The amplitude of the sinusoidially oscillating electric field of a plane wave is 60 V/m. Then the amplitude of magnetic field is

• 2 × 10² T

• 6 × 10⁷ T

• 6 × 10² T

• 2 × 10^-7 T

An α-particle with a specific charge of 2.5 × 10⁷ C kg^-1 moves with a speed of 2 x 10⁵ ms^-1 in a perpendicular magnetic field of 0.05 T. Then the radius of the circular path described by it is

• 8 cm

• 4 cm

• 16 cm

• 2 cm

A cyclotron can be used to accelerate

• α-particles

• β-particles

• neutrons

• neutrino

A straight conductor of length l carrying a current I, is bent in the form of a semicircle. The magnetic field (in tesla) at the centre of the semicircle is

• $\frac{{\mathrm{\pi }}^2\mathrm{I}}{\mathrm{l}} \times {10}^{-7}$

• $\frac{\mathrm{\pi I}}{\mathrm{l}} \times {10}^{-7}$

• $\frac{\mathrm{\pi l}}{{\mathrm{l}}^2} \times {10}^{-7}$

• $\frac{{\mathrm{\pi l}}^2}{\mathrm{l}} \times {10}^{-7}$

Which of the following graphs represents variation of magnetic field B with distance r for  straight long wire carrying current ? 

86.

The force between two magnetic poles is F. If the distance between the poles and pole strengths of each pole are doubled, then the force experienced is

• 2 F

• $\frac{\mathrm{F}}{2}$

• $\frac{\mathrm{F}}{4}$

• F

A magnet of length 0.1 m and pole strength 10^-4 A-m is kept in a magnetic field of 30 Wb/m² at an angle 30°. The couple acting on it is 

• 7.5 × 10^-4 Nm

• 3.0 × 10^-4 Nm

• 1.5 × 10^-4 Nm

• 6.0 × 10^-4 Nm

A circular coil carrying a current has a radius R. The ratio of magnetic induction at the centre of the coil and at a distance equal to $\sqrt{3}\mathrm{R}$ from the centre of the coil on the axis is

• 1 : 1

• 1 : 2

• 8 : 1

• 1 : 8

The force on a conductor of length l placed in a magnetic field of magnitude B and carrying a current I is given by (θ is the angle, the conductor makes with the direction of B)

• F = I l B sin θ

• F = I² l B² sin θ

• F = I l B cos θ

• $\mathrm{F} = \frac{{\mathrm{I}}^2\mathrm{l}}{\mathrm{B}} \sin \mathrm{\theta }$

A needle made of bismuth is suspended freely in a magnetic field. The angle which the needle makes with the magnetic field is

• 0°

• 45°

• 90°

• 180°