Question 1 1 / -0
A needle made of bismuth is suspended freely in a magnetic field. The angle which the needle makes with the magnetic field is
Solution
Bismuth is a diamagnetic substance. So, when placed in an external magnetic field, it rotates such that its axis becomes perpendicular to the magnetic field.
Question 2 1 / -0
The magnetic susceptibility of a material of a rod is 449. Permeability of vacuum is 4π x 10-7 henry/metre. The absolute permeability of the material of the rod (in henry/metre) is
Question 3 1 / -0
A magnet of length 10 cm and magnetic moment 1 Am2 is placed along the side AB of an equilateral triangle ABC. If the length of side AB is 10 cm, then the magnetic field at point C is
Solution
Let m be the pole strength of each pole of the magnet figure. The magnetic field at C due to the N-pole is given by
Question 4 1 / -0
A magnetic dipole is acted upon by two magnetic fields which are inclined to each other at an angle of 75°. One of the fields has a magnitude of
10
-2 T. The dipole attains stable equilibrium at an angle of 30° with this field. What is the magnitude of the other field?
Solution
Refer to the figure given below: Let
(= 30°) be the angle between the magnetic moment vector M and the field vector B
1 (= 1.5
10
-2 T). Then, as shown in the figure, the angle between M and the other field B
2 will be
= 75° - 30° = 45°.
The field exerts a torque
= M
B
1 on the dipole and the field B
2 exerts a torque
= M
B
2 , where M is the magnetic moment of the dipole. Since the dipole is in stable equilibrium, the net torque
must be zero, i.e. the two torques must be equal and opposite. In terms of magnitudes, we have mB
1 sin
= mB
2 sin
or,
Question 5 1 / -0
A steel wire of length 'l' has a magnetic moment M. It is bent into a semicircular arc. The new magnetic moment is
Solution
If m is pole strength, then m =
When the wire is bent into a semicircular arc, the separation between the two poles changes from l to 2r, where r is radius of the semicircular arc.
Since l = π r or r = l/π, the new magnetic moment of the stell wire,
M' = m × 2r =
Question 6 1 / -0
A bar magnet of magnetic moment 2.0 JT-1 lies aligned with the direction of a uniform magnetic field of 0.25 T. What is the work done to turn the magnet so as to align its magnetic moment opposite to the field direction?
Solution
Potential energy of the dipole =
=
= 2
2.0
0.25 = 1.0 J
Question 7 1 / -0
Directions: Assertion: Diamagnetic materials can exhibit magnetism.Reason: Diamagnetic materials have permanent magnetic dipole moment.
Solution
Some materials when placed in an external magnetic field, acquire a very low magnetism in a direction opposite to the field. These materials when brought near the end of a strong magnet, they get repelled. These materials are diamagnetic and this property of materials is called the diamagnetism. This property is generally found in materials in which there are even number of electrons in their atoms (or molecules) and pairs are formed with two electrons. In each pair, the spin of one electron is in direction opposite to the spin of other electron. Hence, the electrons of each pair completely cancel the magnetic moment of each other. Thus, the net magnetic moment of each atom of such material is zero. Hence, option (c) is true.
Question 8 1 / -0
A magnet of magnetic moment M and length 2l is bent at its mid-point such that the angle of bending is 60°. Now, the magnetic moment is
Question 9 1 / -0
Directions:
Solution
A compass is simply a needle shaped magnet that is mounted so it can rotate freely about a vertical axis. When it is held in a horizontal plane, the north pole end of the needle point, generally, towards the geomagnetic north pole (really a south magnetic pole remember). Thus, true geographic north direction cannot be found by using a compass needle. Now vertical plane passing through the magnetic axis of earths magnet is called magnetic meridian.
Question 10 1 / -0
Two identical thin bar magnets are placed mutually at right angles, such that the north pole of one touches the south pole of the other. The length and the pole strength of each bar are L and m, respectively. The resultant magnetic moment of the system is
Question 11 1 / -0
A circular coil of 20 turns and radius 20 cm is mounted vertically with its plane in magnetic meridian. A small magnetic needle is placed at the centre of the coil. When a current is passed through the coil, the needle is deflected through 45o . If the horizontal component of Earth's field is 0.34 x 10-4 T, the current in the coil is:
Question 12 1 / -0
Which of the following quantities is/are used to determine the strength of Earth`s magnetic field at a point on Earth's surface?
Solution
By measuring the horizontal component of Earth`s magnetic field and angle of dip, the strength of Earth`s magnetic field can be found.
