Science and Technology: UPSC Previous Year Questions (Prelims)

Explanation

Monsanto, an American multinational company, developed an insect-resistant cotton variety known as Bt cotton. This transgenic cotton incorporates a gene from the bacterium Bacillus thuringiensis (Bt), enabling the plant to produce Bt toxins that are effective against specific insect pests, particularly lepidopteran larvae like the cotton bollworm. The introduction of the Bt gene allows the cotton to produce its own insecticide, reducing the need for external chemical insecticides.

Explanation

Statement 1 is correct: At the mean position, the pendulum has maximum velocity and is moving in a straight line for an instant with no force along its path and hence producing zero acceleration.

Statement 2 is correct: In one complete oscillation, the bob passes through the mean position twice - once moving in each direction, hence attaining the maximum velocity twice in a complete oscillation.

Statement 3 is incorrect: At the extreme position, the bob has zero velocity as it stops for an instant before reversing the direction. However, at this point the acceleration is maximum because bob completely comes under the force of gravity and hence produces maximum acceleration.

Statement 4 is correct: In an ideal situation, due to air resistance and internal friction, the amplitude of oscillation decreases with time.

Explanation

The motion of an object traveling in a straight line can be represented on a distance-time graph, with the shape of the graph depending on whether the motion is uniform (straight line) or non-uniform (curved line). Uniform motion represents a constant change in displacement per second. The distance time graph for uniform motion is a straight line, that is, equal distance is covered by the equal units of time.

Option (d) represents the graph where change in displacement per second is constant. Therefore, velocity is constant and hence it performs uniform motion.

Explanation

Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is defined as the amount of heat that passes through a unit area of a material over a unit of time, when there is a temperature difference across the material. Assertion (A) is true: When copper and glass are heated to the same temperature, copper feels hotter to the touch because it has a much higher thermal conductivity than glass. Copper transfers heat to your skin more quickly than glass which makes it feel hotter. Reason (R) is true: While it is true that copper has a higher density than glass, the reason copper feels hotter is due to its thermal conductivity, not its density. R is Not a Correct Explanation of A: The reason (R) correctly states that copper is denser than glass, but density alone does not determine how hot an object feels to the touch. The primary factor is thermal conductivity, which determines how quickly heat is transferred from the object to your skin. Directions: The following item consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below.

Explanation

The boiling point of a substance is the temperature at which its vapour pressure equals the external atmospheric pressure, causing it to change from a liquid to a gas. Assertion (A) is true: The boiling point of water decreases with increasing altitude because atmospheric pressure decreases at higher altitudes. At lower pressure the water molecules require less energy to escape into the vapor phase, so water boils at a lower temperature. For example, at sea level, water boils at 100°C, but at higher altitudes, it can boil at temperatures as low as 90°C or even lower. Reason (R) is false: Atmospheric pressure decreases with increasing altitude. As you go higher in the atmosphere, the air becomes less dense, and the pressure exerted by the atmosphere decreases. This is why mountaintops have lower atmospheric pressure compared to sea level.

Explanation

When water is heated from to, it exhibits an unusual behavior due to the anomalous expansion of water. Between 0°C and 4°C, water contracts, meaning its volume decreases as the temperature increases. At 4°C, water reaches its minimum volume (maximum density). Above 4°C the water starts expanding which means its volume increases as the temperature increases further. This behavior is quite unique to water and is crucial for the survival of aquatic life in cold climates.

Explanation

Option (b) is correct: Clouds act as a blanket by trapping and reflecting back the longwave infrared radiation (heat) emitted by the Earth’s surface. This phenomenon is known as the greenhouse effect. On cloudy nights, this trapped heat keeps the surface warmer compared to clear nights when heat escapes more easily into space.

Option (a), (c) and (d) are incorrect:

Clouds do not block "cold waves" from descending on earth. The temperature of the Earth’s surface is primarily influenced by the balance of incoming solar radiation and outgoing terrestrial radiation, not by "cold waves" from the sky. Clouds do not produce heat. They are formed by the condensation of water vapor and do not have any internal mechanism to generate heat. Clouds do absorb some heat from the atmosphere but their primary role in warming the Earth’s surface at night is to reflect back the heat radiated by the Earth and not to absorb and send heat from the atmosphere.

Explanation

When light waves pass from air to glass, the wavelength and velocity are affected. Velocity decreases because the speed of light in glass is less than in air due to the higher refractive index of glass. Wavelength also decreases because it is directly proportional to the velocity of light in the medium (=v/f, where ‘f’ is the frequency). Frequency remains unchanged because it is determined by the source of the light and does not depend on the medium.

Explanation

Nuclear fission is the process in which a heavy atomic nucleus, such as uranium-235 or plutonium-239, splits into two smaller nuclei, releasing a large amount of energy.

Statement 1 is correct: Each fission reaction releases multiple neutrons, which are crucial for sustaining the chain reaction.

Statement 2 is correct: The neutrons produced in fission reactions are used to initiate further fission events.

Statement 3 is correct: Graphite (a moderator) slows down fast neutrons, increasing the likelihood that they will cause further fission reactions, thus making reaction sustainable over longer periods.

Statement 4 is incorrect: Not every neutron released will necessarily cause further fission, as some neutrons may escape the reactor or fail to cause fission if not properly moderated.

Explanation

The order of filling up orbitals in an atom is governed by Aufbau’s principle, which states that electrons occupy orbitals in order of increasing energy levels, starting from the lowest energy orbital first. The sequence is determined by the (n + l) rule, where n is the principal quantum number and l is the azimuthal quantum number. If two orbitals have the same (n + l) value, the orbital with the lower n value is filled first.