General Science Previous Year Questions (PYQ) with Answers – Page 78

The energy gained is the charge of the electron multiplied by the potential difference it crosses. Energy W = qV.

A perfectly horizontal rope has no upward component of tension, so no finite tension can hold up the 200 N weight with the rope fully straight.

Polythene is an addition polymer: ethene molecules join through their double bonds without losing any small molecule. Polymers form in two main ways.

Quartz is a crystalline solid, and crystals show different physical properties in different directions, which is called anisotropy. In a crystalline solid the particles are arranged in a regular, repeating pattern, so the arrangement looks different along different directions.

Brine is a concentrated solution of sodium chloride, common salt, in water. Brine is the starting point of the chlor-alkali process: passing electricity through it gives sodium hydroxide, chlorine gas and hydrogen gas, which is why the process is named after chlorine and alkali.

An LED is a semiconductor p–n junction diode that gives out light when current flows through it in the forward direction.

It contains hydrochloric acid and has a pH of about 1.5 to 2, the lowest of the four, so it has the most hydrogen ions.

A nanometre is 10⁻⁹ m and an ångström is 10⁻¹⁰ m, so one nanometre holds ten ångströms.

Toothpaste is mildly basic so that it can neutralise the acid that causes tooth decay. Bacteria in the mouth break down sugar and food particles left on the teeth and produce acids.

For an ideal gas in a fixed volume at a fixed temperature, pressure is proportional to the number of molecules, so half the molecules give half the pressure.

The stinging hairs on nettle leaves break on touch and inject methanoic acid, also called formic acid, into the skin, which causes the burning pain.

Outside a long straight wire the field is B = μ₀I/(2πr), so at a fixed distance r it is directly proportional to the current I. Hans Christian Oersted found in 1820 that a current deflects a compass needle, showing that a current produces a magnetic field.

A body in equilibrium has no net force and no net moment on it, while neither momentum nor kinetic energy is conserved in every situation.

The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.

Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing. Humans hear sound between about 20 Hz and 20 kHz.

A simple periscope has two plane mirrors, and light reaches the eye only by bouncing off them. In a simple periscope, two plane mirrors are fixed parallel to each other at the two bends of a long tube, each tilted at 45° to its length.

A mixture of carbon monoxide and hydrogen is called synthesis gas, or syngas, because it is the starting material for making methanol and many other compounds.

One nanometre is 10⁻⁹ metre, and since one metre is 100 (10²) centimetres, it equals 10⁻⁷ centimetre. The prefix nano means one billionth.

Saturated open-chain hydrocarbons are the alkanes, and every alkane fits the formula CₙH₂ₙ₊₂. In a saturated hydrocarbon every carbon atom is joined to others only by single bonds, so each carbon holds the largest possible number of hydrogen atoms.

A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.