Q. No.A vibration magnetometer placed in a magnetic meridian has a small bar magnet. The magnet executes oscillations with a time period of 2 s in the earth's horizontal magnetic field of 24 T. When a horizontal field of 18 T is produced opposite to the earth's field by placing a current-carrying wire, the new time period of the magnet will be:
1. 1 s
2. 2 s
3. 3 s
4. 4 s
A magnet is parallel to a uniform magnetic field. If it is rotated by 60°, the work done is 0.8 J. How much work is done in moving it 30° further?
1.
2. 0.4 J
3. 8 J
4. 0.8 ergs
A closely wound solenoid of 2000 turns and area of cross-section 1.5 × 10–4 m2 carries a current of 2.0 A. It is suspended through its center and perpendicular to its length, allowing it to turn in a horizontal plane in a uniform magnetic field 5 × 10–2 tesla making an angle of 30° with the axis of the solenoid. The torque on the solenoid will be:
1. 3 × 10–3 Nm
2. 1.5 × 10–3 Nm
3. 1.5 × 10–2 Nm
4. 3 × 10–2 Nm
A bar magnet of length \(‘l’\) and magnetic dipole moment \(‘M’\) is bent in the form of an arc as shown in the figure. The new magnetic dipole moment will be:
| 1. | \(\frac{3M}{\pi}\) | 2. | \(\frac{2M}{l\pi}\) |
| 3. | \(\frac{M}{ 2}\) | 4. | \(M\) |
A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from:
| 1. | the current source |
| 2. | the magnetic field |
| 3. | the lattice structure of the material of the rod |
| 4. | the induced electric field due to the changing magnetic field. |
A frog can be levitated in a magnetic field produced by a current in a vertical solenoid placed below the frog. This is possible because the body of the frog behaves as:
| 1. | Paramagnetic | 2. | Diamagnetic |
| 3. | Ferromagnetic | 4. | None of these |
| Assertion (A): | Magnetic flux linked with a closed surface is always zero. |
| Reason (R): | Magnetic monopole does not exist. |
| 1. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| 2. | Both (A) and (R) are true but (R) is not the correct explanation of (A). |
| 3. | (A) is true but (R) is false. |
| 4. | (A) is false but (R) is true. |
Select the incorrect statement regarding magnetic material.
| 1. | The magnetic dipole moment of each atom of para and ferromagnetic substance is zero and that of diamagnetic material is non-zero in the absence of external magnetizing force |
| 2. | When the diamagnetic substance is brought near the north or south pole of a bar magnet, it is repelled |
| 3. | Magnetic susceptibility of diamagnetic substances is temperature independent and that of para and ferromagnetic substances is temperature-dependent |
| 4. | All of these |
A bar magnet has coercivity . It is desired to demagnetize it by inserting it inside a solenoid 12 cm long and having 60 turns. The current that should be sent through the solenoid is:
1. 2 A
2. 4 A
3. 6 A
4. 8 A
A uniform magnetic field, parallel to the plane of the paper existed in space initially directed from left to right. When a bar of soft iron is placed in the field parallel to it, the lines of force passing through it will be represented by:
| 1. |  |
2. |  |
| 3. |  |
4. |  |
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