Physics Questions and Answers

Answer: C

A choke is used to reduce the current in an AC circuit.

It is an inductor, whose reactance \(X_L = 2\pi f L\) opposes alternating current. Because the reactance is 90° out of phase with the current, it limits the current while dissipating almost no power - unlike a resistor, which would waste energy as heat.

Answer: D

The order is IV, III, I, II - visible light, ultraviolet, X-rays, gamma rays.

Frequency rises as wavelength falls: visible light at \(10^{14}\ \mathrm{Hz}\), ultraviolet \(10^{16}\), X-rays \(10^{18}\) and gamma rays above \(10^{19}\). Since \(E = h\nu\), the energy and the penetrating power rise in the same order.

Answer: A

Newton's first law states that every body maintains its state of rest or of uniform motion in a straight line unless acted on by an external force.

It defines inertia and identifies the inertial frames in which the second and third laws hold. It is not a special case of \(F = ma\) but a statement about which frames the second law applies in.

Answer: C

Constantan is the ohmic conductor.

An ohmic conductor obeys \(V = IR\) with a constant resistance, so its V-I graph is a straight line. Constantan, a copper-nickel alloy, is prized because its resistance hardly changes with temperature. A transistor, an electrolyte and a thermistor are all non-ohmic.

Answer: B

Alternating current is not used in electroplating.

Electroplating needs the metal ions to travel consistently to one electrode and deposit there. Alternating current would reverse the direction every half cycle, plating and stripping in turn, so direct current is essential.

Answer: C

The IC chip is made of silicon.

Silicon is a semiconductor whose conductivity can be controlled precisely by doping, it is abundant as sand, and it forms a stable insulating oxide, \(\mathrm{SiO_2}\), that makes the layered fabrication of integrated circuits possible.

Answer: C

This is Kirchhoff's law of thermal radiation.

At a given temperature and wavelength, the emissivity of a surface equals its absorptivity, so a good absorber must be a good emitter. It is why a matt black surface both absorbs and radiates most strongly, and why a perfect absorber - a black body - is also the perfect emitter.

Answer: C

Infrared rays are electromagnetic.

They are oscillating electric and magnetic fields and travel through vacuum at \(3\times10^{8}\ \mathrm{m\,s^{-1}}\). Sound and ultrasonic waves are mechanical and need a medium; cathode rays are a stream of electrons, which are particles.

Answer: D

This is Newton's law of cooling.

\[\frac{dQ}{dt} \propto (T - T_s)\] It holds well for small temperature differences, where the loss is chiefly by convection. For large differences the radiative term dominates and Stefan's fourth-power law takes over.

Answer: D

This constant speed is the terminal velocity.

The body accelerates until the viscous drag and buoyancy together balance the weight, after which the net force is zero. For a sphere obeying Stokes' law, \[v_t = \frac{2r^2(\rho - \sigma)g}{9\eta}\]