Physics Questions and Answers

Answer: A

Energy is generated in the Sun by Nuclear fusion.

In the Sun's core, at about 15 million kelvin, hydrogen nuclei fuse to form helium. The small loss of mass in each reaction is converted into a large amount of energy, as given by Einstein's relation E = mc².

Nuclear fission, the splitting of heavy nuclei such as uranium, is used in nuclear power stations but does not power the Sun, so options including fission are wrong.

Answer: B

A light year is a unit of distance.

It is the distance light travels in a vacuum in one year, about \(9.46 \times 10^{12}\) km. Astronomers use it for distances between stars.

The word year makes 'time' the obvious trap, but a light year measures how far, not how long. It is not a unit of light itself or of light's intensity (which is measured in candela).

Answer: B

The best conductor of heat among the options is silver.

Silver has the highest thermal conductivity of any metal, about 430 W/m K, because its free electrons carry heat very efficiently. It is ahead even of copper.

Mercury is a metal but a liquid one, and conducts heat far less well, roughly a fiftieth as well as silver. Leather is an insulator, and benzene, an organic liquid, is a poor conductor too.

Answer: B

The atom bomb is based on nuclear fission.

In an atom bomb a neutron splits a heavy nucleus of uranium-235 or plutonium-239, releasing energy and two or three more neutrons, which split further nuclei. When the mass exceeds the critical mass, this chain reaction runs away in a fraction of a second and releases enormous energy.

Nuclear fusion, the joining of light nuclei, is the basis of the hydrogen (thermonuclear) bomb, not the atom bomb. A chemical reaction powers conventional explosives, and 'atomic collision' is not the mechanism of any bomb.

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.