Physics Questions and Answers

Answer: C

The answer is A is correct but R is not correct.

The Assertion is true: silver is not used for ordinary electric wiring. The Reason is false, because silver is the best electrical conductor of all metals, slightly better than copper.

Silver is avoided because it is far too expensive (and soft) for wiring; copper conducts almost as well at a fraction of the cost, and aluminium is used for overhead lines because it is light. So the real reason is cost, not poor conductivity.

Answer: C

The answer is A is correct but R is not correct.

The Assertion is true: presbyopia develops with age as the eye lens loses flexibility and the ciliary muscles weaken, so the power of accommodation falls and near objects become hard to see.

The Reason has the bifocal lens upside down. In a bifocal lens the upper part is a concave lens for distant vision and the lower part is a convex lens for near vision (reading). The Reason swaps them, so it is incorrect.

Answer: D

The correct statements are a, d and e.

Both uniform and non-uniform motion can be shown on graphs. Uniform motion gives a straight-line distance-time graph, and non-uniform motion gives a curved one. So a and d are true, and b and c, which deny this, are false.

Statement e is the definition of velocity: the displacement of an object per unit time, in a given direction.

Every option other than D includes b or c, so they are wrong.

Answer: B

The answer is Both A and R are correct but R is not the correct explanation of A.

The Assertion is true: every reflecting surface, plane or curved, obeys the laws of reflection, and every refracting surface obeys the laws of refraction.

The Reason is also true: the absolute refractive index of a medium is the speed of light in vacuum divided by its speed in that medium. But a definition of refractive index does not explain why surfaces obey the laws of reflection and refraction, so R is not the explanation of A.

Answer: B

The curie is a unit of radioactivity.

One curie is \(3.7\times10^{10}\) disintegrations per second, originally the activity of one gram of radium-226. The SI unit is the becquerel, one disintegration per second.

Answer: B

The distance is \(\lambda/4\).

In a standing wave, consecutive nodes are half a wavelength apart, and an antinode sits exactly midway between two nodes. The node-to-antinode distance is therefore half of that, \(\lambda/4\).

Answer: A

The time of ascent is less than the time of descent.

Air resistance always opposes motion. Going up it acts downwards, so the retardation is \(g + a\) and the ball slows faster; coming down it acts upwards, so the acceleration is only \(g - a\). The larger effective acceleration on the way up means less time is spent there.

Answer: D

Fat separates because of the centrifugal effect.

In the rotating drum, the denser skimmed milk is thrown to the outside and the lighter fat globules are displaced inwards, so the cream collects near the axis. In the rotating frame this is described as a centrifugal force; in the ground frame it is the denser fluid's greater inertia.

Answer: C

The maxwell is the unit of magnetic flux.

It is the CGS unit: one maxwell is one line of flux, and \(1\ \mathrm{Wb} = 10^{8}\ \mathrm{Mx}\). The SI unit is the weber.

Answer: C

Escape velocity is the minimum velocity with which a body must be projected to escape the earth's gravitational field.

Equating kinetic energy to the depth of the gravitational well: \[\tfrac{1}{2}mv^2 = \frac{GMm}{R} \Rightarrow v = \sqrt{2gR} = 11.2\ \mathrm{km\,s^{-1}}\] It does not depend on the mass of the escaping body, nor on the direction of projection.