31.

The resistance of a bulb filament is 100 Ω at a temperature of 100°C. If its temperature coefficient of resistance be 0.005 per °C, its resistance will become 200 Ω at a temperature of

• 200°C

• 300°C

• 400°C

• 400°C

In an AC generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B. The maximum value of emf generated in the coil is ‘

• N.A.B.ω

• N.A.B.R.ω

• N.A.B

• N.A.B

The flux linked with a coil at any instant ‘t’ is given by
φ= 10t²- 50t +250
The induced emf at t = 3 s is

• 190 V

• −190 V

• -10 V

• -10 V

The threshold frequency for a metallic surface corresponds to an energy of 6.2 eV, and the stopping potential for a radiation incident on this surface 5 V. The incident radiation lies in

• X-ray region

• ultra-violet region

• infra-red region

• infra-red region

An electric bulb is rated 220 volt − 100 watt. The power consumed by it when operated on 110 volt will be

• 50 watt

• 75 watt

• 40 watt

• 40 watt

A long solenoid has 200 turns per cm and carries a current i. The magnetic field at its centre is 6.28 × 10⁻² Weber/m². Another long solenoid has 100 turns per cm and it carries a current i/3. The value of the magnetic field at its centre is

• 1.05 × 10⁻⁴ Weber/m²

• 1.05 × 10⁻² Weber/m²

• 1.05 × 10⁻³ Weber/m²

• 1.05 × 10⁻³ Weber/m²

Two spherical conductors A and B of radii 1 mm and 2 mm are separated by a distance of 5 cm and are uniformly charged. If the spheres are connected by a conducting wire then in equilibrium condition, the ratio of the magnitude of the electric fields at the surface of spheres A and B is

• 1 : 4

• 4 : 1

• 1:2

• 1:2

An inductor (L = 100 mH), a resistor (R = 100 Ω) and a battery (E = 100 V) are initially connected in series as shown in the figure. After a long time the battery is disconnected after short-circuiting the points A and B. The current in the circuit 1 mm after the circuit is

• 1 A

• 1/e A

• e A

• e A

A material ‘B’ has twice the specific resistance of ‘A’. A circular wire made of ‘B’ has twice the diameter of a wire made of ‘A’. Then for the two wires to have the same resistance, the ratio AA /AB of their respective lengths must be

• 2

• 1

• 1/4

• 1/4

Which of the following units denotes the dimensions ML² /Q², where Q denotes the electric charge?

• Weber (Wb)

• Henry (H)

• Wb/m²

• Wb/m²