General Science Previous Year Questions (PYQ) with Answers – Page 7

The correct answer is A, Total internal reflection. When light travels from a denser medium to a rarer one and strikes the boundary at an angle greater than the critical angle, it does not pass out at all but is reflected completely back. An optical fibre uses exactly this. It has a thin glass or plastic core of higher refractive index, covered by a cladding of lower refractive index, so light entering one end keeps bouncing off the walls and travels long distances, even around bends, with very little loss. That is why fibres carry internet and telephone signals and are used in endoscopes to see inside the body. The Indian-born physicist Narinder Singh Kapany is called the father of fibre optics. Option B is wrong because diffraction is the bending of light around small obstacles or through narrow slits. Option C is wrong because scattering spreads light in many directions, which makes the sky look blue. Option D is wrong because refraction is only the bending of light as it enters a new medium, while a fibre works by stopping light from escaping. Exam tip: optical fibre, mirage and a sparkling diamond all work by total internal reflection.

The correct answer is C, Pascal. Pressure is force acting per unit area, so its SI unit is the newton per square metre, which is given the special name pascal (Pa) after the French scientist Blaise Pascal, who stated the law of transmission of pressure in liquids used in hydraulic brakes and lifts. One pascal is a very small pressure, so larger units are common: 1 bar = 10⁵ Pa, 1 atmosphere = 101,325 Pa = 760 mm of mercury, and weather reports use the hectopascal (1 hPa = 100 Pa), while blood pressure is still read in mm Hg. Option A is wrong because the volt is the SI unit of electric potential difference (1 V = 1 J/C). Option B is wrong because the ampere is the SI base unit of electric current. Option D is wrong because the ohm is the SI unit of electrical resistance (1 Ω = 1 V/A). Exam tip: pressure = force ÷ area, N/m² = pascal; and the electrical trio: volt for voltage, ampere for current, ohm for resistance.

The correct answer is C, 500 W. The work done against gravity is mgh = 40 × 10 × 5 = 2000 joule, and power is work divided by time, so 2000 ÷ 4 = 500 watt.

Power is the rate of doing work, measured in watt, where one watt is one joule per second. In climbing stairs the useful work is the gain in potential energy, mgh, where only the vertical rise counts and the length of the staircase does not matter. If the same girl ran up in 2 seconds the work would be the same 2000 joule but the power would double to 1000 watt, which shows that power depends on how fast the work is done.

Option A is wrong because 100 W comes from dividing by the wrong quantity. Option B is wrong because 200 W would need 10 seconds. Option D is wrong because 2000 is the work in joule, not the power in watt.

Exam tip: work = mgh, power = work ÷ time, and one horsepower is about 746 watt.

The correct answer is C, zero. Constant velocity means no acceleration, and by Newton's second law force equals mass into acceleration, so the net force must be zero.

Newton's first law says a body continues in its state of rest or of uniform motion in a straight line unless an external force acts on it, which is why motion at a steady speed in a fixed direction needs no force to keep it going. Force is needed only to change the speed or the direction. A car moving at a steady 60 km per hour on a straight road is in the same state: the engine's push only balances friction and air resistance, so the forces cancel and the net force is zero. Such a body is said to be in equilibrium.

Options A, B and D are wrong because any net force in a direction would produce acceleration and so change the velocity, which contradicts the given condition of constant velocity.

Exam tip: constant velocity means zero acceleration and therefore zero net force, not zero force at all.

The correct answer is B, Platinum. Platinum is the least reactive of the four, so it is found in nature in the free or native state as the metal itself.

A metal occurs free only when it does not react with the air, water and other substances around it. In the reactivity series the metals at the bottom - gold, platinum and silver - are called noble metals and are found native; those higher up occur as oxides, sulphides or carbonates and must be extracted from their ores. This is why gold nuggets are found in river sand while aluminium has to be won from bauxite by electrolysis.

Option A is wrong because copper is mostly mined as copper pyrites and other ores, though small native deposits do exist. Option C is wrong because iron is always found as an oxide such as haematite or magnetite. Option D is wrong because potassium is so reactive that it burns on contact with water and is never found free.

Exam tip: gold, platinum and silver occur native; the more reactive a metal, the more firmly it stays locked in its ore.

