Physics Questions and Answers

Answer: C

A solar eclipse occurs when the moon comes between the sun and the earth.

The moon's shadow falls on the earth, and only those in the narrow path of the umbra see totality. It can happen only at new moon, and not at every new moon, because the moon's orbit is tilted about 5° to the ecliptic.

Answer: D

It loses heat by conduction, convection and radiation.

Conduction carries heat into the metal table, convection carries it away in rising currents of warm air, and radiation carries it off as infrared. All three act together whenever a hot body sits in air on a solid surface.

Answer: C

One watt-hour equals \(3.6\times10^{3}\) J.

\[1\ \mathrm{Wh} = 1\ \mathrm{W}\times 3600\ \mathrm{s} = 3600\ \mathrm{J}\] A kilowatt-hour, the unit on an electricity bill, is a thousand times this, \(3.6\times10^{6}\ \mathrm{J}\).

Answer: C

II and III - refraction and internal reflection.

Sunlight entering a raindrop is refracted and dispersed into its colours, reflects once off the far inner surface, and is refracted again on leaving. It emerges at about 42° to the incoming ray, which is why the bow is an arc of fixed angular radius. Diffusion plays no part.

Answer: D

All of the above.

Radio waves pass through cloud, dust and the daylight sky, which stop or swamp visible light, so a radio telescope works around the clock and in any weather. It also detects sources - pulsars, quasars, cold hydrogen clouds - that emit little or no visible light at all.

Answer: B

Planets are non-luminous heavenly bodies.

They shine only by reflecting sunlight, unlike stars, which generate their own light by fusion. Planets also appear as small discs rather than points, which is why they shine steadily while stars twinkle.

Answer: A

Moment of inertia and moment of a force do not share dimensions.

Moment of inertia is \(\mathrm{ML^2}\) while moment of a force, or torque, is \(\mathrm{ML^2T^{-2}}\). The other three pairs do match: work and torque, angular momentum and Planck's constant \((\mathrm{ML^2T^{-1}})\), and impulse and momentum \((\mathrm{MLT^{-1}})\).

Answer: A

Radiocarbon is produced by collisions between fast neutrons and nitrogen nuclei in the upper atmosphere.

\[\mathrm{^{14}_{7}N + ^1_0n \rightarrow ^{14}_{6}C + ^1_1H}\] The neutrons come from cosmic rays. The carbon-14 oxidises to \(\mathrm{CO_2}\), enters the food chain, and decays with a half-life of 5,730 years once the organism dies - which is the basis of radiocarbon dating.

Answer: C

Silver is the best conductor.

Its resistivity is \(1.59\times10^{-8}\ \Omega\,\mathrm{m}\), just below copper's. Copper is used for wiring nonetheless because silver is far more expensive and tarnishes in air.

Answer: D

Because planets are nearer the earth and so present an extended disc rather than a point.

A star is effectively a point source, so the whole of its light is deflected together by a turbulent air cell and the brightness flickers. A planet's disc is a collection of many such points, and their random fluctuations average out, so the light appears steady.