compulsory · CSS study notes
I. Physical Sciences
CSS GSA — Paper 7, Part-I, Section I (Physical Sciences)
This is the index for Physical Sciences. Each sub-topic below has its own sub-page with full notes, MCQ traps, and self-test questions.
Sub-topics
Past paper practice
- Natural Hazards, Renewable/Non-Renewable Energy, and Universe/Cosmology topics recur in almost every past paper — highest priority for revision
Past Paper Questions (FPSC GSA Paper 7, 2018–2025)
Each question gives a Full Answer (write this in the exam, in full sentences) and Quick Notes (bullets for fast revision).
2025 — Part-I MCQs
- 'Blue dwarfs' and 'Red giants': (B) Young star and Old star — hotter/bluer stars are younger main-sequence stars; red giants are an older, evolved stage.
- Most major Tsunamis are produced by earthquakes with hypocenters less than: (A) 30 km — shallow-focus quakes transmit far more energy to the seabed.
- Energy source caused by uneven heating of Earth's surface: (A) Wind.
- Global winds towards the western direction: (A) Trade winds.
- In fiber structure, light guided through the core due to total internal: (A) Reflection.
2025 — Q2(b) [5] Electromagnetic Radiations — types and applications
Full Answer: Electromagnetic radiation is energy that travels through space as coupled, oscillating electric and magnetic fields, requiring no medium to travel and moving at the speed of light in a vacuum. The EM spectrum, in order of increasing frequency and energy (decreasing wavelength), consists of: radio waves, used for broadcasting and wireless communication; microwaves, used for cooking and satellite/radar communication; infrared radiation, used for night-vision/thermal imaging, remote controls, and IR therapy; visible light, which enables vision and photography; ultraviolet radiation, used for sterilization, vitamin D synthesis, and forensic analysis; X-rays, used for medical imaging and airport security screening; and gamma rays, the highest-energy form, used in cancer radiotherapy and sterilization.
Quick Notes:
- EM waves = oscillating E + B fields, no medium needed, travel at speed of light
- Order (low to high energy): Radio → Microwave → Infrared → Visible → UV → X-ray → Gamma
- Applications: Radio (broadcast), Microwave (cooking/radar), IR (night vision), UV (sterilize), X-ray (imaging/security), Gamma (radiotherapy)
2025 — Q2(c) [5] Hurricanes — formation and damage
Full Answer: Hurricanes form over warm tropical oceans, where sea surface temperature is at least 26.5°C, as a low-pressure system intensifies through the latent heat released when moist air rises and condenses; the system organizes into a large rotating storm with a calm central region called the eye, surrounded by the eyewall where the strongest winds and rainfall occur. Hurricanes cause damage through high sustained winds (classified Category 1–5 on the Saffir-Simpson scale), storm surge that floods coastal areas, and torrential rainfall causing inland flooding, together capable of billions of dollars in damage and significant loss of life.
Quick Notes:
- Forms over warm ocean water (≥26.5°C), fuelled by latent heat of condensation
- Structure: calm eye + eyewall (strongest winds/rain)
- Damage: high winds (Cat 1–5), storm surge, inland flooding
2024 — Q2(c) [5] Middle Latitude Cyclones vs Tornadoes
Full Answer: Middle-latitude (extratropical) cyclones are large-scale, cold-core low-pressure weather systems that form along fronts where warm and cold air masses collide in temperate zones; they can span hundreds of kilometers, last several days, and bring widespread rain and wind over a broad area. Tornadoes, by contrast, are small-scale but extremely intense, short-lived (usually minutes) rotating funnels of air that extend from a severe thunderstorm cloud down to the ground, causing highly localized but very severe destruction along a narrow path.
Quick Notes:
- Mid-latitude cyclone: large-scale, forms at fronts, lasts days, widespread rain/wind
- Tornado: small-scale, lasts minutes, extremely intense, narrow destructive path
2024 — Q2(d) [5] Ionic vs Covalent Bonding, with examples
Full Answer: Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom, creating oppositely charged ions that are held together by strong electrostatic attraction — for example, in sodium chloride (NaCl), sodium loses an electron to become Na⁺ and chlorine gains it to become Cl⁻. Covalent bonding occurs when atoms, typically both non-metals, share pairs of electrons so that each atom completes its outer electron shell — examples include water (H₂O), carbon dioxide (CO₂), and oxygen gas (O₂).