Question 13 1 / -0
When a material is subjected to a small magnetic field H, the intensity of magnetisation is proportional to
Solution
The intensity of magnetisation (Ad ) is defined as the magnetic moment per unit volume of the material. The magnitude of M depends upon the magnetisation current, which is proportional to H. Hence, the correct choice is (2).
Question 14 1 / -0
A bar magnet used in a vibration magnetometer is heated, so as to reduce its magnetic moment by 36%. The time period of the magnet (neglecting the changes in the dimension of the magnet) :
Question 15 1 / -0
Directions: The following question has four choices out of which only one is correct.
A bar magnet of length 3 cm has points A and B along its axis at distances of 24 cm and 48 cm, respectively, on the opposite sides.
Ratio of the magnitudes of the magnetic fields at these points will be
Question 16 1 / -0
A magnet makes 30 oscillations/min in Earth's magnetic field. If the magnetic field is doubled, the period of oscillation of magnet is
Question 17 1 / -0
A paramagnetic substance of susceptibility 3 x 10-4 is placed in a magnetic field of 4 x 10-4 Am-1 . What is the intensity (in Am-1 ) of magnetisation?
Question 18 1 / -0
A bar magnet is oscillating in Earth's magnetic field with a period T. What happens to its period and motion if its mass is quadrupled?
Solution
Time period of the bar magnet in the external magnetic field is
If the mass of the bar magnet is increased 4 times , then the Moment of Inertia is also become 4 times so Time period will become twice but the motion remains simple harmonic.
Question 19 1 / -0
Two short bar magnets of magnetic moment 'M' each are arranged at the opposite corners of a square of side 'd', such that their centres coincide with the corners and their axes are parallel. If the like poles are in the same direction, then the magnetic field at any of the other corners of the square is
Solution
Refer to the following figure. Let us find the net magnetic field at corner C of the square ADBC. For the magnet at corner B, the point C is on the axial line at a distance d from the centre of the magnet. For a short magnet, the magnetic field at C is given by
.
For the magnet at corner A, the point C is on the equatorial line at a distance d from its centre. For a short magnet, the magnetic field at C due to this magnet is given by
Since their like poles are in the same direction, so the net magnetic field at C is
B = B
1 - B
2 Hence, the correct choice is (1).
Question 20 1 / -0
The effective length of a magnet is 31.4 cm and its pole strength is 0.5 A-m. What will be the magnetic moment if it is bent in the form of a semicircle?
Solution
If L is length of the magnet, then magnetic moment, M = mL (where m is the pole strength).
When same magnet is bent into a semicircle of radius r, then
Separation between the poles = 2r
New magnetic moment of the bar magnet is:
Question 21 1 / -0
The force between two magnetic poles is F. If the distance between the poles and pole strength of each pole are doubled, then the force experienced is
Solution
Force between the two poles is given by:
; where k is the constant of proportionality, and m
1 and m
2 are the pole strengths with separation 'r' between them.
For m
1 = m
2 ,
If the pole strength and the separation are doubled, then the new force between the poles is F.
Question 22 1 / -0
Which of the following has the highest magnetic permeability?
Solution
Ferromagnetic substances have a very high (of the order 1000 or more) and positive susceptibility (
). Now,
= 1 +
.
Hence, the correct choice is (3).
Question 23 1 / -0
A toroidal solenoid has 3000 turns and a mean radius of 10 cm. It has a soft iron core of relative permeability 2000. What is the magnitude of the magnetic field in the core, when a current of 1 A is passed through the solenoid?
Solution
The magnetic field in the core is given by:
B =
nI; where,
is the permeability of soft iron and n is the number of turns per unit length of the solenoid
Now,
Hence, the correct choice is (4).
Question 24 1 / -0
A magnetic needle, suspended by a highly flexible and highly temperature-resistant thread, is vibrating in Earth's magnetic field. If the temperature of the needle is increased by 500°C, then
Solution
At high temperature, magnetism of the magnetic needle will decrease. Hence, there will be a decrease in the magnetic moment of the needle.
Time period of oscillation,
.
As the magnetic moment decreases, the time period of oscillation increases.
Question 25 1 / -0
For a paramagnetic material, the dependence of magnetic susceptibility X on absolute temperature T is given by (C is a constant)
Solution
According to Curie's law, susceptibility is related to temperature T as
Where, C is Curie's constant.
Hence, the correct choice is (2).
×
Sign in
Profile
Signout
Get latest Exam Updates 
