Physics Questions and Answers

Answer: A

A solar cell converts solar energy into electrical energy.

Photons above the band-gap energy free electron-hole pairs in a semiconductor, and the built-in field at the p-n junction separates them, driving a current through an external circuit. It is the photovoltaic effect.

Answer: A

At minimum deviation, the angle of emergence equals the angle of incidence.

The ray passes symmetrically through the prism, parallel to its base, so \(i = e\) and \(r_1 = r_2\). That symmetry gives the prism formula \[n = \frac{\sin\left(\frac{A+D_m}{2}\right)}{\sin\left(\frac{A}{2}\right)}\]

Answer: C

Sunlight reaches us in about 8 minutes.

\[t = \frac{d}{c} = \frac{1.5\times10^{11}}{3\times10^{8}} = 500\ \mathrm{s} \approx 8\ \text{minutes 20 seconds}\] So we always see the sun as it was eight minutes ago.

Answer: C

Pure water freezes at 32 °F.

\[F = \frac{9}{5}C + 32 = \frac{9}{5}(0) + 32 = 32\] Fahrenheit set zero at the temperature of a brine mixture, which is why the freezing point of water falls at 32 rather than at 0 on his scale.

Answer: D

A ball pen works on capillarity and surface tension.

Ink is drawn along the narrow tube by capillary action and held at the tip by surface tension, so it does not run out. Rolling the ball picks up a film of ink and lays it on the paper.

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

The SI unit of temperature is the kelvin.

It is one of the seven base units, and the scale starts at absolute zero, so no negative values occur. A kelvin and a degree Celsius are the same size, and \(T(\mathrm{K}) = t(^\circ\mathrm{C}) + 273.15\).

Answer: C

The temperature decreases slightly.

Evaporation removes the fastest molecules from the surface, lowering the average kinetic energy of those left behind, and the latent heat of vaporisation is drawn from the water itself. The drop is small because heat flows in from the surroundings to replace it.

Answer: C

It will bend towards the left, that is towards the iron.

Copper's coefficient of linear expansion is about \(17\times10^{-6}\ \mathrm{K^{-1}}\) against iron's \(12\times10^{-6}\). On heating, the copper strip lengthens more, and since the two are welded the strip must curve with copper on the outside - so the free end swings towards the iron side.

Answer: C

Heat flows according to the temperatures of the two objects.

It always passes from the hotter to the colder body, whatever their masses or heat contents, until both reach the same temperature. This is the content of the second law of thermodynamics, and the reason the zeroth law makes temperature meaningful.