General Science Mixed Quiz: Set 63

The correct answer is B, helium. An alpha particle consists of two protons and two neutrons bound together, which is exactly the nucleus of a helium-4 atom, so it carries a charge of plus two and a mass number of four. Being heavy and doubly charged it ionises the air strongly but loses energy quickly, and a sheet of paper or a few centimetres of air stops it. Option A is wrong because the hydrogen nucleus is a single proton, which is emitted in some nuclear reactions but is not an alpha particle. Option C is wrong because a lithium nucleus has three protons. Option D is wrong because the deuterium nucleus, the deuteron, has one proton and one neutron and is used in heavy water, not in alpha decay.

The correct answer is B, Concrete. Concrete is the building material made by mixing cement with coarse aggregate such as gravel, fine aggregate such as sand, and water.

The cement and water react in a process called hydration, which sets into a hard mass and binds the aggregate together. A common ratio for ordinary work is one part cement, two parts sand and four parts gravel, written as 1:2:4. Concrete is strong in compression but weak in tension, so steel bars are embedded in it to make reinforced cement concrete, the material of most modern buildings, bridges and dams.

Option A, mortar, is cement or lime with sand and water only, with no coarse aggregate, and is used to join bricks. Option C, slurry, is any thin, pourable mix of a solid in a liquid. Option D, a moderator, is the material that slows neutrons in a nuclear reactor.

Exam tip: mortar has no gravel; concrete has gravel; adding steel gives RCC.

The correct answer is B, about 1.1 eV. Silicon is the most widely used semiconductor and its forbidden energy gap, the separation between the top of the valence band and the bottom of the conduction band, is about 1.1 electron volts at room temperature. Option A is wrong because 0.7 electron volts is the band gap of germanium, the other elemental semiconductor, and the two values are the pair most often swapped in examinations. Option C is wrong because a gap of that size belongs to a wide band gap material rather than to silicon. Option D is wrong because 5.5 electron volts is the band gap of diamond, which is quoted as the standard example of an insulator. Gallium arsenide, for comparison, has a gap of about 1.43 electron volts.

The correct answer is A, 1 and 4. Hooke's law holds only as far as the limit of proportionality, and that limit is either equal to or lower than the elastic limit, so statements 1 and 4 are the correct pair. Hooke's law says stress is proportional to strain, which is the straight part of the stress-strain graph; the point where the curve leaves the straight line is the limit of proportionality. A little beyond it the material still springs back to its original shape, and the last point at which it does so is the elastic limit. For most metals the two points lie very close together and for an ideal material they coincide. B is wrong because statement 2 extends the law past the straight part of the graph, where stress and strain are no longer proportional. C and D are wrong because statement 3 says always less, while the two limits may also be equal. Exam tip: order the four points as proportionality limit, elastic limit, yield point, ultimate stress.

The correct answer is B, the mass number falls by 4 and the atomic number by 2. An alpha particle takes away two protons and two neutrons, so the parent loses four units of mass number and two units of atomic number and becomes an element two places earlier in the periodic table; uranium-238, for instance, becomes thorium-234. Option A is wrong because no decay removes two units of mass with one of charge. Option C is wrong because that describes beta minus decay, in which a neutron turns into a proton and an electron is emitted, so the charge rises by one while the mass number stays the same. Option D is wrong because that is gamma emission, in which the nucleus only sheds surplus energy and neither number changes.

The correct answer is C, fish farming. Pisciculture is the rearing of fish in ponds, tanks and other managed waters, the Latin word piscis meaning fish. It is a branch of aquaculture and India is one of the largest fish producers in the world; the Blue Revolution is the name given to the planned growth of fish output, and rohu, catla and mrigal are the common Indian carps raised in ponds. A is wrong because the rearing of honey bees is apiculture, which gives honey and beeswax. B is wrong because the rearing of silkworms is sericulture, in which the mulberry-fed Bombyx mori gives most of the world's silk. D is wrong because the rearing of birds is aviculture, and the keeping of fowl for eggs and meat is poultry farming. Exam tip: learn the culture words together - pisciculture fish, apiculture bees, sericulture silkworms, aviculture birds, horticulture fruits and vegetables, floriculture flowers.

The correct answer is A, rectifier. A p-n junction conducts freely in forward bias, where the potential barrier is reduced, and blocks current in reverse bias, where only a tiny leakage current flows. This one-way conduction is exactly what is needed to turn alternating current into direct current, so the diode's main use is as a rectifier, in half-wave, full-wave or bridge arrangements. Option B is wrong because amplification needs a three-terminal device such as a transistor; a two-terminal diode cannot supply power gain. Option C is wrong because an oscillator requires an amplifier with positive feedback, although special diodes can support oscillation. Option D is wrong because a transformer works by mutual induction between coils, handles only alternating current, and contains no semiconductor at all.