Quick Notes:
- Ionic = electron transfer, metal + non-metal, forms ions (e.g. NaCl)
- Covalent = electron sharing, non-metal + non-metal (e.g. H₂O, CO₂, O₂)
2024 — Q3(c) [5] Kepler's Laws of Planetary Motion
Full Answer: Kepler's three laws describe planetary motion around the Sun. The Law of Orbits states that every planet moves in an elliptical orbit with the Sun at one focus of the ellipse. The Law of Areas states that a line joining a planet to the Sun sweeps out equal areas in equal intervals of time, which means a planet moves faster when closer to the Sun (perihelion) and slower when farther away (aphelion). The Law of Periods states that the square of a planet's orbital period is directly proportional to the cube of the semi-major axis of its orbit (T² ∝ a³), meaning planets farther from the Sun take proportionally much longer to complete one orbit.
Quick Notes:
- Law 1 (Orbits): elliptical orbit, Sun at one focus
- Law 2 (Areas): equal areas in equal time → faster near Sun, slower far away
- Law 3 (Periods): T² ∝ a³
2024 — Q4(d) [5] Radioactivity — laws and two radioactive elements
Full Answer: Radioactivity follows two key laws. First, the rate of decay at any instant is directly proportional to the number of undecayed radioactive atoms present at that time, which produces an exponential decay curve described by N = N₀e⁻λt. Second, each radioactive isotope has a fixed, characteristic half-life — the time taken for half of the atoms in a sample to decay — which remains constant regardless of temperature, pressure, or the chemical form of the substance. Common examples of radioactive elements are Uranium (U-238) and Radium (Ra-226).
Quick Notes:
- Law 1: decay rate ∝ number of undecayed atoms (exponential decay)
- Law 2: fixed half-life per isotope, unaffected by external conditions
- Examples: Uranium (U-238), Radium (Ra-226)
2024 — Q5(c) [5] Types of Earthquake Waves
Full Answer: Earthquake waves are of two main categories. Body waves travel through the Earth's interior and include P-waves (Primary waves), which are the fastest, compressional waves able to travel through solids, liquids, and gases, and S-waves (Secondary waves), which are slower shear waves that can only travel through solids. Surface waves travel along the Earth's surface and include Love waves and Rayleigh waves; although they are lower-frequency and arrive later than body waves, they cause the most damage to buildings and infrastructure because of their larger amplitude and rolling/side-to-side motion.
Quick Notes:
- Body waves: P-waves (fastest, all states of matter, compressional), S-waves (slower, solids only, shear)
- Surface waves: Love waves, Rayleigh waves — arrive last but cause most damage
2024 — Q5(d) [5] Longitudinal waves, Electromagnetic waves, and Gamma radiation
Full Answer: Longitudinal waves are waves in which particle vibration occurs parallel to the direction the wave travels, producing alternating regions of compression and rarefaction — sound waves are the classic example. Electromagnetic waves, by contrast, are transverse waves consisting of oscillating electric and magnetic fields perpendicular to each other and to the direction of travel; they require no medium and all travel at the speed of light, together forming the electromagnetic spectrum. Gamma radiation is the highest-energy, shortest-wavelength type of electromagnetic wave, emitted from the nucleus of unstable radioactive atoms; it is highly penetrating and is used in medicine for cancer radiotherapy and for sterilizing medical equipment.
Quick Notes:
- Longitudinal wave: vibration parallel to travel direction (e.g. sound)
- EM wave: transverse, oscillating E+B fields, no medium needed, speed of light
- Gamma ray: highest-energy EM wave, from unstable nuclei, used in radiotherapy/sterilization
2023 — Q2(a) [5] Ceramic and Nano-ceramic Materials
Full Answer: Ceramics are inorganic, non-metallic solid materials, typically oxides, nitrides, or clays, known for being hard, brittle, and highly resistant to heat and chemical attack. Nano-ceramics are ceramics engineered with grain sizes under 100 nanometres; this much finer grain structure limits crack propagation and improves densification during sintering, giving nano-ceramics greater strength, toughness, and even some ductility compared to conventional coarse-grained ceramics. Applications of ceramic materials include cutting tools, biomedical implants, electrical insulators, thermal barrier coatings, and everyday tiles and pottery.
