Physics Questions and Answers

Answer: C

The highest temperature is reached in a valley below sea level.

With the valve open the cooker is simply an open vessel, so water boils at the temperature at which its vapour pressure equals the surrounding pressure. Atmospheric pressure is greatest at the lowest altitude, so the boiling point - and the internal temperature - is highest there.

Answer: C

Supersonic planes fly at a speed greater than the speed of sound.

That is a Mach number above 1, or more than about \(343\ \mathrm{m\,s^{-1}}\) at sea level. The aircraft outruns its own pressure waves, which pile into a shock cone heard on the ground as a sonic boom.

Answer: C

Because the cohesion of mercury exceeds its adhesion to glass.

A liquid spreads when it is more attracted to the surface than to itself, as water is on clean glass. Mercury's cohesive forces are much stronger, so it draws itself into the shape of least surface area - a sphere - and its meniscus curves upward at the edges rather than down.

Answer: C

Its light takes 4.2 years to reach us.

Alpha Centauri is 4.2 light years away, and a light year is precisely the distance light covers in a year. So we see the system as it was more than four years ago - and there is no way to see it as it is now.

Answer: A

The lux is the SI unit of illuminance, the intensity of illumination.

\[1\ \mathrm{lx} = 1\ \mathrm{lm\,m^{-2}}\] The lumen measures luminous flux and the candela luminous intensity at the source; the lux measures how much of that flux lands on a surface.

Answer: A

Intensity of sound has an objective existence.

Intensity is the energy carried per unit area per unit time and can be measured with an instrument, independently of any listener. Loudness is the subjective sensation it produces, which varies with frequency and from person to person.

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

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

Henri Becquerel discovered natural radioactivity, in 1896.

He found that uranium salts fogged a wrapped photographic plate without any exposure to sunlight, so the radiation came from the element itself. He shared the 1903 Nobel Prize with Pierre and Marie Curie, who coined the word radioactivity.

Answer: C

One nanometre equals \(10^{-9}\) m.

The prefix nano- means one thousand-millionth. For scale, an atom is a few tenths of a nanometre across and visible light has wavelengths of 400 to 700 nm.