Physics Questions and Answers

Answer: A

True. This is Kepler's first law, the law of orbits.

Each planet moves in an ellipse with the sun at one of the two foci. The orbits of the planets are very nearly circular - the earth's eccentricity is only 0.017 - which is why the departure from a circle went unnoticed until Kepler analysed Tycho Brahe's observations of Mars.

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: C

Lightning conductors are made of copper.

Copper conducts almost as well as silver at far less cost, so it carries the enormous discharge current to earth without melting, and it resists corrosion out of doors. The conductor runs from a pointed rod above the roof to a plate buried in moist earth.

Answer: D

The parsec is a unit of astronomical distance.

It is the distance at which one astronomical unit subtends one arcsecond - hence parallax-second. One parsec is 3.26 light years, or \(3.086\times10^{16}\) m, and it is the unit astronomers actually use, light years being reserved for popular accounts.

Answer: D

Prithvi is the Indian surface-to-surface missile.

It was the first missile developed under the Integrated Guided Missile Development Programme, with a range of 150 to 350 km depending on the variant. Nag is an anti-tank missile and Akash a surface-to-air missile.

Answer: B

The distance is \(\lambda/4\).

In a standing wave, consecutive nodes are half a wavelength apart, and an antinode sits exactly midway between two nodes. The node-to-antinode distance is therefore half of that, \(\lambda/4\).

Answer: C

The instrument is a periscope.

Two mirrors or right-angled prisms set at 45° at the ends of a tube turn the light through 90° twice, so the viewer sees over an obstruction - or, in a submarine, above the surface while the hull stays submerged.

Answer: C

There are three common states of matter.

Solid, liquid and gas differ in how tightly the intermolecular forces hold the particles: fixed lattice sites, close but mobile, or far apart and free. Plasma and the Bose-Einstein condensate are usually counted as further states, but under ordinary conditions three is the answer.

Answer: A

The weight of a body is maximum at the poles.

Two effects work together: the earth is flattened at the poles, so the surface is closer to the centre and \(g\) is larger; and there is no centrifugal effect from rotation on the axis. Weight falls by about half a per cent from pole to equator, and falls further with altitude.

Answer: A

Because the moon's periods of rotation and revolution are equal.

Both take about 27.3 days, so the moon turns once on its axis in exactly the time it takes to go once round the earth, and keeps the same face towards us. This synchronous or tidally locked rotation is the result of tidal friction over billions of years.