A candidate connects a moving coil ammeter A and a moving coil voltmeter V and a resistance R as shown. If the voltmeter reads 20 V and the ammeter reads 4 A, then R is

• equal to 5Ω

• greater than 5Ω

• less than 5Ω

• none of these

A wire of length L has resistance R. It is bent at mid-point such that the two halves make an angle 90° with each other. The new resistance would be.

• R

• $\sqrt{2}$ R

• $\frac{\mathrm{R}}{\sqrt{2}}$

• $\frac{R}{4}$

The ratio of magnetic potentials due to a magnetic dipole in the 'end on position' to that in the 'broad side on position' for the same distance from it is

• zero

• infinite

• 1

• 2

In the unmagnetized state magnetic domains of a magnetic substance are oriented at

• 60°

• 90°

• randomly

• 150°

When a ferromagnetic material is heated above its Curie temperature it

• gets demagnetised

• becomes diamagnetic

• behaves like a paramagnetic substance

• remains unaffected

A DC circuit contains 10Ω of resistance in series with 10 H coil. The impedance of the circuit is

• 10 Ω

• 20 Ω

• 1 Ω

• zero

Two coils of self-inductance L₁ and L₂ are placed so close together that effective flux in one coil is completely linked with the other. If M is the mutual inductance between them, then

• M = L₁L₂

• M = L_1/L₂

• M = L

• $\mathrm{M} = \sqrt{{\mathrm{L}}_1{\mathrm{L}}_2}$

38.

In an L-R circuit, the current increases to three fourth of its maximum value in 4 s, then the time constant of the circuit is

• 2log_e 2

• $\frac{4}{{\log }_{\mathrm{e}} 2}$

• $\frac{2}{{\log }_{\mathrm{e}} 2}$

• $\frac{{\log }_{\mathrm{e}} 2}{2}$

Two identical galvanometers are taken, one is to be converted into an ammeter and other into a milliammeter. The shunt of milliammeter compared to ammeter is

• less

• more

• zero

• none of these

If the vibrations of a string are to be increased by a factor of two, the tension in the string should be made

• twice

• four times

• eight times

• half