The correct answer is B, Galileo Galilei. Galileo was the first to argue that, with air resistance removed, every body falls with the same acceleration regardless of its mass. He reached this by rolling balls down inclined planes and timing them, and the idea is linked in popular accounts with his experiment from the Leaning Tower of Pisa. This overturned the older belief of Aristotle that heavier bodies fall faster. The acceleration he described is g, about 9.8 metres per second squared near the earth. A is wrong because Einstein built on this in the general theory of relativity through the equivalence principle, long afterwards. C is wrong because Edison was an inventor of the electric lamp and phonograph and did no work on falling bodies. D is wrong because Newton, born the year Galileo died, later explained why this happens through his law of gravitation. Exam tip: remember the order - Galileo found that all bodies fall alike, Newton explained the cause.

The correct answer is C, gamma rays. Gamma radiation is electromagnetic, carries no charge and no rest mass, and therefore passes through matter far more easily than charged particles; thick lead or concrete is needed to reduce it. Option A is wrong because the alpha particle, though the most strongly ionising of the three, is stopped by a sheet of paper or a few centimetres of air precisely because it interacts so strongly. Option B is wrong because beta particles, being light electrons, penetrate more than alpha but are stopped by a few millimetres of aluminium. Option D is wrong because the three differ widely. The useful rule is that penetrating power increases from alpha to beta to gamma while ionising power decreases in the same order.

The correct answer is C, Cleaning. In electroplating the article to be coated is made the cathode, the plating metal serves as the anode, and both are dipped in an electrolyte containing a salt of that metal. When current flows, the metal ions travel to the cathode and settle on it. The coating can bond only if the surface is chemically clean, so grease, oil, rust and oxide are stripped off first by degreasing and by pickling in acid; any film left behind makes the plating peel, blister or come out patchy. Option A is wrong because buffing only brings up a shine on a surface that is already clean. Option B is wrong because soldering joins two metals with a filler alloy and has nothing to do with preparing a surface for plating. Option D is wrong because polishing smooths the surface mechanically but does not remove the oily and oxide films that spoil adhesion. Exam tip: the order is clean first, then plate, and the article is always the cathode.

The correct answer is C, Zener diode. A Zener diode is heavily doped so that breakdown occurs at a low and sharply defined reverse voltage. Beyond that point the voltage across it stays almost constant even when the current through it changes a great deal, which is precisely the property a voltage regulator needs. Option A is wrong because a light emitting diode is worked in forward bias and converts electrical energy into light. Option B is wrong because a photodiode is indeed reverse biased, but it is used to detect light rather than to hold a voltage steady, and it is not operated in breakdown. Option D is wrong because a solar cell works with no external bias at all and generates its own electromotive force from incident light.

The correct answer is A, Ashwagandha. Ashwagandha, whose botanical name is Withania somnifera, is called winter cherry in English because of the small red berry it bears inside a papery cover; it is also called Indian ginseng. The part used is the root, which in Ayurveda is given as a tonic for weakness, stress and sleeplessness, and the plant's Sanskrit name comes from the horse-like smell of the fresh root. It is a hardy shrub of dry regions, so Rajasthan, Madhya Pradesh and Gujarat are the main growing states; the Nagauri type from Nagaur in Rajasthan and the crop around Mandsaur and Neemuch in Madhya Pradesh are the best known. B is wrong because rose is an ornamental and perfume crop, not a winter cherry. C is wrong because lily is a garden flower grown from bulbs. D is wrong because chandni is an ornamental flowering shrub grown for its white flowers. Exam tip: Ashwagandha, Withania somnifera, winter cherry or Indian ginseng, root used.

The correct answer is A, as a moderator. Heavy water, in which the hydrogen of ordinary water is replaced by deuterium, slows the fast neutrons released in fission to low speeds at which they are far more likely to split further uranium nuclei, and it absorbs very few neutrons in doing so, which is why Indian pressurised heavy water reactors can use natural uranium. In such reactors heavy water also serves as coolant. Option B is wrong because the fuel is uranium or plutonium. Option C is wrong because control rods are made of neutron absorbers such as cadmium or boron, whose purpose is the opposite, to take neutrons out of the reaction. Option D is wrong because shielding is provided by thick concrete and lead around the core.

