Physics Questions and Answers

Answer: B

The frequency is the same but the wavelength is smaller in water.

Frequency is fixed by the source and cannot change at a boundary. Since \(v = f\lambda\) and light slows to \(c/1.33\) in water, the wavelength must shrink in the same proportion - which is exactly why the ray bends.

Answer: D

A step-up transformer increases voltage.

The secondary has more turns than the primary: \[\frac{V_s}{V_p} = \frac{N_s}{N_p}\] Power is conserved apart from losses, so the current falls in the same proportion, and the frequency is unchanged. Stepping voltage up for transmission is what cuts \(I^2R\) losses in the line.

Answer: D

The colours arise from multiple reflection and interference.

Light reflected from the front surface of the film and light reflected from the back travel different distances and interfere. The path difference depends on both thickness and wavelength, so different colours cancel or reinforce at different points, giving the shifting bands.

Answer: B

It will weigh 2 N.

Weight is \(W = mg\), and the mass is unchanged, so the weight falls in the same proportion as \(g\): \[W_{moon} = \frac{12}{6} = 2\ \mathrm{N}\] The mass remains \(12/9.8 \approx 1.22\) kg on both bodies.

Answer: B

Newton's second law states that the rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction of the force.

\[F = \frac{dp}{dt} = \frac{d(mv)}{dt} = ma\] for constant mass. The law gives force its quantitative meaning and defines the newton as the force that gives 1 kg an acceleration of \(1\ \mathrm{m\,s^{-2}}\).

Answer: C

Lightning heats the air to about 30,000 °C.

That is roughly five times the temperature of the sun's surface. The air expands explosively at that heat, and the shock wave it sends out is what we hear as thunder.

Answer: C

Impurities are added to increase the semiconductor's electrical conductivity.

Doping with a pentavalent element such as phosphorus adds free electrons, giving n-type material; a trivalent one such as boron adds holes, giving p-type. Either raises the carrier concentration by orders of magnitude, and putting the two together makes the p-n junction.

Answer: B

Angular momentum is the vector.

It is \(\vec{L} = \vec{r} \times \vec{p}\), a cross product, so it has a direction - along the axis of rotation, by the right-hand rule. Heat, time and work are all scalars.

Answer: D

Water has the highest specific heat of these.

It is \(4186\ \mathrm{J\,kg^{-1}\,K^{-1}}\), against about 2400 for alcohol and 2100 for kerosene - among the highest of any common substance, because much of the energy goes into breaking hydrogen bonds. This is why water is used as a coolant and why coastal climates are mild.

Answer: A

Because mica is a good conductor of heat but a bad conductor of electricity.

That combination is exactly what an iron needs: the heating element must be electrically isolated from the sole plate while passing its heat through. Mica also splits into thin sheets and withstands high temperature without decomposing.