Physics Questions and Answers

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.

Answer: C

The water level remains the same.

A floating ice cube displaces its own weight of water. When it melts it becomes exactly that weight of water, which occupies exactly the volume it was displacing, so the level does not change. This is why melting sea ice does not by itself raise sea level, whereas melting land ice does.

Answer: B

Both acquire the same momentum.

By the impulse-momentum theorem \(Ft = \Delta p\), so equal forces acting for equal times give equal changes in momentum whatever the masses. Their velocities, accelerations and kinetic energies all differ.

Answer: C

Red has the longest wavelength of the visible colours.

The visible range runs from about 400 nm at the violet end to 700 nm at the red end, so red has the longest wavelength and, since \(E = hc/\lambda\), the lowest energy. White is not a spectral colour but a mixture of all of them.

Answer: A

\(\mathrm{C_2H_6O}\) - ethanol - is the non-electrolyte.

It dissolves in water as whole molecules, producing no ions, so the solution does not conduct. HCl ionises completely, and \(\mathrm{Na_2CO_3}\) and NaCl are ionic solids that dissociate on dissolving.