The correct answer is C, Thallophyta. Litmus is a purple dye taken from lichens, and in the classification used in school science lichens are placed in Thallophyta, the group of simple plants whose body is an undifferentiated thallus with no true root, stem or leaf. Algae and fungi belong to the same group, and a lichen is in fact a partnership of an alga and a fungus living together. Litmus is a natural indicator: it stays purple in a neutral solution, turns red in acid and blue in base, and is used both as a solution and as litmus paper. Option A is wrong because gymnosperms are well developed plants with naked seeds, such as pine and cycas. Option B is wrong because pteridophytes, the ferns, are the first plants with vascular tissue. Option D is wrong because bryophytes such as moss and Marchantia are called the amphibians of the plant kingdom. Exam tip: litmus is red in acid, blue in base and purple when neutral.

The correct answer is B, base. The base of a transistor is made very thin and only lightly doped so that most of the carriers injected by the emitter pass straight through it to the collector instead of recombining in it. This is what gives the transistor its large current transfer ratio. Option A is wrong because the emitter is the most heavily doped region, since its job is to supply carriers in plenty. Option C is wrong because the collector is moderately doped and is made physically the largest region so that it can dissipate the heat generated there. Option D is wrong because unequal doping is essential; it is also the reason a transistor cannot be built by simply joining two separate diodes back to back, since no thin common base would exist.

The correct answer is D, Ar and N2. An ordinary filament bulb is filled with a mixture of argon and nitrogen because both are chemically unreactive and will not burn the white-hot tungsten filament. The filament works at about 2,500 degrees Celsius, and in air it would oxidise within seconds. The inert filling also raises the pressure inside the glass, which slows the evaporation of tungsten atoms, so the filament lasts longer and the glass blackens less. Argon is preferred among the noble gases because it is the cheapest, making up nearly one per cent of air. A is wrong because hydrogen is highly inflammable and would be a danger inside a hot bulb. B is wrong for the same reason, as the hydrogen in it makes the mixture unsafe. C is wrong because the oxygen in it would oxidise and break the filament at once. Exam tip: tungsten filament, melting point about 3,400 degrees Celsius, in an argon-nitrogen filling.

The correct answer is B, cadmium and boron. Both absorb neutrons strongly, so pushing the rods into the core removes neutrons and slows the chain reaction, while withdrawing them allows the reaction to build up; this is how the power of a reactor is controlled and how it is shut down. Option A is wrong because graphite and heavy water are moderators, which slow neutrons without absorbing them, the opposite requirement. Option C is wrong because lead and concrete form the biological shield that keeps radiation from escaping the reactor building. Option D is wrong because uranium and plutonium are the fissile fuels that supply the neutrons in the first place. Candidates should fix moderator, control rod, coolant and shield to their own materials. A reactor therefore holds four distinct materials in four distinct roles, and a question that names any one of them is really asking which role it fills.

The correct answer is B, NAND and NOR. They are called universal gates because either one of them, used repeatedly, can build every other logic gate, including NOT, AND, OR, XOR and XNOR. This matters in practice because a manufacturer can lay down a whole integrated circuit using only one kind of gate. Option A is wrong because AND and OR, along with NOT, are the basic gates, and AND and OR by themselves cannot produce an inversion. Option C is wrong for the same reason; OR and NOT together can produce NOR and from it the rest, but the two gates are not individually universal in the sense asked. Option D is wrong because XOR and XNOR are derived gates built from the basic ones and are not universal.

The correct answer is B, the nuclear fusion of hydrogen nuclei. In the core of the Sun, at temperatures of millions of degrees, hydrogen nuclei fuse in stages to form helium, and the small amount of mass lost in the process appears as energy according to Einstein's mass-energy relation. Option A is wrong because fission needs heavy nuclei such as uranium, which are not the fuel of a star. Option C is wrong because chemical burning releases only a few electron volts per reaction, nowhere near enough to keep the Sun shining for billions of years, and there is no oxygen in the core to burn with. Option D is wrong because radioactive decay of heavy elements warms the interior of the Earth to some extent but is not the source of solar energy.

The correct answer is C, 100 Hz. A full-wave rectifier conducts during both halves of the input cycle, since one pair of diodes handles the positive half and the other pair the negative half. The output therefore contains two pulses for every cycle of the input, so its fundamental ripple frequency is twice the input frequency, that is 100 hertz for a 50 hertz supply. Option A is wrong because the frequency is doubled, not halved. Option B is wrong because 50 hertz is the ripple frequency of a half-wave rectifier, which conducts in only one half of each cycle; this is the standard pair of values examiners test. Option D is wrong because the output of a rectifier is pulsating direct current and its ripple is zero only after a filter has smoothed it.