Quick Notes:
- Ceramics: inorganic, non-metallic, hard/brittle/heat-resistant
- Nano-ceramics: grain size <100nm → stronger, tougher, more ductile than regular ceramics
- Uses: cutting tools, implants, insulators, coatings, tiles
2023 — Q2(b) [5] Black Hole — formation and discovery
Full Answer: A black hole forms when a massive star, several times the mass of the Sun, exhausts its nuclear fuel and its core collapses under gravity in a supernova explosion, compressing the stellar remnant beyond the density of a neutron star into a singularity surrounded by an event horizon, the boundary beyond which nothing, not even light, can escape. Black holes were inferred indirectly for decades from their gravitational effects on nearby stars and from X-ray emissions produced by matter in their accretion disks; the first direct image of a black hole was captured by the Event Horizon Telescope in 2019 (of the galaxy M87), and merging black holes were confirmed through gravitational-wave detection by LIGO in 2015.
Quick Notes:
- Formed from collapse of a massive star's core after a supernova → singularity + event horizon
- Discovery: indirect evidence (gravity, X-rays) for decades → first direct image by Event Horizon Telescope (2019) → gravitational waves confirmed by LIGO (2015)
2023 — Q2(c) [5] Applications of Electromagnetic Radiations
Full Answer: Each type of electromagnetic radiation has distinct practical applications. Ultraviolet radiation is used for sterilization and disinfection, and in forensic analysis. Infrared radiation is used for night-vision and thermal imaging, and in remote controls. Microwaves are used for cooking and for satellite/radar communication. Radio waves are used for broadcasting and wireless communication. X-rays are used for medical imaging and airport security screening.
Quick Notes:
- UV → sterilization, forensics
- Infrared → night vision/thermal imaging, remote controls
- Microwaves → cooking, satellite/radar
- Radio waves → broadcasting, wireless comms
- X-rays → medical imaging, security screening
2023 — Q2(d) [5] Wildfire — types, causes, spread, prevention
Full Answer: Wildfires occur in three main types: surface fires, which burn litter and undergrowth close to the ground; crown fires, which spread rapidly through tree canopies and are the most intense and destructive; and ground/peat fires, which smoulder slowly underground in organic soil. Causes include natural triggers like lightning strikes as well as human activity such as campfires, discarded cigarettes, and arson, with prolonged dry, hot, and drought conditions making an area far more susceptible. The spread of a wildfire is driven mainly by wind direction and speed, the amount of available fuel (dry vegetation), and terrain, since fires spread faster uphill. Prevention measures include controlled/prescribed burning to reduce fuel buildup, maintaining firebreaks, deploying early-warning detection systems, running public awareness campaigns, and practicing sustainable forest management.
Quick Notes:
- Types: surface fire, crown fire (most intense), ground/peat fire (smoulders underground)
- Causes: lightning, human activity (campfires, cigarettes, arson) + dry/hot/drought conditions
- Spread: wind, fuel load, terrain (faster uphill)
- Prevention: controlled burns, firebreaks, early warning, awareness, forest management
2023 — Q4(d) [5] Layers of the Atmosphere, Auroras, and Satellite Orbits
Full Answer: The atmosphere is divided into layers — troposphere, stratosphere, mesosphere, thermosphere, and exosphere — classified according to the temperature gradient with altitude, which itself depends on which type of solar radiation is being absorbed at each height (for example, ozone absorbing UV radiation in the stratosphere). Auroras form in the thermosphere/ionosphere, roughly 100–300 km above the surface, where charged particles from the solar wind interact with Earth's magnetosphere and collide with atmospheric gases, causing them to glow. Regarding satellites, low-Earth-orbit satellites such as the ISS and Hubble Space Telescope orbit within the thermosphere, while geostationary satellites orbit much higher, at roughly 36,000 km, in the exosphere/free space beyond the named atmospheric layers.
Quick Notes:
- Layers (by temperature gradient): Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
- Auroras form in thermosphere/ionosphere (~100–300 km) — solar wind + magnetosphere + atmospheric gases
- LEO satellites (ISS, Hubble) in thermosphere; geostationary satellites (~36,000 km) in exosphere
2022 — Q2(a) [5] Volcanoes — causes and effects of eruptions
Full Answer: Volcanic eruptions occur mainly at tectonic plate boundaries, both convergent and divergent, and at isolated hotspots, when magma, dissolved gas, and pressure build up beneath the crust and are eventually released explosively or effusively through a vent. The effects of an eruption include the death of plants and animals from lava flows and toxic volcanic gases, destruction of property and infrastructure, ash fall that damages crops and causes respiratory illness in humans, and the potential to trigger secondary hazards such as tsunamis and earthquakes, as well as short-term global climatic cooling caused by aerosols and ash blocking sunlight.
