21.

If E and B represent electric and magnetic field vectors of an electromagentic wave, the direction of propagation of the wave is along

• E × B

• B × E

• E

• B

Two point charges A = +3 nC and B=+ 1 nC are placed 5 cm apart in air. The work done to move charge B towards A by 1 cm is

• 2.0 × 10^-7 J

• 2.7 × 10^-7 J

• 1.21 × 10^-7 J

• 1.35 × 10^-7 J

A proton, a deuteron and an α-particle are projected perpendicular to the direction of a uniform magnetic field with same kinetic energy. The ratio of the radii of the circular paths described by them is

• $\sqrt{2} : \sqrt{2} : 1$

• $\sqrt{2} : 1 : 1$

A bar magnet is allowed to fall vertically through a copper coil placed in a horizontal plane. The magnet falls with a net acceleration, is

• Zero

• = g

• < g

• > g

A galvanometer of resistance 50 Ω is connected to a battery of 3 V along with. a resistance of 2950 Ω in series shows full-scale deflection of 30 divisions. The additional series resistance required to reduce the deflection to 20 divisions is

• 2950 Ω

• 1500 Ω

• 4450 Ω

• 7400 Ω

In the A.C. circuit shown, keeping 'K' pressed, if an iron rod inserted into the coil, the bulb in the circuit

• glows less brightly

• glows with same brightness (as before the rod is inserted)

• gets damaged

• glows more brightly

A magnetic dipole of magnetic moment 6 × 10^-2 A-m² and moment of inertia 12 x 10^-6 kg-m² performs oscillations in a magnetic field of 2 x 10^-2 T. The time taken by the dipole to complete 20 oscillations is (π ≈ 3)

• 18 s

• 6 s

• 36 s

• 12 s

4 x 10¹⁰ electrons are removed from a neutral metal sphere of diameter 20 cm placed in air. The magnitude of the electric field (in NC^-1) at a distance of 20 cm from its centre is

• 640

• 5760

• Zero

• 1440

A cylindrical conductor of diameter 0.1 mm carries a current of 90 mA. The current density (in Am^-2) is (π ≈ 3)

• 12 × 10⁷

• 3 × 10⁶

• 6 × 10⁶

• 2.4 × 10⁷

Which of the following properties is 'False' for a bar magnet ?

• It doesn't produce magnetic field.

• Its like poles repel and unlike poles attract.

• Its poles cannot be separated

• It points in North-South direction when suspended.