Physics Questions and Answers

Answer: A

The choke in a tube light is there to induce a high voltage.

When the starter's contacts open, the sudden collapse of current in the inductor produces a large back emf, \[e = -L\frac{di}{dt}\] of several hundred volts, which strikes the arc. Once the tube is conducting, the same choke's reactance limits the current.

Answer: C

The ampere-hour measures amount of charge.

Charge is current times time: \[1\ \mathrm{Ah} = 1\ \mathrm{A} \times 3600\ \mathrm{s} = 3600\ \mathrm{C}\] Battery capacity is quoted in ampere-hours because it says how long the battery can supply a given current.

Answer: B

The light year is a unit of distance.

It is the distance light travels in one year: \[d = ct = 3\times10^{8}\times 3.156\times10^{7} \approx 9.46\times10^{15}\ \mathrm{m}\] The word 'year' in the name misleads - it measures length, not time.

Answer: A

The order of decreasing wavelength is radio waves, infrared, visible light, X-rays.

The electromagnetic spectrum runs from radio waves at kilometres down to gamma rays at \(10^{-12}\) m. Radio is longest, then microwave, infrared, visible, ultraviolet, X-ray and gamma. Since \(E = hc/\lambda\), energy rises in exactly the reverse order.

Answer: B

Growth of the baby in the uterus is followed by ultrasound.

High-frequency sound above 20 kHz is reflected at boundaries between tissues of different density, and the echoes are timed to build the image. Unlike X-rays and gamma rays it is non-ionising, so it does not harm the developing foetus.

Answer: A

A man jumping from a moving train is thrown forward.

His whole body shares the train's forward velocity. On landing, the ground brings his feet to rest at once while the upper body keeps moving by inertia, so he pitches forward. Running a few steps in the direction of motion is the way to avoid it.

Answer: B

Pressure is the scalar.

Although it arises from force, pressure at a point in a fluid acts equally in all directions and has no direction of its own, so it is specified by a single number. Force, velocity and acceleration are all vectors.

Answer: C

Because diamond has a very high refractive index, 2.42.

That gives a critical angle of only 24.4°, so light entering the stone is totally internally reflected many times before it can escape, and emerges concentrated through the crown. The high dispersion also splits the light into colours, giving the fire. A cut that respects those angles is what makes the sparkle.

Answer: B

Because rolling friction is much less than sliding friction.

In rolling, the point of contact is momentarily at rest relative to the ground, so the surfaces do not scrape; only a small deformation at the contact loses energy. Rolling friction is typically a hundredth of sliding friction.

Answer: A

The sun's energy comes from nuclear fusion.

Four hydrogen nuclei fuse to helium through the proton-proton chain: \[\mathrm{4\,^1_1H \rightarrow ^4_2He + 2e^+ + 2\nu + 26.7\ MeV}\] The mass of the helium nucleus is slightly less than that of the four protons, and the difference appears as energy by \(E = \Delta m c^2\).