Magnetism and Matter Practice Questions - DPP for JEE

 Page 1


1. (c) Initial magnetic moment of each magnet = m × l.
As is clear from Fig., S
1
 and N
2
 neutralize each other.
Effective distance between,
N
1
 and S
2
 = 
?  
2. (a) The time period of oscillation of a freely suspended magnet is given
by,
Thus, 
Given, T = 4 sec, ,
So, 
or,
or,
3. (b) t = MB sin ?
t = iAB sin 90º
? 
Also, A = 1/2 (BC) (AD)
Page 2


1. (c) Initial magnetic moment of each magnet = m × l.
As is clear from Fig., S
1
 and N
2
 neutralize each other.
Effective distance between,
N
1
 and S
2
 = 
?  
2. (a) The time period of oscillation of a freely suspended magnet is given
by,
Thus, 
Given, T = 4 sec, ,
So, 
or,
or,
3. (b) t = MB sin ?
t = iAB sin 90º
? 
Also, A = 1/2 (BC) (AD)
But  
?
4. (a) M = 60 Am
2
 = 1.2 × 10
–3
 Nm, B
H
 = 40 × 10
–6
 Wb/m
2
  
 1.2 × 10
–3
 = 60 × 40 × 10
–6
 sin ?
  = sin 30°
 ? = 30°
5. (b) Electro magnet should be amenable to magnetisation and 
demagnetization
? retentivity and coercivity should be low.
6. (b) where 
When the magnet is cut into three pieces the pole strength will remain
the same and
M.I. (I') = 
Page 3


1. (c) Initial magnetic moment of each magnet = m × l.
As is clear from Fig., S
1
 and N
2
 neutralize each other.
Effective distance between,
N
1
 and S
2
 = 
?  
2. (a) The time period of oscillation of a freely suspended magnet is given
by,
Thus, 
Given, T = 4 sec, ,
So, 
or,
or,
3. (b) t = MB sin ?
t = iAB sin 90º
? 
Also, A = 1/2 (BC) (AD)
But  
?
4. (a) M = 60 Am
2
 = 1.2 × 10
–3
 Nm, B
H
 = 40 × 10
–6
 Wb/m
2
  
 1.2 × 10
–3
 = 60 × 40 × 10
–6
 sin ?
  = sin 30°
 ? = 30°
5. (b) Electro magnet should be amenable to magnetisation and 
demagnetization
? retentivity and coercivity should be low.
6. (b) where 
When the magnet is cut into three pieces the pole strength will remain
the same and
M.I. (I') = 
We have, Magnetic moment (M)
= Pole strength (m) × l
?  New magnetic moment,
? .
7. (a) Given that : B
1
 = 1.2 × 10
–2
 T, orientation of dipole with the field B
1
,
?
1
 = 15°
Hence, orientation of dipole with B
2
,
?
2
 = 60° – 15° = 45° (figure)
As the dipole is in equilibrium, therefore, the torque on the dipole due to
the two fields must be equal and opposite. If M be the magnetic
dipole moment of the dipole, then
t
1
 = t
2
or MB
1
 sin ?
1
 = MB
2
 sin ?
2
or,     B
2
 = 
=  = 4.4 × 10
–3
 Tesla
8. (a) Here, 
At neutral point,
Page 4


1. (c) Initial magnetic moment of each magnet = m × l.
As is clear from Fig., S
1
 and N
2
 neutralize each other.
Effective distance between,
N
1
 and S
2
 = 
?  
2. (a) The time period of oscillation of a freely suspended magnet is given
by,
Thus, 
Given, T = 4 sec, ,
So, 
or,
or,
3. (b) t = MB sin ?
t = iAB sin 90º
? 
Also, A = 1/2 (BC) (AD)
But  
?
4. (a) M = 60 Am
2
 = 1.2 × 10
–3
 Nm, B
H
 = 40 × 10
–6
 Wb/m
2
  
