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

The Gaussian surface:

Assertion    :  A potentiometer of longer length is used for accurate measurement.

Reason        : The potential gradient for a potentiometer of longer length with a given source of e.m.f. becomes small.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If the assertion and reason both are false.

A magnet can only repel another magnet. So, ‘X’ is a surer test of magnetism. Identify X.

If no external voltage is applied across P-N junction, there would be

(a) No electric field across the junction

(b) An electric field pointing from N-type to P-type side across the junction

(c) An electric field pointing from P-type to N-type side across the junction

(d) A temporary electric field during formation of P-N junction that would subsequently disappear

In the circuit shown here, what is the value of the unknown resistor R so that the total resistance of the circuit between points P and Q is also equal to R

(a) 3 ohms

(b)

(c) ohms

(d) 10 ohms

Gravitational force has only on sign but coulomb force has:

At temperature 0K, the germanium behaves as a/ an:

The figure illustrate how B, the flux density inside a sample of unmagnetised ferromagnetic material varies with B₀, the magnetic flux density in which the sample is kept. For the sample to be suitable for making a permanent magnet

(a) OQ should be large, OR should be small

(b) OQ and OR should both be large

(c) OQ should be small and OR should be large

(d) OQ and OR should both be small

Two equally charged, identical metal spheres A and B repel each other with a force 'F'. The spheres are kept fixed with a distance 'r' between them. A third identical, but uncharged sphere C is brought in contact with A and then placed at the mid-point of the line joining A and B. The magnitude of the net electric force on C is

(a) F

(b) 3F/4

(c) F/2

(d) F/4

Stopping potential is the negative voltage on the collector plate at which the photoelectric current just becomes __________.