The correct answer is D, Intermolecular forces are weak. Melting and boiling only have to break the weak forces between the molecules, not the strong bonds inside them, so little heat is needed.

A covalent bond is formed by the sharing of electrons and is itself strong, but it acts only within the molecule. Separate molecules are held to one another by weak van der Waals forces, and these give way at low temperature, which is why water, ammonia, carbon dioxide and most organic compounds melt and boil easily. Ionic compounds such as sodium chloride behave in the opposite way: a strong electrostatic force runs through the whole crystal, so their melting points are very high. Giant covalent solids like diamond and silicon carbide are the exception, since their bonds form a continuous network.

Options A, B and C are wrong because they all claim the forces between molecules are medium or strong, which would raise the melting and boiling points instead of lowering them.

Exam tip: strong bond inside the molecule, weak force between molecules, so covalent compounds melt at low temperature.

The correct answer is C, 24 J. By the work-energy theorem the work done equals the change in kinetic energy, which is half of 1.0 into 8 squared minus 4 squared, that is 0.5 × (64 - 16) = 24 joule.

Kinetic energy is given by half m v squared, so at 4 m/s the object has 8 joule and at 8 m/s it has 32 joule; the force must supply the difference of 24 joule. Note that doubling the speed makes the kinetic energy four times, because the speed is squared. The work-energy theorem holds whatever the path taken, which is why the question can be solved without knowing the force or the distance.

Option A is wrong because 8 joule is only the starting kinetic energy. Option B is wrong because 32 joule is the final kinetic energy, not the change. Option D is wrong because 16 joule comes from taking half of the difference of the squares of the speeds incorrectly.

Exam tip: work done = final kinetic energy minus initial kinetic energy, and doubling speed makes kinetic energy four times.

The correct answer is D, Lead and tin. Solder is an alloy of lead and tin, usually in nearly equal parts.

An alloy is a mixture of a metal with one or more other elements, made to gain a property the pure metal lacks. Solder is useful because the mixture melts at a far lower temperature than either lead or tin alone, so it flows easily and is used to join wires and electrical joints. Other alloys asked in papers are brass, made of copper and zinc, bronze of copper and tin, stainless steel of iron, chromium and nickel, and German silver of copper, zinc and nickel.

Option A is wrong because lead with copper is not the soldering alloy. Option B is wrong because tin with iron gives tin plating, used for tin cans. Option C is wrong because lead and iron together are not used as solder.

Exam tip: solder is lead plus tin and melts at a low temperature; brass is copper and zinc, bronze is copper and tin.

The correct answer is D, uterus. The zygote divides as it travels down the fallopian tube and the resulting ball of cells gets implanted in the thick lining of the uterus.

Fertilisation itself takes place in the fallopian tube, where a sperm fuses with the ovum to form the zygote. Repeated division turns it into an embryo, which reaches the uterus in about a week and sinks into the endometrium, the soft inner wall prepared by hormones. There the placenta develops and supplies the growing embryo with food and oxygen from the mother's blood while carrying away waste. Human pregnancy lasts about nine months.

Option A is wrong because the cervix is only the narrow neck of the uterus that opens into the vagina. Option B is wrong because the ovary produces the ovum and hormones but does not hold the embryo. Option C is wrong because the vagina is the passage that receives the sperm and later serves as the birth canal.

Exam tip: fertilisation in the fallopian tube, implantation in the uterus, nourishment through the placenta.

The correct answer is B, 19.8 × 10⁶ J. One kilowatt hour is 3.6 × 10⁶ joule, so 5.5 kWh = 5.5 × 3.6 × 10⁶ = 19.8 × 10⁶ joule.

The kilowatt hour is the commercial unit of electrical energy, the unit in which the electricity board bills a house, and it is often called one unit of electricity. It comes from energy = power × time, so one kilowatt of power drawn for one hour means 1000 watt × 3600 second = 3600000 joule. The joule is the SI unit of energy, and one watt is one joule per second.

Option A is wrong because 14.0 × 10⁶ J is about 3.9 kWh. Option C is wrong because 14.4 × 10⁸ J would be 400 kWh, a hundred times too large. Option D is wrong because 14.4 × 10⁵ J is only 0.4 kWh.

