Moving Charges and Magnetism · Question 29

A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B', at radial distance a2 and 2a respectively, from the axis of the wire is

1. 1
2. 4
3. 14
4. 12

A square loop ABCD carrying a current i, is placed near and coplanar with a long straight conductor XY carrying a current I, the net force on the loop will be [figure]

1. 2 0 IiL3
2. 0 IiL2
3. 2 0 Ii3
4. 0 Ii2

A long wire carrying a steady current is bent into a circular loop of one turn. The magnetic field at the centre of the loop is B. It is then bent into a circular coil of n turns. The magnetic field at the centre of this coil of n turns will be

1. nB
2. n2B
3. 2nB
4. 2n2B

An electron is moving in a circular path under the influence of a transverse magnetic field of 3.57 10-2 T. If the value of e/m is 1.76 1011 C kg-1, the frequency of revolution of the electron is

1. 1 GHz
2. 100 MHz
3. 62.8 MHz
4. 6.28 MHz

A circuit contains an ammeter, a battery of 30 V and a resistance 40.8 ohm all connected in series. If the ammeter has a coil of resistance 480 ohm and a shunt of 20 ohm, the reading in the ammeter will be

1. 2 A
2. 1 A
3. 0.5 A
4. 0.25 A

A proton and an alpha particle both enter a region of uniform magnetic field B, moving at right angles to the field B. If the radius of circular orbits for both the particles is equal and the kinetic energy acquired by proton is 1 MeV, the energy acquired by the alpha particle will be

1. 1.5 MeV
2. 1 MeV
3. 4 MeV
4. 0.5 MeV

A rectangular coil of length 0.12 m and width 0.1 m having 50 turns of wire is suspended vertically in a uniform magnetic field of strength 0.2 weber/m2. The coil carries a current of 2 A. If the plane of the coil is inclined at an angle of 30 with the direction of the field, the torque required to keep the coil in stable equilibrium will be

1. 0.24 Nm
2. 0.12 Nm
3. 0.15 Nm
4. 0.20 Nm

An electron moving in a circular orbit of radius r makes n rotations per second. The magnetic field produced at the centre has magnitude

1. 0 n2 er
2. 0 n e2r
3. 0 n e2 r
4. Zero

A wire carrying current I has the shape shown in adjoining figure. [figure] Linear parts of the wire are very long and parallel to X-axis while semicircular portion of radius R is lying in Y-Z plane. Magnetic field at point O is

1. B = - 0 I4 R ( i + 2 k )
2. B = 0 I4 R ( i - 2 k )
3. B = 0 I4 R ( i + 2 k )
4. B = - 0 I4 R ( i - 2 k )

In an ammeter 0.2% of main current passes through the galvanometer. If resistance of galvanometer is G, the resistance of ammeter will be

1. 1499 G
2. 499500 G
3. 1500 G
4. 500499 G