⚛️ Physics

📌 0. Exam Pattern & 2025 Analysis (2 min read)

• 10 Science questions: Physics 4 + Chemistry 3 + Biology 3. Physics = 4 questions in every shift (the most within Science).
• Topic-wise count of the official 2025 paper (16–21 Jan 2026, 12 shifts, 48 Physics questions):
  • Magnetism & EMI — 11 questions (Fleming rule, F = BIL, flux, Lenz, solenoid) — the most important!
  • Electricity/Electrostatics/Semiconductor — 7 | Light (mirror/lens) — 6 | Heat & Kinetic theory — 5
  • Units — 4 | Force/Rotation (moment of inertia) — 4 | Work-Energy — 3 | Gravitation, Pressure, Sound — 2 each | Motion, Nuclear — 1 each.
• ⚠️ Many questions were of NCERT class 11-12 level (moment of inertia, kinetic theory, point dipole, band theory). Short numericals (P = 1/f, R = 2f, parallel resistance, F = BIL) also came — remember the formulas.

📌 1. Units & Measurement (7 min read)

1.1 Fundamental Quantities & the 7 SI Units

Base (fundamental) quantities do not depend on any other quantity. The SI (International System, 1960) has 7 base units:

Supplementary: plane angle — radian (rad); solid angle — steradian (sr).

1.2 Derived Units

Quantities formed by multiplying/dividing base units = derived — velocity (m/s), acceleration (m/s²), force, area, volume, density.

1.3 Must-know Conversions

1.4 Scalar & Vector

• Scalar — magnitude only: distance, speed, mass, time, work, energy, power, temperature, pressure, electric current.
• Vector — magnitude + direction: displacement, velocity, acceleration, force, weight, momentum, electric field, magnetic field.
• ⚠️ Current has a direction, yet it is a scalar (because it does not follow the laws of vector addition).

📌 2. Motion (5 min read)

2.1 Basic Concepts

• Distance — the total path covered (scalar). Displacement — the straight-line distance between the starting and final points, with direction (vector). Displacement ≤ distance; after one full round of a circle, displacement = 0.
• Speed = distance/time; Velocity = displacement/time. Acceleration = change in velocity/time (m/s²). Decreasing velocity = retardation.
• Average speed (over two equal distances at v₁ and v₂) = 2v₁v₂/(v₁ + v₂) — not the simple average!
• Uniform circular motion: speed stays the same, but direction keeps changing → velocity changes → it is accelerated motion. The direction at every point is along the tangent.

2.2 Equations of Motion — uniform acceleration

v = u + at  |  s = ut + ½at²  |  v² = u² + 2as  (u = initial velocity, v = final velocity, a = acceleration, s = displacement)

• Free fall: a = g ≈ 9.8 m/s². In a vacuum, heavy and light objects (a feather and a coin) fall together (Galileo). In air the feather falls slowly — air resistance.
• Projectile: throwing at 45° gives the maximum range.

2.3 Graphs — a question surely comes

• x-t graph parallel to the time axis → object at rest; v-t graph parallel to the time axis → uniform velocity (a = 0).

📌 3. Force, Newton's Laws & Rotation (8 min read)

3.1 Newton's 3 Laws of Motion

• First law — Law of Inertia: an object at rest stays at rest, and a moving object keeps moving in a straight line with uniform velocity, until an external unbalanced force acts on it. It gives the definition of force. Inertia depends on mass — a heavier object has more inertia.
  • Sudden brakes in a moving bus → passengers lean forward (inertia of motion).
  • A stationary bus suddenly starts → passengers fall backward (inertia of rest).
  • Beating a blanket with a stick separates the dust; shaking a tree makes fruits fall; hitting a carrom coin from below leaves the one above in place.
• Second law — F = ma: the rate of change of momentum equals the applied force: F = Δp/Δt. It gives the formula/measure of force. Unit newton.
  • A cricketer pulls his hands back while taking a catch → time (Δt) increases → force decreases → no injury. Similarly, mattresses/sand in high jump, airbags in cars, shock absorbers.
• Third law — Action-Reaction: every action has an equal and opposite reaction; both act on different objects (so they do not cancel).
  • Rocket (gas down → rocket up), recoil of a gun, the boat moves back when you jump to the shore, swimming, walking (we push the ground backward).

3.2 Momentum & Conservation of Momentum

• Momentum p = mv (kg·m/s, vector). Impulse = F × Δt = Δp.
• Conservation of linear momentum: if there is no external force, the total momentum of the system is constant. Rocket propulsion, recoil of a gun, explosions — are based on this (and on the 3rd law).