Exam tip: 1 kWh = 3.6 × 10⁶ J = one unit of electricity; multiply the number of units by 3.6 million to get joule.

The correct answer is C, 10 Joule. Potential difference is defined as the work done for each unit of charge carried between two points, so V = W/Q and therefore W = QV. Here the charge is 2 coulomb and the potential difference is 5 volt, which gives work of 2 into 5, that is 10 joule. The definition is the whole question: one volt is one joule per coulomb, so a charge of two coulomb moving across five volt must take ten joule of work. Option A, 0.4 joule, comes from dividing the charge by the voltage instead of multiplying. Option B, 2.5 joule, comes from dividing the voltage by the charge, which gives volt per coulomb and not an energy at all. Option D, 20 joule, is double the right value and would need a charge of 4 coulomb or a potential difference of 10 volt. Exam tip: W = QV, and one volt means one joule of work for every coulomb of charge.

The correct answer is B, Kinetic energy + Potential energy. The mechanical energy of a body is the sum of the energy of its motion and the energy of its position. Kinetic energy is half m v squared and gravitational potential energy is mgh, and their total stays the same as long as only gravity acts and friction is absent. That is the law of conservation of mechanical energy: a ball thrown upward loses kinetic energy and gains exactly as much potential energy, and a falling ball does the reverse, which is why a simple pendulum keeps swinging between the same two heights. When friction or air resistance acts, part of the mechanical energy turns into heat and sound, so it is no longer conserved, although the total energy of the system still is. Option A is wrong because chemical energy is stored in the bonds of fuels and food. Option C is wrong because heat is the energy of random molecular motion and is counted separately. Option D is wrong because electrical energy comes from moving charges. Exam tip: mechanical energy equals KE plus PE, and it stays constant only when no friction acts.

The correct answer is B, kinetic energy. A stretched bow stores elastic potential energy, and when the string is released that store becomes the kinetic energy of the flying arrow. Potential energy is the energy held by a body because of its position or its shape. Pulling the string changes the shape of the bow, so the work done by the archer is stored as elastic potential energy, the same kind of store found in a compressed spring or a wound watch spring. On release the bow springs back to its old shape and pushes the arrow forward, so the stored energy turns into energy of motion. A small part does escape as the twang of sound and as heat, but the main change is potential to kinetic. Option A is wrong because chemical energy is stored in the bonds of fuels and food, not in a bent bow. Option C is wrong because the sound is only a small loss, not the main conversion. Option D is wrong because heat is likewise a minor loss here. Exam tip: a stretched bow, a compressed spring and a drawn catapult all store elastic potential energy.

The correct answer is C, 240 J. Work done against gravity is weight multiplied by height, so 120 × 2 = 240 joules. Work is done when a force moves a body along the direction of that force, and it is measured as W = F × s. Here the boy must at least match the weight of the box, which is 120 N, and he moves it through 2 m, so the work done is 240 J. This work is stored in the box as gravitational potential energy, and the box would give back the same 240 J if it fell to where it started. Note that the weight is already given in newtons, so there is no need to multiply by g again. Option A is wrong because 60 J divides the weight by the height instead of multiplying. Option B is wrong because 120 J is only the weight in newtons, with the height left out. Option D is wrong because 130 J matches no step of the calculation. Exam tip: W = F × s, and work done against gravity is mgh, where mg is the weight.

The correct answer is A, 6.67 m/s². Find each mass from F = ma, add the masses, then divide the same force by the total. The first block gives m1 = 5 ÷ 10 = 0.5 kg and the second gives m2 = 5 ÷ 20 = 0.25 kg. Tied together they move as one body of mass 0.5 + 0.25 = 0.75 kg, and the same 5 N force now produces an acceleration of 5 ÷ 0.75, which is about 6.67 m/s². The joint acceleration is always less than the smaller of the two single accelerations, because the same push has to move more mass. Option B is wrong because 1.67 m/s² matches no step of this working. Option C is wrong because 5.67 m/s² would need a total mass of about 0.88 kg. Option D is wrong because 4.67 m/s² would need a total mass of about 1.07 kg. Exam tip: for one force on tied blocks the reciprocals add, that is 1 by a equals 1 by a1 plus 1 by a2.

