General Science Previous Year Questions (PYQ) with Answers – Page 87

The speed of light in a medium is inversely proportional to its refractive index, so the ratio of speeds is the inverse ratio of the refractive indices.

The so-called lead of a pencil is a mix of graphite and clay; it contains no lead metal. Graphite is a crystalline form, or allotrope, of carbon.

A liquid has no shape of its own, so it must be measured inside a vessel that expands at the same time.

G has the same value everywhere in the universe, for any two masses. The relation is Newton's law of universal gravitation: every two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

The Kelvin and Fahrenheit scales read the same at about 574.25, which is about 301 degrees Celsius. The scales are linked by K = C + 273 and F = (9/5)C + 32.

The lens of the human eye is a converging (convex) lens, not a diverging one, so statement A is not correct.

The rod becomes negative because it gains electrons from the wool. When two materials are rubbed together, only electrons move, because they are the loosely held outer particles of atoms.

This is the Clausius statement of the second law. Rudolf Clausius stated that heat cannot by itself pass from a colder body to a hotter one; work must be done to make it do so.

Bats send out ultrasonic squeaks and listen for the echoes to locate obstacles and prey. This method is called echolocation.

A rainbow forms when raindrops split white sunlight into its seven colours. When sunlight enters a raindrop, it is refracted, and because each colour bends by a different amount, the colours spread apart.

Washing soda is the crystalline form of sodium carbonate, Na2CO3·10H2O, sodium carbonate decahydrate. Sodium carbonate is made from common salt by the Solvay (ammonia-soda) process.

Hair turns grey because the chemical make-up of the hair changes as the body stops producing the pigment melanin, so new substances form and the change cannot be reversed.

The resistance of a conductor depends on its material and shape, not on how much current flows through it. For a cylindrical conductor, R = ρL/A, where ρ is the resistivity of the material, L the length and A the area of cross-section.

Oxygen makes up about 21 per cent of dry air by volume, second only to nitrogen. Dry air near the ground is about 78 per cent nitrogen, 21 per cent oxygen and 0.93 per cent argon, which is the third most abundant gas.

In an insulator the electrons are tightly bound to their atoms, so there are almost no free electrons to carry a current.

Zinc gives up electrons more readily than iron, so it corrodes first and keeps the iron safe. Coating iron or steel with a thin layer of zinc is called galvanisation.

Both stress and pressure are force per unit area, so both are measured in newton per square metre, or pascal (Pa).

A step-up transformer raises an alternating voltage, using more turns in its secondary coil than in its primary. A transformer works on electromagnetic induction and only with alternating current.

Heat makes the copper wire expand a little, and a longer pendulum swings a little more slowly. The time period of a simple pendulum is T = 2π√(L/g), so it grows with the square root of the length.

This is the law of conservation of energy: energy only changes from one form to another. In any isolated system the total energy stays the same.