Electric Charges and Fields — Physics STD 12 Science — Question

An image of the sun is formed by a lens of focal length of 30 cm on the metal surface of a photoelectric cell and a photoelectric current I is produced. The lens forming the image is then replaced by another of the same diameter but of focal length 15 cm. The photoelectric current in this case is

(a)

(b) I

(c) 2I

(d) 4I

A nichrome wire 50 cm long and one square millimetre cross-section carries a current of 4A when connected to a 2V battery.  The resistivity of nichrome wire in ohm metre is

(a) 1

(b) 4

(c) 3

(d) 2

Internal forces can change:

1. The linear momentum but not the kinetic energy.
2. The kinetic energy but not the linear momentum.
3. Linear momentum as well as kinetic energy.
4. Neither the linear momentum nor the kinetic energy.

A long solenoid has a radius a and number of turns per unit length n. If it carries a current i, then the magnetic field on its axis is directly proportional to:

The mass number of a nucleus is:

The figure shows a network of currents. The magnitude of currents is shown here. The current I will be

(a) 3 A

(b) 9 A

(c) 13 A

(d) 19 A

An ammeter whose resistance is 180 Ω gives full scale deflection when current is 2 mA. The shunt required to convert it into an ammeter reading 20 mA (in ohms) is

(a) 18

(b) 20

(c) 0.1

(d) 10

Three equal resistances each of value R are joined as shown in the figure. The equivalent resistance between M and N is

(a) R

(b) 2R

(c)

(d)

A parallel plate capacitor is charged and the charging battery is then disconnected. If the plates of the capacitor are moved further apart by means of insulating handles, then

(a) The charge on the capacitor increases

(b) The voltage across the plates decreases

(c) The capacitance increases

(d) The electrostatic energy stored in the capacitor increases

A heater coil is cut into two equal parts and only one part is now used in the heater. The heat generated will now be

(a) One fourth

(b) Halved

(c) Doubled

(d) Four times