Physics Questions and Answers

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: B

An optical fibre works by total internal reflection.

The core has a slightly higher refractive index than the cladding, so light striking the boundary beyond the critical angle is reflected entirely back into the core, and travels the length of the fibre in a zigzag with almost no loss.

Answer: A

Because metals contain free electrons.

In the metallic bond, the valence electrons are delocalised over the whole lattice rather than bound to individual atoms. An applied field sets this electron sea drifting, which is the current. The same electrons also carry heat, which is why good electrical conductors are good thermal ones.

Answer: C

An LED emits visible light.

When electrons cross the forward-biased p-n junction and recombine with holes, the energy released appears as photons. The colour is set by the band gap of the semiconductor: gallium arsenide phosphide for red, indium gallium nitride for blue.

Answer: A

A mirage is due to unequal heating of different parts of the atmosphere.

Hot ground warms the air just above it, making it optically rarer, so a gradient of refractive index forms. Light from the sky curves gradually downward through these layers and is finally totally internally reflected, so it appears to come from the ground.

Answer: A

Isotopes have the same number of protons but different numbers of neutrons.

The proton count fixes the element and its chemistry; the differing neutron count changes only the mass. Carbon-12, carbon-13 and carbon-14 are examples, all with six protons.

Answer: D

Neutrons are not emitted in ordinary radioactive decay.

Radioactive substances emit alpha particles, beta particles (electrons) and gamma rays (electromagnetic radiation). Free neutrons come from fission and from certain induced reactions, not from spontaneous decay.

Answer: B

Normal atmospheric pressure is maintained by air pumps.

At cruising altitude the outside pressure is only about a quarter of that at sea level, too low to sustain consciousness. Bleed air from the engines is compressed and fed into the cabin, which is held at a pressure equivalent to about 2,400 m.

Answer: D

\(\mathrm{ML^2T^{-2}}\) is the dimensional formula of a couple, or torque.

Torque is force times perpendicular distance: \(\mathrm{MLT^{-2}} \times \mathrm{L} = \mathrm{ML^2T^{-2}}\). Work and energy share these dimensions, though torque is a vector and they are scalars. Moment of inertia is \(\mathrm{ML^2}\) and pressure \(\mathrm{ML^{-1}T^{-2}}\).

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, \(3.6\times10^{6}\ \mathrm{J}\).