The correct answer is A, 275 J. Kinetic energy is half m v squared, so half of 22 multiplied by 5 multiplied by 5 gives 275 joules. Kinetic energy is the energy a body has because of its motion. It rises in direct proportion to the mass but with the square of the speed, so doubling the speed makes the energy four times larger, which is why a fast vehicle needs a much longer distance to stop than a slow one. The joule is the SI unit of energy and equals one newton metre; one joule is the work done when a force of one newton moves a body through one metre. Option B is wrong because 110 J is mass multiplied by speed, which gives momentum, not energy, and in different units. Option C is wrong because 1100 J leaves out the half in the formula, that is it works out m v squared. Option D is wrong because 2750 J is ten times the correct value. Exam tip: write half m v squared carefully, because the half and the squared speed are where marks are usually lost.

The correct answer is B, 0 J. The body is thrown up at 20 m/s and gravity takes away 10 m/s of speed every second, so after 2 seconds its speed is zero. Using v = u − gt, we get v = 20 − 10 × 2 = 0 m/s. Kinetic energy is half m v squared, and with v = 0 the kinetic energy is also zero. At that instant the body is at the highest point of its path and all of its energy is potential energy, mgh = 2 × 10 × 20 = 400 J, since the body has risen 20 m. A moment later it begins to fall and the kinetic energy grows again. Option A is wrong because 100 J needs a speed of 10 m/s, which the body has after only one second. Option C is wrong because 400 J is the kinetic energy at the moment of throwing, and the potential energy at the top. Option D is wrong because 200 J matches no point of this motion. Exam tip: at the highest point of a vertical throw the speed, and so the kinetic energy, is zero.

The correct answer is A, Liquid. At normal atmospheric pressure water stays liquid between 0 °C and 100 °C, and 12 °C lies inside that range. Water freezes into ice at 0 °C, which is its melting point, and boils into steam at 100 °C, which is its boiling point, when the pressure is one atmosphere. Between these two fixed points the molecules have enough energy to slide over one another but not enough to fly apart, so water flows and takes the shape of its container while keeping a fixed volume. Water does evaporate slowly at 12 °C from an open surface, but that happens only at the top layer and does not change the state of the water below. Option B is wrong because ionic is not a state of matter at all; it describes a kind of bond or compound. Option C is wrong because water becomes a gas, that is steam, only above 100 °C. Option D is wrong because water turns solid only at or below 0 °C. Exam tip: remember 0 °C and 100 °C as the fixed points of water at one atmosphere.

The correct answer is C, 16 times the initial value. Kinetic energy depends on the square of the momentum, so the momentum grows as the square root of the energy. For a body of mass m, the kinetic energy can be written as p squared divided by 2m, where p is the linear momentum. If the mass does not change, p equals the square root of 2mE, so p is proportional to the square root of E. When the kinetic energy becomes 256 times, the momentum becomes the square root of 256, that is 16 times its earlier value, and the speed also becomes 16 times. Option A is wrong because a body cannot gain energy while its momentum stays the same. Option B is wrong because 8 times the momentum would make the energy 64 times, not 256 times. Option D is wrong because 32 times the momentum would make the energy 1024 times. Exam tip: E equals p squared by 2m, so if the energy becomes n times the momentum becomes root n times, and if the momentum becomes n times the energy becomes n squared times.

The correct answer is B, The stamen and pistil. The stamen and the pistil make the male and female gametes, so they are the essential or reproductive organs of a flower. The stamen is the male part and has an anther that produces pollen grains and a filament that holds the anther up. The pistil, also called the carpel, is the female part and has three portions: the stigma that receives the pollen, the style, and the ovary that holds the ovules. After pollination and fertilisation the ovary grows into the fruit and the ovules become seeds. Sepals and petals are called accessory organs because they only protect the bud and attract insects for pollination. Option A is wrong because the sepal is only the green leaf-like part that covers the bud. Option C is wrong because neither sepals nor petals take any part in making gametes. Option D is wrong because the petal is accessory, even though the pistil named with it is essential. Exam tip: stamen and pistil are essential, sepal and petal are accessory.