Current Electricity — Physics STD 12 Science — Question

The back e.m.f. induced in a coil, when current changes from 1 ampere to zero in one milli‑second, is 4 volts, the self inductance of the coil is

(a) 1 H

(b) 4 H

(c)  H

(d) 4  H

A transformer is employed to reduce 220 V to 11 V. The primary draws a current of 5 A and the secondary 90 A. The efficiency of the transformer is

(a) 20%

(b)  40%

(c) 70%

(d)  90%

How does the red shift confirms that the universe is expanding

(a) Due to Wien's law	(b) Due to Stefan's law
(c) Due to Kirchhoff's law	(d) Due to Doppler's effect

A pendulum clock keeping correct time is taken to high altitudes:

1. It will keep correct time.
2. Its length should be increased to keep correct time.
3. Its length should be decreased to keep correct time.
4. It cannot keep correct time even if the length is changed.

Choose the correct statement:

A resistor and an inductor are connected to an ac supply of 120V and 50 Hz. The current in the circuit is 3A. If the power consumed in the circuit is 108W, then the resistance in the circuit is.

Two batteries of e.m.f. 4V and 8 V with internal resistances 1 W and 2 W are connected in a circuit with a resistance of 9 W as shown in figure.  The current and potential difference between the points P and Q are

(a)  and 3V

(b)  and 4V

(c)  and 9V

(d)  and 12 V

Three plates of common surface area A are connected as shown. The effective capacitance will be

(a)

(b)

(c)

(d)

Self induction of a solenoid is

(a) Directly proportional to current flowing through the coil

(b) Directly proportional to its length

(c) Directly proportional to area of cross-section

(d) Inversely proportional to area of cross-section

Rutherford’s a-particle experiment showed that the atoms have: