized weakly along the direction of the field.
3) Magnetic lines of forces prefer to pass through paramagnetic substances rather then the
air.
4) As we remove the magnetic field from it the substance gets demagnetized.
.5) Paramagnetic substances move from the weaker field to stronger field.
6) M has a positive value, so susceptibility  = M / H is small & positive.
7) Permeability > 1.
8) Magnetism is produced along the direction of the field.
Ferromagnetic: - The substances, which are strongly attracted by the magnets, are called
ferromagnetic substances. Or Substances which when placed in a magnetic field are
strongly magnetized along the direction of the external field called ferromagnetic
substances. E.g. Fe, Ni, Co, Gadolinium (Gd), Dysprosium .
Properties: - Similar to those paramagnetic substances1] Strongly attracted by a magnet.
2] Ferromagnetism is not found in liquids.
3] M has a large positive value.
4] Susceptibility  = M / H has very high positive value.
5] Magnetism still remains even after removal of the external field. 6] Ferromagnetism decreases
with increase in temperature it is maximum at absolute zero & becomes zero at curie’s temp. 7]
The temp at which magnetism is completely lost is called curie temperature 770 C for iron, 356 C
for nickel & 1075 C for Co.15 C for Gadolinium. Above Tc substances becomes paramagnetic. 8] 
>>1.
2. Magnetic dipole moment (M) of the current loop: -Let a circular wire carry
current in clockwise direction, then direction of
magnetic field will be such that lower face will act as
North pole and upper face will act as South pole. Thus a
current loop behaves as a magnet i.e. magnetic dipole
as shown in figure
Torque acting on a current carrying coil placed in the magnetic field B is given by  = IAB sin
- - -(i)
But, the torque acting on a magnetic dipole placed in the magnetic field is given by  = MB
sin  - - -(ii)
Comparison of eqn. (i) and (ii) gives the magnetic dipole moment of current loop M B sin 
= IA B sin 
Hence dipole moment M = I A

If the loop has n turns, then M = n I A

In vector form M  n I A
Where n is number of turns of loop, I is the current and A is the area of loop.
Atom as a Magnetic Dipole: -In an atom, electrons revolve around the nucleus in circular
orbits. The movement of the electron in circular orbit around the nucleus in anticlockwise is
equivalent to the flow of current in the orbit in clockwise direction. Thus the orbit of
electrons is considered as tiny current loop.
If an electron of mass ‘m’ revolves in anti-clockwise direction with velocity ‘v’ in circular
orbit of radius ‘r’ as shown in figure, the angular momentum vector L acts along normal to
the plane of orbit in upward direction and its magnitude is according to Bohr’s Theory is
nh
Where n = 1, 2, 3 - - L
 mvr - - - - (1)
2
Orbital motion of the electron is equivalent to the flow of current (clockwise) so current,
I = Charge of electron / Time taken to complete 1 revolution = e / T
But time period T 
2 r
d

v
v
So I 
Magnetic moment of a current loop. M = N I A = 1
M 
evr
2
- (3)
Dividing eq3 by eq1
e
- - 2 r / v
- (2)
ev
  r2
2 r
Hence
M
e v r/2

nh
mvr
2

e L
In vector form M 
2m

 ve sign shows that electron is negatively charged. Above equation show that M and L are
opposite to each other.
 eh 

Hence magnetic moment of electron due to its orbital motion is M  n 
4  m
OR M = nB.
Here
eh
  B is the least value of magnetic moment. This natural unit of magnetic
4 m
moment is called Bohr Magneton. Its value is B = 9.27  10 –24 Am2. The magnetic dipole
moment associated with an atom due to orbital motion of an electron in the first orbit of
hydrogen atom is called Bohr Magneton B.
The electron also posses magnetic moment due to its spin motion. So total magnetic
dipole moment of the electron is vector sum of the magnetic moment due to orbital motion
& spin motion.
ANSWER OF VALUE BASE QUESTION (4 MARKS)
1. (I) caring attitude, sensitive towards society, concern for others
(II) Electromagnet- temporary magnet. Hysteresis curve has small are, small coercivity, and small
retentivity.
2. (I) Awareness, concern for conservation of energy and fossil fuel sharing the knowledge
(II) Susceptibility is independent of the temperature as they have no permanent dipoles.
3. (a) Sympathy, responsibility, helping and self reliance.
(b) The magnetic form is uniform and force acting on the two ends of needle is equal and opposite
so the net force is zero. However the torque acts on the needle.
In case of nail of iron nail, there is an induced magnetism, the induced south pole in the nail be
closer to the north pole experiences a large attractive force then the induced north pole so the
nail experience both a net attractive force and torque.
HOTS
1. Diamagnetism is due to orbital motion of an electron developing magnetic moments opposite to
applied field and hence is not much affected by the temperature. Paramagetism and
ferromagnetism is due to alignments of the atomic magnetic moments in the direction of applied
field. As the temperature increases this alignment is disturbed and hence susceptibility of both
decrease as the temperature increase.
2. (a) Slope of I-H graph gives the susceptibility=(I/H) of the material.
For the material A, the slope is +ve and smaller, it is likely to be paramagnetic.
Slope of I-H graph gives the susceptibility= (I/H) of the material.
For the material B, the slope is +ve and larger, it is likely to be ferromagnetic.
(b) The I-T graph for the paramagnetic material A is shown in the figure.
I
T (Temperature)
3. The electron in the Hydrogen atom is moving with the speed of 2.3x106m/s in an orbital of radius
.53A0. Calculate the magnetic moment of the revolving electron.
2. The orbital magnetic moment of the electron M =e v r/2,
After calculation, we get
M=9.75 x 10-24Am2
3. The magnetic moment of a proton and an electron are respectively given by
eh
eh
µp = 4πm and µe = 4πM
as m>>>M, so µp<<µe.
Where m and M are the masses of proton and electron respectively.
That is why effect of spin and magnetic moment of proton is neglected in the magnetism of the
material.