The magnetic moment of a current (I) carrying circular coil of radius (r) and number of turns (n) varies as

• $\frac{1}{{\mathrm{r}}^2}$

• $\frac{1}{\mathrm{r}}$

• r

• r²

The cyclotron frequency of an electron  gyrating in a magnetic field of  1 T approximately

• 28 MHz

• 280 MHz

• 2.8 GHz

• 28 GHz

An electric dipole placed in a non-uniform electric field experiences

• both a torque and a net force

• only a force but no torque

• only a torque but no net force

• no torque and no net force

Three charges are placed at the vertices of an equilateral triangle of side a as shown in the given figure. The force experienced by the charge placed at the vertex A in a direction normal to BC is

• Q² / 4πε₀a²

• - Q^{2 (4πε₀a2<)}

• zero

• Q² / 2πε₀a²

The earth's magnetic field at a  given point is 0.5 × 10^-5 Wb/m². This field is to be annulled by magnetic induction at the centre of a circular conducting loop of radius 5.0 cm. The current required to be flown in the loop nearly

• 0.2 A

• 0.4 A

• 4 A

• 40 A

The difference in lengths of a mean solar day and a sideral day is about 

• 1 min

• 4 min

• 15 min

• 56 min

The coefficient of mutual inductance, when magnetic flux changes by 2 x 10^-2 Wb and current changes by 0.01 A is

• 2 H

• 4 H

• 3 H

• 8 H

268.

Assertion:  We cannot think of a magnetic field configuration with three poles. 

Reason:  A bar magnet does exert a torque on itself due to its own field.

• If both the assertion and reason are true and reason is a correct explanation of the assertion.

• If both assertion and reason are true but assertion is not a correct explanation of the assertion.

• If the assertion is true but the reason is false.

• If both assertion and reason are false

At a place earth's magnetic field, 5 × 10⁵ Wb/m² is acting perpendicular to a coil of radius R = 5 cm. If  μ_ο / 4$\mathrm{\pi }$ = 10⁷ , then how much current is induced in circular loop?

• 0.2 A

• 0 A

• 4 A

• 40 A

A wire length l carries a steady current. It is bent first to form a circular plane loop of one turn. The magnetic field at the centre of the loop is B. The same length is now bent more sharply give a to double loop of smaller radius. The  magnetic field at the centre caused by the same is

• B

• B/4

• 4B

• B/2