Quick Notes:
- Occurs at plate boundaries (convergent/divergent) + hotspots — magma/gas pressure release
- Effects: kills life (lava, gases), destroys property, ash → crop damage + respiratory illness, can trigger tsunami/earthquake, causes short-term cooling
2022 — Q2(b) [5] Renewable vs Non-Renewable Energy; Wind, Solar, Biofuels
Full Answer: Renewable energy sources, such as solar, wind, hydro, biomass, and geothermal energy, are naturally replenished and do not run out with use, whereas non-renewable sources, such as coal, oil, gas, and nuclear fuel, exist in finite quantities and are depleted faster than they can form. Wind energy is generated when moving air turns turbine blades, which drive a generator to produce electricity. Solar energy is generated when photovoltaic cells directly convert sunlight into electricity. Biofuels are derived from organic matter such as crops or waste, through processes like burning, fermentation, or decomposition, and are considered roughly carbon-neutral because the biomass absorbed an equivalent amount of CO₂ while growing.
Quick Notes:
- Renewable (solar, wind, hydro, biomass, geothermal) = naturally replenished
- Non-renewable (coal, oil, gas, nuclear) = finite
- Wind → turbine blades + generator; Solar → photovoltaic cells; Biofuel → organic matter, roughly carbon-neutral
2022 — Q2(c) [5] Tornado — formation and effects
Full Answer: A tornado is a violently rotating column of air that extends from the base of a severe thunderstorm cloud down to the ground. It forms when wind shear (winds changing speed or direction with height) causes a horizontal rotating column of air within the storm to be tilted into a vertical position by the thunderstorm's strong updraft, which then tightens and intensifies the rotation. The effects of a tornado include the destruction of homes and infrastructure, uprooted trees, and flying debris that can cause serious injury or death, typically concentrated along a narrow but extremely intense damage path.
Quick Notes:
- Forms from wind shear → horizontal rotation tilted vertical by thunderstorm updraft
- Effects: destroyed buildings, uprooted trees, flying debris — narrow but very intense path
2022 — Q2(d) [5] Factors Affecting Climate Variation
Full Answer: Several factors affect temperature variation in a place's climate: latitude is the most significant, since areas near the equator receive more direct sunlight and are warmer while polar regions are colder; altitude, since temperature falls with increasing height above sea level; distance from large water bodies, since water moderates temperature swings compared to inland areas; and ocean currents, since warm currents raise coastal temperature and humidity while cold currents lower them. Precipitation is affected mainly by topography, through the rain-shadow effect on the leeward (downwind) side of mountain ranges, and by prevailing wind patterns that carry moisture from oceans to land.
Quick Notes:
- Temperature factors: latitude (biggest factor), altitude, distance from water, ocean currents
- Precipitation factors: topography (rain-shadow effect), prevailing winds
2022 — Q4(d) [5] Radioactivity — Natural vs Artificial
Full Answer: Natural radioactivity is the spontaneous decay of naturally occurring unstable isotopes, such as Uranium-238, without any external trigger; it was discovered by Henri Becquerel in 1896. Artificial (induced) radioactivity is produced by bombarding a stable nucleus with particles such as neutrons or alpha particles inside a nuclear reactor or particle accelerator, converting it into a radioactive isotope; this was first achieved by Irène and Frédéric Joliot-Curie in 1934, and is used today to produce medical isotopes and in nuclear power generation.
Quick Notes:
- Natural: spontaneous decay of unstable isotopes (e.g. U-238) — discovered by Becquerel, 1896
- Artificial: induced by bombarding stable nuclei with particles — first done by Joliot-Curies, 1934
- Artificial radioactivity used for medical isotopes, nuclear power
2021 — Q2(a) [5] Star vs Planet; Magnitude; Color-Temperature Correlation
Full Answer: Stars are self-luminous bodies that generate their own light and energy through nuclear fusion in their cores, whereas planets are not self-luminous and are only visible because they reflect light from a star. Magnitude is the measure of a celestial object's brightness: apparent magnitude describes how bright an object appears from Earth, while absolute magnitude describes its true, intrinsic brightness as it would appear from a standard distance; lower or negative magnitude values indicate a brighter object. A star's color correlates directly with its surface temperature: the hottest stars (Type O, over 30,000K) appear blue, cooling progressively through white, yellow, and orange, down to the coolest stars (Type M, under 3,500K), which appear red.
