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: C

The densest element of the four is Gold.

Gold has a density of about 19.3 g/cm³, mercury about 13.5, iron about 7.9 and sodium only about 0.97, light enough to float on water.

Mercury is often guessed because it is a heavy liquid, but gold is denser by nearly half again.

Answer: A

Most asteroids lie between Mars and Jupiter.

The main asteroid belt occupies the gap between the orbits of Mars and Jupiter, roughly 2.2 to 3.3 astronomical units from the Sun. Ceres, the largest body in it, is classed as a dwarf planet.

Some asteroids cross the orbits of the inner planets, and others share Jupiter's orbit, but the great majority are in the main belt, so the other pairs are wrong.

Answer: C

The planet is Neptune.

A planet's period grows with its distance from the sun (Kepler's third law). Neptune is the farthest of the four and takes about 165 years to go round once.

Uranus takes about 84 years, Saturn about 29.5 years and Jupiter about 12 years, in that order of decreasing distance.

In order from the sun the outer planets are Jupiter, Saturn, Uranus and Neptune, so their periods increase in that order too.

Answer: A

The bulk modulus of a perfectly rigid body is Infinite.

Bulk modulus is \(K = -\frac{\Delta P}{\Delta V / V}\), the pressure change divided by the fractional change of volume. A perfectly rigid body does not change its volume at all, so \(\Delta V = 0\) and \(K\) becomes infinite.

Zero bulk modulus would mean the body compresses without resistance, the opposite of rigidity; unity and a finite low value have no special meaning here.

Answer: A

Both 1 and 2 are correct.

The coefficient of linear expansion is \(\alpha = \frac{\Delta L}{L\,\Delta T}\) and the coefficient of volume expansion is \(\gamma = \frac{\Delta V}{V\,\Delta T}\). In each, a ratio of like quantities (length to length, volume to volume) is divided by a temperature change, so the dimension is \([K^{-1}]\), that is, per kelvin.

They differ in size (\(\gamma \approx 3\alpha\)) but not in dimension.

Answer: A

Statement I is correct, statement II is wrong.

Light travels at about \(3 \times 10^{8}\) m/s in air and about \(2.25 \times 10^{8}\) m/s in water, so statement I is true.

Refractive index is \(n = \frac{c}{v}\): the slower the light, the higher the index. Water's index (about 1.33) is greater than air's (about 1.0003), so statement II is false. In fact the truth is the reverse of statement II, which is why it cannot explain statement I.