51.

Two long conductors, separated by a distance d carry current I₁ and I₂ in the same direction. They exert a force F on each other. Now the current in one of them increased to two times and its direction reversed. The distance is also increased to 3d. The new value of the force between them is

• −2F 

• F/3

• −2F/3

• −2F/3

The length of a magnet is large compared to its width and breadth. The time period of its width and breadth. The time period of its oscillation in a vibration magnetometer is 2 s. The magnet is cut along its length into three equal parts and three parts are then placed on each other with their like poles together. The time period of this combination will be

• 2 s

• 2/3 s

• 2√3 s

• 2√3 s

The materials suitable for making electromagnets should have

• high retentivity and high coercivity

• low retentivity and low coercivity

• high retentivity and low coercivity

• high retentivity and low coercivity

In an LCR series a.c. circuit, the voltage across each of the components, L, C and R is 50 V. The voltage across the LC combination will be

• 50 V

• 50√2 V

• 100 V

• 100 V

A coil having n turns and resistance 4R Ω. This combination is moved in time t seconds from a magnetic field W₁ weber to W₂ weber. The induced current in the circuit is

In a uniform magnetic field of induction B a wire in the form of semicircle of radius r rotates about the diameter of the circle with angular frequency ω. The axis of rotation is perpendicular to the field. If the total resistance of the circuit is R the mean power generated per period of rotation is

In a LCR circuit capacitance is changed from C to 2C. For the resonant frequency to remain unchanged, the inductance should be changed from L to

• 4L

• 2L

• L/2

• L/2

A metal conductor of length 1 m rotates vertically about one of its ends at angular velocity 5 radians per second. If the horizontal component of earth’s magnetic field is 0.3 × 10⁻⁴ T, then the e.m.f. developed between the two ends of the conductor is

• depends on the nature of the metal used

• depends on the intensity of the radiation

• depends both on the intensity of the radiation and the metal used

• depends both on the intensity of the radiation and the metal used

A charged oil drop is suspended in a uniform field of 3 × 10⁴ V/m so that it neither falls nor rises. The charge on the drop will be (take the mass of the charge = 9.9 × 10⁻¹⁵ kg and g = 10 m/s² )

• 3.3 × 10⁻¹⁸ C

• 3.2 × 10⁻¹⁸ C

• 1.6 × 10⁻¹⁸ C

• 1.6 × 10⁻¹⁸ C

A plane convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now this lens has been used to form the image of an object. At what distance from this lens an object be placed in order to have a real image of the size of the object?

• 20 cm

• 30 cm

• 60 cm

• 60 cm