Quick Notes:
- Star = self-luminous (fusion); Planet = reflects light only
- Magnitude: apparent (as seen from Earth) vs absolute (true brightness at standard distance); lower/negative = brighter
- Color-temperature: Blue (hottest, Type O) → White → Yellow → Orange → Red (coolest, Type M)
2021 — Q2(b) [5] 'Semiconductors are the Brains of Modern Electronics'
Full Answer: Semiconductors, such as silicon and germanium, have an electrical conductivity that lies between that of a conductor and an insulator, and crucially, this conductivity can be precisely controlled through a process called doping (adding small amounts of impurity atoms). This controllability is what allows semiconductors to be engineered into diodes, transistors, and integrated circuits (microchips) — the fundamental switching and logic components that carry out all computation inside modern electronic devices such as computers and smartphones. Because these components perform the actual processing and decision-making, semiconductors are described as the "brains" of modern electronics.
Quick Notes:
- Semiconductors (Si, Ge): conductivity between conductor and insulator, controllable via doping
- Enables diodes, transistors, microchips — the components that perform computation
- "Brains" analogy = they carry out the logic/switching inside every electronic device
2021 — Q2(c) [5] Most Popular Theory of the Origin of the Universe
Full Answer: The Big Bang Theory is the most widely accepted scientific explanation for the origin of the universe. It states that the universe began roughly 13.8 billion years ago as an extremely hot, dense point, and has been continuously expanding and cooling ever since. Key evidence supporting this theory includes the observed expansion of galaxies away from each other (redshift, first measured by Edwin Hubble) and the discovery of the Cosmic Microwave Background Radiation by Penzias and Wilson in 1965, which is the residual heat left over from the initial expansion.
Quick Notes:
- Big Bang Theory: universe began ~13.8 billion years ago from a hot, dense point, still expanding
- Evidence: galaxy redshift (Hubble), Cosmic Microwave Background Radiation (Penzias & Wilson, 1965)
2021 — Q2(d) [5] Advantages/Limitations of Renewable Energy; Prospects in Pakistan
Full Answer: Renewable energy offers several advantages: it is clean and low-emission, essentially inexhaustible, and reduces a country's dependence on imported fuel. However, it also has limitations: supply can be intermittent since it depends on weather conditions, the upfront capital cost of installation is high, energy storage remains a technical challenge, and suitability depends heavily on site and geography. Pakistan has strong renewable energy prospects, including significant solar potential demonstrated by projects like the Quaid-e-Azam Solar Park in Bahawalpur, a wind corridor in Sindh around Gharo and Jhimpir, substantial hydropower capacity from its rivers, and biomass potential from agricultural waste.
Quick Notes:
- Advantages: clean, inexhaustible, reduces fuel-import dependence
- Limitations: intermittent (weather-dependent), high upfront cost, storage challenges, geography-dependent
- Pakistan: solar (Quaid-e-Azam Solar Park), wind (Gharo-Jhimpir, Sindh), hydropower, biomass
2021 — Q4(a) [5] Sequence of Atmospheric Strata and What It Depends On
Full Answer: The atmosphere is arranged, from lowest to highest, as the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. This sequence and the boundaries between layers are determined chiefly by the temperature gradient with altitude, which is itself governed by which type of solar radiation is being absorbed at each height — for instance, ozone in the stratosphere absorbing incoming ultraviolet radiation — as well as by the way air density and atmospheric composition change with increasing altitude.
Quick Notes:
- Order: Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
- Depends on: temperature gradient with altitude, radiation absorption (e.g. ozone/UV), changing air density
2021 — Q4(c) [5] Asthenosphere vs Lithosphere; Components of the Lithosphere
Full Answer: The lithosphere is the rigid, outermost solid shell of the Earth, made up of the crust and the uppermost part of the mantle, and it is broken into large tectonic plates. The asthenosphere lies beneath the lithosphere, in the upper mantle; it is a ductile, partially molten layer that can flow slowly over long timescales, and it is the layer on which the rigid lithospheric plates slide, enabling plate tectonics. The components of the lithosphere are the continental crust, the oceanic crust, and the rigid uppermost portion of the mantle.
Quick Notes:
- Lithosphere: rigid outer shell (crust + upper mantle), broken into tectonic plates
- Asthenosphere: below lithosphere, ductile/partially molten, allows plates to slide (plate tectonics)
- Lithosphere components: continental crust, oceanic crust, rigid uppermost mantle
2021 — Q5(d) [5] Natural vs Artificial Satellites; Communication Satellites
Full Answer: A natural satellite is a celestial body that orbits a larger body without any human involvement, such as the Moon orbiting Earth, whereas an artificial satellite is a human-made object placed into orbit, such as the International Space Station or a communication satellite. Communication satellites typically operate in geostationary orbit and function as relays: they receive a signal transmitted from a ground station, amplify it, and retransmit it (through a device called a transponder) to another location on Earth, enabling services such as television broadcasting, telephony, internet access, and GPS-adjacent navigation.
Quick Notes:
- Natural satellite = no human involvement (e.g. Moon); Artificial satellite = human-made (e.g. ISS)
- Communication satellites: geostationary orbit, act as relay via transponder (receive → amplify → retransmit)
- Enables: TV broadcast, telephony, internet, GPS-adjacent services
2020 — Q2(a) [5] Avalanche — types and the most dangerous kind
Full Answer: An avalanche is a rapid flow of snow down a slope. The main types are the loose-snow avalanche (starts from a single point and fans out), the wet-snow avalanche (heavy, water-saturated snow that moves slowly but with immense destructive force), the powder-snow avalanche (fast-moving cloud of fine, dry snow), and the slab avalanche, which is the most dangerous type, occurring when a large, cohesive slab of snow breaks away from the slope all at once and slides at very high speed. Slab avalanches are responsible for the great majority of avalanche fatalities because of their size, speed, and the difficulty of predicting exactly when a slab will release.
Quick Notes:
- Types: loose-snow, wet-snow, powder-snow, slab avalanche
- Slab avalanche = most dangerous — large cohesive mass releases suddenly, high speed, most fatalities
2020 — Q2(b) [5] Global Wind and Pressure Patterns; Features at Higher Altitude
Full Answer: Uneven heating of the Earth's surface by the sun creates a belted pattern of pressure zones around the globe — the Equatorial Low, Subtropical High, Subpolar Low, and Polar High — and air flowing between these pressure belts, deflected by the Earth's rotation (Coriolis effect), forms the global wind belts: the trade winds, the westerlies, and the polar easterlies. At higher altitude, surface friction disappears almost entirely, so winds blow faster and align more closely with the pressure-gradient (geostrophic) balance; this gives rise to fast-moving jet streams near the tropopause (about 10–12 km up), which form where there are strong temperature contrasts between adjacent air masses.
Quick Notes:
- Uneven heating → pressure belts (Equatorial Low, Subtropical High, Subpolar Low, Polar High) → wind belts (trade winds, westerlies, polar easterlies)
- Higher altitude: less friction → faster winds → jet streams near tropopause (~10–12 km), from strong temperature contrasts
2020 — Q2(c) [5] Earthquakes; Shallow-Focus vs Deep-Focus
Full Answer: An earthquake is the sudden release of energy built up from tectonic stress along a fault, which radiates outward as seismic waves. Shallow-focus earthquakes, with a depth of less than 70 km, tend to be the most destructive because their energy reaches the surface with very little attenuation — the largest earthquake ever recorded, the 1960 Valdivia earthquake in Chile (magnitude 9.5), was a shallow-focus event. Deep-focus earthquakes, occurring at depths of 300–700 km, take place in subduction zones and, despite sometimes being very large in magnitude, tend to cause less surface damage because their energy is attenuated (weakened) over the much greater distance it must travel to reach the surface.
Quick Notes:
- Shallow-focus (<70 km): most destructive — e.g. 1960 Valdivia, Chile (M9.5), largest ever recorded
- Deep-focus (300–700 km): occurs in subduction zones, less surface damage due to attenuation
2020 — Q2(d) [5] Renewable vs Non-Renewable Energy; Geothermal and Hydroelectric
Full Answer: Renewable energy sources regenerate naturally and include solar, wind, hydro, geothermal, and biomass energy, while non-renewable sources such as coal, oil, gas, and nuclear fuel are finite and do not replenish on a human timescale. Geothermal energy works by using heat from inside the Earth to convert underground water into steam, which is then used to drive turbines and generate electricity. Hydroelectricity works by using flowing or falling water to rotate turbines inside a dam, which generates electricity through connected generators.
Quick Notes:
- Renewable: solar, wind, hydro, geothermal, biomass (naturally replenished)
- Non-renewable: coal, oil, gas, nuclear (finite)
- Geothermal: underground heat → steam → turbines; Hydroelectric: flowing/falling water → turbines
2020 — Q4(a) [5] Segments of the Atmosphere and Earth's Radiation Balance
Full Answer: The atmosphere's five layers each contribute to balancing incoming solar (shortwave) radiation against outgoing terrestrial (longwave) radiation, which keeps Earth's average temperature roughly stable over time. In the stratosphere, the ozone layer absorbs a large portion of harmful incoming ultraviolet radiation before it reaches the surface. In the troposphere, greenhouse gases such as carbon dioxide, water vapour, and methane trap a portion of the outgoing longwave radiation re-emitted from Earth's surface, warming the lower atmosphere. Together, this balance between absorbed and re-emitted radiation across the atmospheric layers maintains a relatively stable global energy budget, barring disruption from human-driven increases in greenhouse gas concentration.
Quick Notes:
- 5 layers each play a role in Earth's radiation balance (incoming shortwave vs outgoing longwave)
- Stratosphere ozone → absorbs UV; Troposphere greenhouse gases (CO₂, water vapour, methane) → trap outgoing longwave radiation
- Keeps average global temperature roughly stable (until disrupted by human GHG emissions)
2020 — Q4(c) [5] Natural vs Artificial Radioactivity
Full Answer: Natural radioactivity is the spontaneous decay of naturally occurring unstable atomic nuclei, such as those of uranium or radium, occurring without any external trigger. Artificial (induced) radioactivity is created deliberately by bombarding a stable nucleus with particles such as neutrons or alpha particles inside a nuclear reactor or particle accelerator, converting it into a radioactive isotope; this was first demonstrated by Irène and Frédéric Joliot-Curie in 1934.
Quick Notes:
- Natural: spontaneous decay of unstable nuclei (e.g. U, Ra), no external trigger
- Artificial: induced by bombarding stable nuclei with particles (reactor/accelerator) — first done 1934 (Joliot-Curies)
2019 — Q4(d) [5] How Can the Sun Have Such a Strong Gravitational Field If It's Made of Gases?
Full Answer: Gravitational pull depends entirely on an object's total mass, not on its physical state, whether solid, liquid, or gas. The Sun contains 99.86% of the entire Solar System's mass, roughly 333,000 times the mass of Earth, compressed by its own self-gravity into an extremely hot, dense plasma. By Newton's law of gravitation (F = Gm₁m₂/r²), this enormous mass alone produces an equally enormous gravitational field, regardless of the fact that the matter making up the Sun is gaseous/plasma rather than solid.
Quick Notes:
- Gravity depends on mass, not state of matter (solid/liquid/gas)
- Sun = 99.86% of Solar System's mass (~333,000 × Earth's mass)
- Newton's law: F = Gm₁m₂/r² — huge mass → huge gravitational field, even as gas/plasma
2018 — Q2(a) [5] Methods to Estimate the Age of the Universe
Full Answer: Scientists use several independent methods to estimate the age of the universe. The Hubble's Law/redshift method measures the rate of cosmic expansion (the Hubble Constant) from the redshift of distant galaxies and extrapolates backward in time to a single point of origin. Analysis of the Cosmic Microwave Background Radiation involves precisely measuring its temperature and tiny fluctuations and fitting these measurements to Big Bang cosmological models. Radiometric dating of the oldest known meteorites and rocks, using radioactive decay processes such as uranium-lead dating, establishes a minimum age for the Solar System and hence the universe. Finally, dating the oldest stars found in globular star clusters provides an independent lower bound on the age of the universe, since the universe must be at least as old as its oldest stars.
Quick Notes:
- Hubble's Law/redshift: measure expansion rate → extrapolate back to origin
- Cosmic Microwave Background Radiation: temperature/fluctuation analysis fitted to Big Bang models
- Radiometric dating: uranium-lead dating of oldest meteorites/rocks
- Oldest stars in globular clusters: gives a lower age bound
2018 — Q2(b) [5] Formation of a Lunar Eclipse
Full Answer: A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, so that the Earth's shadow, consisting of a darker inner umbra and a lighter outer penumbra, falls across the surface of the Moon. This can only happen at full moon, when the Moon is on the opposite side of Earth from the Sun. However, it does not happen at every full moon, because the Moon's orbital plane is tilted about 5 degrees relative to Earth's orbital plane (the ecliptic), so the Sun, Earth, and Moon are usually not perfectly aligned; a lunar eclipse only occurs when a full moon coincides with the Moon crossing the ecliptic plane.
Quick Notes:
- Lunar eclipse: Earth between Sun and Moon → Earth's shadow (umbra + penumbra) falls on Moon
- Only occurs at full moon, but not every full moon (Moon's orbit tilted ~5° to Earth's orbital plane)
2018 — Q2(c) [5] Dark Energy and Dark Matter
Full Answer: Dark matter, making up roughly 27% of the universe's total mass-energy content, does not emit, absorb, or reflect any light or electromagnetic radiation, and so it cannot be observed directly; its existence is inferred only through its gravitational effects, such as galaxies rotating faster than their visible mass alone could explain. Dark energy, making up roughly 68% of the universe, is a mysterious form of energy that appears to act opposite to gravity, and is responsible for the observed accelerating expansion of the universe.
Quick Notes:
- Dark matter (~27%): invisible, detected only via gravitational effects (e.g. galaxy rotation speeds)
- Dark energy (~68%): causes universe's accelerating expansion, acts opposite to gravity
2018 — Q2(d) [5] Black Hole — Definition and What's Inside
Full Answer: A black hole is a region of space where gravity is so intense that nothing, not even light, can escape once it passes the boundary known as the event horizon. Black holes form from the collapse of a massive star's core after it exhausts its nuclear fuel and undergoes a supernova explosion. What lies inside a black hole is, theoretically, all of its matter crushed into a singularity, an infinitely dense point at the very centre, where the known laws of physics as currently understood break down and cannot reliably describe the true interior state.
Quick Notes:
- Black hole: gravity so strong nothing (even light) escapes past the event horizon
- Forms from collapse of a massive star's core after a supernova
- Inside: theoretically a singularity (infinitely dense point) — known physics breaks down there
2018 — Q4(d) [5] Causes of Tsunami; Tsunami vs Tidal Wave; Worst Tsunami on Record
Full Answer: Tsunamis are mainly caused by undersea (submarine) earthquakes, accounting for roughly 86% of cases, and are also caused by volcanic eruptions and submarine landslides that suddenly displace a large volume of ocean water. A tsunami is unrelated to tides; it is a seismic sea wave caused by sudden displacement of water. A tidal wave, by contrast, is the genuine, regular rise and fall of sea level caused by the gravitational pull of the Moon and Sun, so the two phenomena are physically distinct, despite "tidal wave" being commonly (and incorrectly) used to refer to tsunamis. The worst tsunami on record is generally considered to be the 2004 Indian Ocean tsunami, triggered by a magnitude 9.1 undersea earthquake off the coast of Sumatra, which killed roughly 230,000 people across multiple countries.
Quick Notes:
- Main cause: undersea earthquakes (~86%), also volcanic eruptions, submarine landslides
- Tsunami = seismic sea wave (unrelated to tides); Tidal wave = real tide from Moon/Sun gravity
- Worst on record: 2004 Indian Ocean tsunami (M9.1, off Sumatra, ~230,000 deaths)
2024 — Part-I MCQs (Physical Science items)
- Day and night equal (equinox) on: (B) 21 March and 23 September
- Alum chemical formula: (A) KAl(SO4)2·12H2O
- Circle radius increased 3×, area increases: (C) Nine times (area ∝ r², so 3²=9)
- One degree of longitude at the equator equals: (B) 69 miles
- Isotopes of an element contain equal number of: (A) Protons (isotopes differ in neutrons, same protons)
- Unit for measuring distance (astronomical): (B) light year
- Compound with most carbon atoms per mole (Chloroform/Methanol/Water/Ethanol): (D) Ethanol (C2H5OH has 2 carbon atoms per molecule, more than chloroform or methanol's 1, and water's 0)
- Pencil in water appears bent due to: (C) Refraction
- Earthquake measured by: (C) Seismograph
- Cyclone formed when: (D) warm, moist air rises upward over the ocean
- Heavy water is: (C) Denoted by D2O and contains deuterium
- Ionosphere located within thermosphere extends: (A) From 60–300 km above Earth's surface
- Red light used in traffic signal for danger because: (C) It has long wavelength and high frequency — red light's longer wavelength scatters least in the atmosphere (by Rayleigh scattering), so it remains visible over the greatest distance, making it ideal as a warning/danger signal.
Quick Notes:
- Red light for danger signals → long wavelength → scatters least → visible from farthest away
- Equinoxes: 21 March (spring) and 23 September (autumn)
- Isotopes = same protons, different neutrons
- Ethanol has the most C atoms/mole among the given options (2 per molecule)