3.3 Friction & Circular Motion

• Friction: static > sliding/kinetic > rolling — the least. That is why wheels and ball-bearings are used. To reduce it: lubricant (oil/grease), ball bearings, smooth surfaces. To increase it: treads on tyres, soles of shoes. Walking is not possible without friction.
• Centripetal force — towards the centre of the circle; needed for circular motion (e.g. whirling a stone tied to a string). Centrifugal — an imaginary (pseudo) force: washing machine drier, cream separator.
• Raising the outer edge of a road at a turn = banking of roads; a cyclist leans inward at a turn.

3.4 Advanced (11): Rotation & Moment of Inertia

• Torque = force × perpendicular distance from the axis. A door handle is fixed far from the hinges → more torque with less force. A long spanner = easier.
• Moment of Inertia, I = inertia in rotation. It depends on mass and the distribution of mass about the axis — the farther the mass, the larger I.

• Rolling race on an inclined plane: the one with smaller I/MR² reaches first (mass/radius make no difference): solid sphere > solid cylinder/disc > hollow sphere > ring/hollow cylinder.
• Conservation of angular momentum: an ice skater/diver pulls in the arms → I decreases → spins faster.
• Centre of gravity lower → more stability (luggage kept low in a bus, wide base of a chair).

📌 4. Gravitation (6 min read)

• Newton's universal law of gravitation: F = G m₁m₂ / r² — the force is proportional to the product of the masses and inversely proportional to the square of the distance (inverse-square law). G = 6.67 × 10⁻¹¹ N·m²/kg² — the universal constant (same everywhere).
• Acceleration due to gravity g = GM/R² ≈ 9.8 m/s². g does not depend on the mass of the object.
  • Maximum at the poles, minimum at the equator (Earth is flattened at the poles + rotation).
  • It decreases on going up in height; it also decreases on going down in depth; at the centre of the Earth g = 0.
  • If the Earth stopped rotating, g at the equator would increase.
• Mass — amount of matter, same everywhere, kg, beam balance (tarazu). Weight = mg, it is a force, newton, spring balance. On the Moon g ≈ 1/6 of Earth's → weight 1/6, mass the same.
• Weight in a lift: accelerating upward → more; accelerating downward → less; free fall (rope breaks) → zero (weightlessness). Astronauts in a spacecraft feel weightless for this reason.
• Acceleration of the Moon: the Moon is ~60 R away from the Earth's centre → its acceleration is g/60² = g/3600 — confirmation of the inverse-square law (Newton).
• Tides — mainly due to the gravitation of the Moon (and the Sun).
• Kepler's laws: (1) planets move around the Sun in elliptical orbits, with the Sun at one focus; (2) the line joining a planet and the Sun sweeps equal areas in equal times (areal velocity constant) — a planet is faster near the Sun; (3) T² ∝ r³.
• Escape velocity: Earth — 11.2 km/s; Moon — ~2.4 km/s (that is why the Moon has no atmosphere). Orbital velocity (near the Earth) ≈ 7.9 km/s.
• Geostationary satellite: at a height of ~36,000 km (35,786 km) above the equator, time period 24 hours, rotates from west to east — appears stationary from the Earth (TV, communication, weather).
• Energy of a satellite: in a circular orbit the gravitational force is perpendicular to the velocity → work done by gravity is zero; PE + KE = constant (conservation of energy).

📌 5. Work, Energy & Power (6 min read)

5.1 Work

• W = F × s × cos θ (θ = angle between force and displacement). Unit: joule. Work is done only when a force acts and there is displacement.
• Zero work (θ = 90°): a coolie carrying a load on his head walks on a horizontal platform → work against gravity = 0; work done by gravity on a satellite in a circular orbit = 0; pushing a wall (displacement 0) → work 0.
• Negative work: friction (opposite to motion); gravity on a ball thrown upward.
• Work in lifting against gravity = mgh → depends only on mass and height; not on time or path.

5.2 Energy

• Kinetic energy KE = ½ mv² — moving vehicle, bullet, flowing water. Velocity 2 times → KE 4 times; 3 times → 9 times. KE = p²/2m (p = momentum).
• Potential energy PE = mgh — due to position or shape: water in a dam, a stretched bow, the wound key (spring) of a clock.
• Conservation of energy: energy is neither created nor destroyed — it only changes form. Ball thrown upward: going up KE → PE, coming down PE → KE; total mechanical energy is constant at every point (if there is no air resistance). A pendulum too.
• When a moving car is stopped suddenly, its KE changes into heat (brakes/tyres get hot) and sound — it does not disappear.

