Ray Optics and Optical Instruments — Physics STD 12 Science — Question

An electron is moving along the positive x-axis. You want to apply a magnetic field for a short time so that the electron may reverse its direction and move parallel to the negative x-axis. This can be done by applying the magnetic field along:

1. y-axis.
2. z-axis.
3. y-axis only.
4. z-axis only.

An ideal transformer has 100 turns in the primary and 250 turns in the secondary. The peak value of the ac is 28 V. The r.m.s. secondary voltage is nearest to

(a) 50 V

(b) 70 V

(c) 100 V

(d) 40 V

Electric flux over a surface in an electric field may be:

In order to increase the sensitivity of a moving coil galvanometer, one should decrease

(a) The strength of its magnet	(b) The torsional constant of its suspension
(c) The number of turns in its coil	(d) The area of its coil

The best suitable metal for photoelectric effect is:

Displacement current goes through the gap between the plates of a capacitor when the charge of the capacitor:

Two point charges +8q  and -2q are located at x = 0  and x = L  respectively. The location of a point on the x-axis at which the net electric field due to these two point charges is zero is

(a) 8 L

(b) 4 L

(c) 2 L

(d)

A long straight wire carrying current of 30A is placed in an external uniform magnetic field of induction 4 × 10^–4 T. The magnetic field is acting parallel to the direction of current. The magnitude of the resultant magnetic induction in tesla at a point 2.0 cm away from the wire is

(a) 10–4

(b) 3 × 10–4

(c) 5 × 10–4

(d) 6 × 10–4

LEDs have the following advantages over conventional incandescent low power lamps: