Physics and chemistry in daily life; energy and nuclear technology: previous-year questions
46 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 13 of them came in the last five papers.
How often UPSC asks this
Questions per year, counted from the official papers.
Attempt the questions
Newest papers first. Tap an option; the answer and the reason appear instantly.
To meet its rapidly growing energy demand, some opine that India should pursue research and development on thorium as the future fuel of nuclear energy. In this context, what advantage does thorium hold over uranium?
- 1. Thorium is far more abundant in nature than uranium.
- 2. On the basis of per unit mass of mined mineral, thorium can generate more energy compared to natural uranium.
- 3. Thorium produces less harmful waste compared to uranium.
Which of the statements given above is/are correct?
Answer (d) & explanation
All three are correct per UPSC's key (d). Thorium is about three to four times more abundant than uranium in the Earth's crust, and India holds about a quarter of world reserves in monazite sands of Kerala, Tamil Nadu and Odisha (1). Nearly all mined thorium-232 can be bred into fissile uranium-233, unlike the 0.7% U-235 in natural uranium, so it can yield more energy per tonne mined (2). The thorium cycle produces far fewer long-lived transuranic wastes such as plutonium (3). So (a), (b) and (c) leave out real advantages.
Notice India's plan: thorium is stage three of Bhabha's three-stage programme; the Prototype Fast Breeder Reactor at Kalpakkam began core loading in 2024. Thorium is not fissile itself, so it needs a starter fuel. Trap: 'more energy' holds only via breeding.
Answer as per the official UPSC answer key.
Microbial fuel cells are considered a source of sustainable energy. Why ?
- 1. They use living organisms as catalysts to generate electricity from certain substrates.
- 2. They use a variety of inorganic materials as substrates.
- 3. They can be installed in waste water treatment plants to cleanse water and produce electricity.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). A microbial fuel cell uses living bacteria as catalysts to break down substrates and release electrons, which flow through a circuit as electric current (1). It can be fitted in wastewater treatment plants, where bacteria feed on organic matter in sewage, cleaning the water while generating electricity (3). Statement 2 is wrong: the substrates are mainly organic materials such as glucose, acetate and sewage waste, not a variety of inorganic materials. So options (a), (b) and (d) fail.
Notice why it is called sustainable: it turns waste into power, works at normal temperatures, and does not burn fuel. Related ideas include biogas from anaerobic digestion and bio-electrochemical systems. India's SATAT and GOBARdhan schemes also aim at waste-to-energy. Trap: 'living organisms' in statement 1 is correct, since bacteria are the catalysts, not an error.
Answer as per the official UPSC answer key.
The function of heavy water in a nuclear reactor is to
Answer (a) & explanation
Slowing down neutrons is correct (a). Heavy water (D₂O) is a moderator. Fast neutrons released in fission are too quick to be captured by uranium-235; colliding with deuterium nuclei slows them to thermal speed so they cause further fission and sustain the chain reaction. Heavy water absorbs few neutrons, so it lets reactors run on natural uranium. (b) is the opposite. (c) is incomplete: heavy water also serves as coolant in India's PHWRs, but its defining function asked here is moderation. (d) is wrong: control rods of boron or cadmium absorb neutrons to slow or stop the reaction.
Notice India's fleet: most Indian reactors are Pressurised Heavy Water Reactors, matching stage one of the three-stage programme.
Answer as per the official UPSC answer key.
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason ?
Answer (c) & explanation
Density of water is maximum at 4°C is correct (c). As a lake cools in winter, surface water reaches 4°C, becomes densest and sinks. Colder water then stays on top and freezes, and ice floats because it is lighter than water. So the bottom stays liquid at about 4°C, and fish survive. (a) is a helping factor, since ice is a poor conductor and insulates the water below, but it is not the basic reason. (b) is wrong: heat is lost from the surface, which is why it freezes. (d) fails because (c) is correct.
Notice the wider idea: this anomalous expansion of water between 0°C and 4°C is unusual among liquids. It also makes water pipes burst in frost and helps break rocks (frost weathering).
Answer as per the official UPSC answer key.
What is the difference between a CFL and an LED lamp ?
- 1. To produce light, a CFL uses mercury vapour and phosphor while an LED lamp uses semi-conductor material.
- 2. The average life span of a CFL is much longer than that of an LED lamp.
- 3. A CFL is less energy-efficient as compared to an LED lamp.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). A CFL passes current through mercury vapour, whose ultraviolet light makes a phosphor coating glow, while an LED emits light from a semiconductor chip (1). LEDs give more light per watt, so a CFL is less energy-efficient (3). Statement 2 is reversed: an LED lasts much longer, about 25,000 hours or more, against roughly 8,000 for a CFL. So (a), (b) and (d) fail.
Notice the policy: India's UJALA scheme (2015) distributed over 36 crore LED bulbs, cutting their price sharply, and the Street Lighting National Programme replaced street lamps with LEDs. CFLs also pose a mercury disposal problem. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
An artificial satellite orbiting around the Earth does not fall down. This is so because the attraction of Earth
Answer (d) & explanation
Option (d) is correct. A satellite is always 'falling' towards Earth, but it moves sideways so fast that it keeps missing it. Earth's gravity supplies the centripetal acceleration that bends its straight path into a circle. (a) is wrong: gravity certainly exists in orbit; at the ISS height it is still about 90% of surface gravity. (b) is wrong: the Moon's pull is tiny and does not cancel Earth's. (c) is wrong: gravity provides acceleration towards the centre, not the forward speed; the speed comes from the launch.
Notice the numbers: a low Earth orbit needs about 7.8 km/s; a geostationary satellite at about 35,786 km circles once in 24 hours. Astronauts float because they are in free fall, not because gravity is absent.
Answer as per the official UPSC answer key.