5.3 Power

• P = W / t (rate of doing work); unit watt (W) = J/s. 1 kW = 1000 W; 1 HP = 746 W.
• The home electricity bill is in kWh ("units") — this is a unit of energy: 1 kWh = 3.6 × 10⁶ J. For example, a 100 W bulb for 10 hours = 1 kWh = 1 unit.

📌 6. Pressure, Buoyancy & Fluids (7 min read)

6.1 Pressure

• Pressure = Force / Area (P = F/A), unit pascal (N/m²), scalar. Less area → more pressure: sharp knife, pointed nail, injection needle. More area → less pressure: wide straps of a school bag, wide tyres of a truck, wide feet of a camel, walking on sand.
• Liquid pressure = hρg — increases with depth → the wall of a dam is thicker at the bottom; divers need a special suit.
• Atmospheric pressure: 1 atm ≈ 1.013 × 10⁵ Pa ≈ 76 cm Hg. Measured with a barometer (Torricelli). It decreases with height → nosebleeds on mountains, ink leaking from a fountain pen in an aeroplane. A sudden fall of mercury in a barometer → storm.
• Pascal's law: pressure applied to an enclosed liquid is transmitted equally in all directions → hydraulic lift, hydraulic brake, hydraulic press.

6.2 Buoyancy & Archimedes' Principle

• Buoyant force — the upward force exerted by a fluid. Archimedes: this force = the weight of the fluid displaced by the object = ρ_fluid × V_submerged × g. So it depends not on the mass of the object, but on the volume and density of the displaced fluid.
• The "apparent loss of weight" of an object in a liquid = the upward buoyant force.
• Law of floatation: density of the object < that of the liquid → it floats; more → it sinks. A ship (made of iron) is hollow, so it displaces more water → it floats; an iron nail sinks.
• Ice (density ~0.92 g/cm³) floats on water, with about 9/10 of it submerged. Sea water is denser → floating in it is easier. The density of water is maximum at 4°C.
• Relative density = density of the substance / density of water (no unit) — measured with a hydrometer; for milk, a lactometer.

6.3 Bernoulli, Surface Tension, Capillarity, Viscosity

• Bernoulli's principle: where the speed of a liquid/air is higher, the pressure is lower → lift of an aeroplane, roofs blown off in a storm, being pulled when standing near a fast train, sprayer/atomizer.
• Surface tension: the surface of a liquid behaves like a stretched membrane → drops are round, insects walk on water, a needle can float on water. Soap/detergent and heat reduce surface tension (hot soapy water cleans better).
• Capillarity: rise of a liquid in a narrow tube — oil in the wick of a lantern/lamp, blotting paper, a towel soaking up water. Mercury goes down in the tube.
• Viscosity: the "thickness" of a liquid (honey > water). Heating decreases the viscosity of liquids and increases that of gases.

📌 7. Heat & Temperature (8 min read)

7.1 Temperature Scales

• Heat = a form of energy (joule/calorie) that flows from a hotter to a colder object. Temperature = the degree of hotness (kelvin). Measured with a thermometer.
• Conversion formula: C/5 = (F − 32)/9 = (K − 273)/5 (exact value: K = C + 273.15).

• Clinical thermometer: 35°C–42°C (has a kink — mercury does not fall back). Lab thermometer: −10°C–110°C. Mercury freezes at −39°C → for extreme cold, an alcohol thermometer. For very high temperatures (furnace, Sun) → pyrometer.

7.2 Specific & Latent Heat

• Q = m c ΔT. Water has the highest specific heat (~4186 J/kg·K, 1 cal/g·°C) → it holds heat for a long time: car radiator coolant, hot water bottle (fomentation), moderate climate at the seashore.
• Latent heat: heat absorbed/released during a change of state without any change in temperature. For melting ice (fusion) ≈ 3.34 × 10⁵ J/kg (80 cal/g); for boiling water (vaporisation) ≈ 22.5 × 10⁵ J/kg (~540 cal/g).
  • Latent heat of vaporisation is more than that of fusion because forming a gas requires separating the molecules completely (fully breaking the intermolecular forces).
  • A burn from steam at 100°C is more dangerous than from water at 100°C — extra latent heat.
  • Ice at 0°C cools more than water at 0°C.
• Cooling by evaporation: earthen pot (matka), sweat, cooler.

7.3 Heat Transfer

• Black/rough surfaces are good absorbers and emitters (radiator); white/shiny ones are poor. White clothes in summer, dark-coloured clothes in winter.
• Thermos flask (Dewar) — vacuum between two walls (stops conduction + convection) + shiny surface (stops radiation).
• Stefan's law (12): energy radiated by a body E ∝ T⁴ (T in kelvin). T doubled → E 16 times.
• Thermal resistance R = L / (kA) — proportional to thickness (L), inversely proportional to area and conductivity (k).
• Thermal expansion: gaps between rail tracks, electric wires sag in summer. Anomalous expansion of water — it contracts when heated from 0°C to 4°C; density is max at 4°C → a lake freezes from the top, fish stay alive below.

7.4 Advanced (11): Kinetic Theory of Gases

• Average kinetic energy of gas molecules = (3/2) kT → proportional only to absolute temperature (K). T halved → KE halved.
• Speeds: v_rms = √(3kT/m) > v_avg = √(8kT/πm) > v_p (most probable) = √(2kT/m).

📌 8. Sound (6 min read)

8.1 Nature of Sound

• Sound = a mechanical wave — it needs a medium to travel; it does not travel in a vacuum (in space, astronauts talk by radio). In air it is a longitudinal wave — particles vibrate back and forth.
• Compression = particles close together, high pressure / high density. Rarefaction = particles far apart, low pressure / low density.
• v = f × λ (speed = frequency × wavelength).

8.2 Speed of Sound

• Speed depends on the properties of the medium (elasticity and density) and temperature; not on amplitude or frequency.
• Solid > Liquid > Gas. NCERT (25°C): steel ~5960 m/s, aluminium ~6420 m/s, water (distilled) ~1498 m/s, air ~346 m/s. In air, 331 m/s at 0°C, 344 m/s at 22°C.
• Speed increases with temperature; it also increases with humidity.
• Lightning is seen first, thunder is heard later — light (3 × 10⁸ m/s) is much faster than sound.

8.3 Characteristics of Sound

8.4 Reflection, Echo etc.

• Echo: the effect of sound stays in the ear for ~0.1 s → for a clear echo, the wall must be at least 17.2 m away (344 m/s × 0.1 s ÷ 2).
• Reverberation: sound persisting due to repeated reflections — sound-absorbing materials in auditoriums (curtains, fibre board).
• Stethoscope, megaphone, sound board — based on multiple reflection of sound.
• SONAR (Sound Navigation And Ranging) — ultrasonic waves for sea depth, submarines. RADAR — radio waves (not sound).
• Doppler effect: the pitch of an approaching source rises, that of a receding one falls (train whistle; police speed gun).
• Supersonic — faster than sound; sonic boom. Mach number = speed of the object / speed of sound.
• Resonance: soldiers do not march in step on a bridge (risk of it breaking by matching the bridge's natural frequency).

📌 9. Light — Mirrors, Lenses, Eye, Phenomena (12 min read)

9.1 Basics

• Light = an electromagnetic wave — no medium is needed. Speed in vacuum c = 3 × 10⁸ m/s (the fastest). From the Sun to the Earth ~8 min 20 s.
• Light travels in a straight line → shadows, eclipses, pinhole camera (inverted image).

9.2 Reflection & Mirrors

• Laws: angle of incidence = angle of reflection; the incident ray, the reflected ray and the normal lie in the same plane.
• Plane mirror: image is virtual, erect, same size, as far behind the mirror as the object is in front, laterally inverted — that is why "AMBULANCE" is written reversed (as a mirror image) on the front of an ambulance, so that it reads correctly in the rear-view mirror of the vehicle ahead. To see the full image, the height of the mirror = half the height of the person.
• Two plane mirrors at angle θ: number of images n = 360/θ − 1 (when 360/θ is even). 90° → 3; 60° → 5; parallel → infinite (multiple reflection in a kaleidoscope).
• Spherical mirror: R = 2f. Parallel rays meet at the focus (F) after reflection from a concave mirror. Mirror formula: 1/v + 1/u = 1/f.

9.3 Refraction

• Light bends when going from one medium to another because its speed changes. Refractive index n = c / v. Rarer → denser: it bends towards the normal.
• Examples: a pencil looks broken in water, a pond looks shallower, twinkling of stars (atmospheric refraction), the Sun is seen ~2 minutes before sunrise and ~2 minutes after sunset.

• Optical density ≠ mass density: kerosene is lighter than water but optically denser (1.44 > 1.33).
• Total Internal Reflection (TIR): from denser → rarer, angle > critical angle. Sparkle of a diamond (critical angle ~24°), optical fibre, mirage.

9.4 Lenses

• Convex lens = converging: parallel rays pass through the focus. Concave lens = diverging.
• Lens formula: 1/v − 1/u = 1/f. Power P = 1/f (f in metres), unit dioptre (D). Convex +, concave −. For example +5 D → f = 1/5 = 0.2 m (20 cm); −2 D → concave, f = −50 cm.
• Convex lens: object at 2F → image at 2F (same size); between F and 2F → beyond 2F, enlarged, real and inverted; between F and the lens → virtual, erect, enlarged (magnifying glass).
• Concave lens: the image of a real object is always virtual, erect, diminished. So only a convex lens (or a concave mirror) can give an enlarged real image.

9.5 Human Eye & Defects of Vision

• Cornea (most refraction), Iris (colour of the eye; controls the size of the pupil), Pupil, Lens (convex; ciliary muscles change its focal length = power of accommodation), Retina (screen; image is real and inverted). Rods = dim light, Cones = colour. Where the optic nerve leaves = blind spot.
• Healthy eye: near point 25 cm, far point infinity. Persistence of vision ~1/16 s (cinema is based on this).

9.6 Dispersion & Scattering

• A prism splits white light into 7 colours — VIBGYOR (Hindi mnemonic: बैं नी आ ह पी ना ला). Violet bends the most, red the least. Shown by Newton.
• Rainbow: refraction + dispersion + internal reflection in water droplets; always seen in the direction opposite to the Sun.
• Scattering (Rayleigh): short wavelengths (blue) scatter more → the sky is blue. The Sun looks red at sunrise/sunset. Danger signals are red (scatters the least, visible from far). To astronauts the sky looks black. Clouds are white (large particles scatter all colours). Tyndall effect — the path of light becomes visible in a colloid.
• Raman effect (1928) — C.V. Raman, Nobel 1930; 28 Feb = National Science Day.

📌 10. Electricity (10 min read)

10.1 Charge & Current

• The unit of charge is the coulomb; the charge of one electron e = 1.6 × 10⁻¹⁹ C. Like charges repel, unlike charges attract.
• Coulomb's law: F ∝ q₁q₂/r² (inverse square).
• Electric current I = Q/t, unit ampere. The direction of conventional current is opposite to the flow of electrons. Ammeter in series, voltmeter in parallel.
• Potential difference V = W/Q, unit volt.
• Near an infinite plane sheet, the field E = σ/2ε₀ → proportional to the surface charge density σ.
• Electric dipole: p = q × 2a. "Point dipole" = q → ∞ and 2a → 0 while p stays finite.

10.2 Ohm's Law & Resistance

• V = IR (at constant temperature). V–I graph = a straight line passing through the origin. Unit ohm (Ω).
• Non-ohmic (do not obey the law): diode, transistor, electrolyte. Metal wires (copper, nichrome) are ohmic.
• R = ρL/A — proportional to length, inversely proportional to area; ρ = resistivity (depends on the material and temperature). The resistance of a metal increases with temperature.
• Stretching a wire to n times its length (same volume) → A becomes 1/n → R = n² times. Double → 4 times.
• The best conductor is silver, then copper, gold, aluminium. Electric wires are made of copper/aluminium.
• Nichrome (Ni-Cr-Fe alloy) — high resistivity, high melting point → element of heaters, irons, geysers. Bulb filament = tungsten (melting point ~3422°C); inert gas (argon/nitrogen) inside the bulb.
• Fuse: a wire of low melting point (tin-lead alloy), connected in series in the circuit; it melts at high current and breaks the circuit. Nowadays MCB.

Example: 3 Ω ‖ 6 Ω → R = (3×6)/(3+6) = 18/9 = 2 Ω.

10.3 Power & Domestic Electricity

• P = VI = I²R = V²/R. Joule's law of heating H = I²Rt → heater, iron, fuse.
• Domestic supply in India: 220 V, 50 Hz AC.
• Wire colours (NCERT): Live — red, Neutral — black, Earth — green. (In the new IEC standard: live brown, neutral blue, earth green-yellow.) Earthing — protects appliances with a metal body (fridge, iron) from shocks.
• A bird sitting on a power line gets no shock — the potential difference between its two feet is almost zero.
• DC (direction does not change — cell, battery) vs AC (direction changes again and again — homes). AC → DC = rectifier (diode); DC → AC = inverter.

10.4 Conductors, Insulators, Semiconductors (Band Theory)

• Conductor: the conduction band and valence band overlap → free electrons.
• Insulator: large energy gap (> 3 eV) — wood, rubber, plastic, glass.
• Semiconductor: small gap (silicon ~1.1 eV, germanium ~0.7 eV); conductivity increases with temperature. n-type = doping with pentavalent (P, As); p-type = trivalent (B, Al, Ga). Diodes, transistors, solar cells, LEDs, chips.

📌 11. Magnetism & Electromagnetic Induction (EMI) (10 min read) — the most in 2025 (11 Qs)!

11.1 Magnets & Field Lines

• A freely suspended magnet comes to rest in the north-south direction. Like poles repel. Near the Earth's geographic north lies its magnetic south pole.
• Field lines: N → S outside the magnet, S → N inside; always closed loops; they never intersect each other; where they are dense, the field is strong.
• The unit of magnetic field (B) is the tesla (T); of magnetic flux, the weber (Wb).
• Materials: Ferromagnetic — iron, cobalt, nickel; paramagnetic — aluminium, platinum; diamagnetic — bismuth, copper, water. Soft iron → electromagnet (becomes a magnet and loses magnetism quickly); steel → permanent magnet.

11.2 Magnetic Effect of Current

• Oersted (1820): a compass needle near a current-carrying wire gets deflected → magnetism from electricity.
• Direction: Right-hand thumb rule.
• Solenoid: the field inside is uniform; the field outside is like a bar magnet / dipole. Current reversed → field direction reversed. Soft iron core → electromagnet (crane, electric bell, MRI).

11.3 Force on a Current-carrying Conductor & Motor

• F = B I L sin θ → maximum at θ = 90° (F = BIL), zero at θ = 0° or 180°. I doubled and B tripled → F 6 times.
• Fleming's Left-hand rule (motor): Forefinger = Field, Middle finger = Current, Thumb = Motion/Force. Remember: F-B-I → thumb F(orce), fore B, middle I.
• Electric motor: electrical → mechanical energy (fan, mixer, washing machine).

11.4 Electromagnetic Induction (EMI)

• Faraday (1831): when the magnetic flux passing through a coil changes, an emf/current is induced. Flux Φ = B A cos θ, θ = angle between B and the normal to the area → flux is maximum when B is perpendicular to the surface.
• Lenz's law: the induced current opposes the cause that produced it (conservation of energy). When a magnet is brought near, the coil repels it.
• Cause of emf in a moving conductor — the Lorentz force (qv × B) on free charges.
• Fleming's Right-hand rule → generator/dynamo (mechanical → electrical).
• Transformer: mutual induction; works only on AC; V_s/V_p = N_s/N_p. Step-up (increases voltage) / step-down (decreases it). Voltage is raised to send electricity far (less loss).

📌 12. Modern Physics Basics (6 min read)

12.1 Electromagnetic Spectrum

In order of increasing wavelength (decreasing frequency/energy):

All EM waves travel at 3 × 10⁸ m/s in vacuum. Sound, alpha and beta are not EM waves.

12.2 Nuclear Physics

• E = mc² (Einstein) — conversion of mass into energy.
• Nuclear fission: a heavy nucleus (U-235) breaks up when hit by a slow neutron → 2 medium nuclei + 2–3 neutrons + ~200 MeV energy → chain reaction. Uncontrolled = atom bomb; controlled = nuclear reactor.
• In a reactor: moderator — heavy water (D₂O)/graphite, slows down neutrons; control rods — cadmium/boron, absorb neutrons; coolant.
• Nuclear fusion: light nuclei (hydrogen isotopes) join to form helium — needs extremely high temperature (~10⁷ K). Energy of the Sun/stars, hydrogen bomb.
• Photoelectric effect: emission of electrons from a metal surface by light — Einstein got the Nobel 1921 for this (not for relativity). Solar cell.

📌 13. Instruments, Inventions & Scientists (6 min read)

13.1 Measuring Instruments

13.2 Police-relevant Devices

13.3 Inventions

13.4 Indian Scientists

13.5 Units Named after Scientists

Newton (force), Joule (work/energy), Watt (power), Pascal (pressure), Ampere (current), Volt (potential), Ohm (resistance), Coulomb (charge), Hertz (frequency), Tesla (magnetic field), Weber (flux), Kelvin (temperature), Farad (capacitance).