 1.2 × 10
–3
 = 60 × 40 × 10
–6
 sin ?
  = sin 30°
 ? = 30°
5. (b) Electro magnet should be amenable to magnetisation and 
demagnetization
? retentivity and coercivity should be low.
6. (b) where 
When the magnet is cut into three pieces the pole strength will remain
the same and
M.I. (I') = 
We have, Magnetic moment (M)
= Pole strength (m) × l
?  New magnetic moment,
? .
7. (a) Given that : B
1
 = 1.2 × 10
–2
 T, orientation of dipole with the field B
1
,
?
1
 = 15°
Hence, orientation of dipole with B
2
,
?
2
 = 60° – 15° = 45° (figure)
As the dipole is in equilibrium, therefore, the torque on the dipole due to
the two fields must be equal and opposite. If M be the magnetic
dipole moment of the dipole, then
t
1
 = t
2
or MB
1
 sin ?
1
 = MB
2
 sin ?
2
or,     B
2
 = 
=  = 4.4 × 10
–3
 Tesla
8. (a) Here, 
At neutral point,
= 5 ab-amp cm.
9. (a) ?
d
 < ?
p
 < ?
f
For diamagnetic substance ?
d
 is small and negative (10
–5
)
For paramagnetic substances ?
p
 is small and positive (10
–3
 to 10
–5
)
For ferromagnetic substanes ?
f
 is very large (10
3
 to 10
5
)
10. (d) PQ
6
 corresponds to the lowest potential energy among all the
configurations shown.
11. (d) tan   
?  
V = 6 × 10
–5
 T
H = 
?  B
total 
=
= 10 × 10
–5
 = 10
–4
T.
12. (b) Ferromagnetic substance has magnetic domains whereas
paramagnetic substances have magnetic dipoles which get attracted
to a magnetic field.
Diamagnetic substances do not have magnetic dipole but in the presence
of external magnetic field due to their orbital motion of electrons
these substances are repelled.
13. (c) Initially magnetic moment of system
 and moment of inertia
Page 5


1. (c) Initial magnetic moment of each magnet = m × l.
As is clear from Fig., S
1
 and N
2
 neutralize each other.
Effective distance between,
N
1
 and S
2
 = 
?  
2. (a) The time period of oscillation of a freely suspended magnet is given
by,
Thus, 
Given, T = 4 sec, ,
So, 
or,
or,
3. (b) t = MB sin ?
t = iAB sin 90º
? 
Also, A = 1/2 (BC) (AD)
But  
?
4. (a) M = 60 Am
2
 = 1.2 × 10
–3
 Nm, B
H
 = 40 × 10
–6
 Wb/m
2
  
 1.2 × 10
–3
 = 60 × 40 × 10
–6
 sin ?
  = sin 30°
 ? = 30°
5. (b) Electro magnet should be amenable to magnetisation and 
demagnetization
? retentivity and coercivity should be low.
6. (b) where 
When the magnet is cut into three pieces the pole strength will remain
the same and
M.I. (I') = 
We have, Magnetic moment (M)
= Pole strength (m) × l
?  New magnetic moment,
? .
7. (a) Given that : B
1
 = 1.2 × 10
–2
 T, orientation of dipole with the field B
1
,
?
1
 = 15°
Hence, orientation of dipole with B
2
,
?
2
 = 60° – 15° = 45° (figure)
As the dipole is in equilibrium, therefore, the torque on the dipole due to
the two fields must be equal and opposite. If M be the magnetic
dipole moment of the dipole, then
t
1
 = t
2
or MB
1
 sin ?
1
 = MB
2
 sin ?
2
or,     B
2
 = 
=  = 4.4 × 10
–3
 Tesla
8. (a) Here, 
At neutral point,
= 5 ab-amp cm.
9. (a) ?
d
 < ?
p
 < ?
f
For diamagnetic substance ?
d
 is small and negative (10
–5
)
For paramagnetic substances ?
p
 is small and positive (10
–3
 to 10
–5
)
For ferromagnetic substanes ?
f
 is very large (10
3
 to 10
5
)
10. (d) PQ
6
 corresponds to the lowest potential energy among all the
configurations shown.
11. (d) tan   
?  
V = 6 × 10
–5
 T
H = 
?  B
total 
=
= 10 × 10
–5
 = 10
–4
T.
12. (b) Ferromagnetic substance has magnetic domains whereas
paramagnetic substances have magnetic dipoles which get attracted
to a magnetic field.
Diamagnetic substances do not have magnetic dipole but in the presence
of external magnetic field due to their orbital motion of electrons
these substances are repelled.
13. (c) Initially magnetic moment of system
 and moment of inertia
I
1
 = I + I = 2I.
Finally when one of the magnet is removed then
M
2
 = M and I
2
 = I
So, 
14. (b) Force on a charged particle is given by F = qvB. Here v = 0 and
also resultant B is zero.
? Force = 0
15. (b)
 = 
= ,  
16. (d) d
1
 = 40°, d
2
 = 30°, d = ?