IGCSE Physics Notes

Examples

• Offset start: Start 1.0 cm, end 3.7 cm → length = 2.7 cm.
• Rolling cylinder (2 turns): Start 2 cm, end 28 cm → total = 26 cm for 2 turns → circumference = 13 cm.

Step-by-step

• 15 N East and 10 N North → perpendicular vectors.
• R = √(15² + 10²) = √325 ≈ 18 N, direction: North-East.

Example

Car from rest to 20 m/s in 10 s: a = (20−0)/10 = 2 m/s²; distance = area = ½×10×20 = 100 m.

Worked Example

Throw up: u=14, a=−9.8. Top time t=14/9.8≈1.43 s; height ≈10 m.

• Q: Why do racing cars have very low chassis?
  A: Lower CG increases stability when cornering.
• Q: A cabinet is more stable with a wider base. Why?
  A: The base of support is larger, so the CG line stays within the base for larger tilts.

Idea

A bouncing ball undergoes a larger change in momentum than one that is simply caught, so the impulse (and peak force) is larger.

1) Gun Recoil

A bullet of mass 0.03 kg leaves a gun at 1000 m/s. The gun’s mass is 1.5 kg. Find the recoil speed of the gun (take forward as +).

1. Total momentum before firing = 0.
2. After: p_\text{bullet} + p_\text{gun} = 0 ⇒ (0.03)(+1000) + (1.5)v_g = 0.
3. v_g = - (0.03×1000)/1.5 = -20 m/s (20 m/s backward).

2) Two Carts (Inelastic)

Cart A (0.80 kg) at +3.0 m/s sticks to Cart B (0.40 kg) at −1.0 m/s. Find their common speed immediately after collision.

1. p_\text{before} = 0.80×(+3.0) + 0.40×(−1.0) = 2.4 − 0.4 = 2.0 kg·m/s.
2. Total mass = 1.20 kg ⇒ v = 2.0/1.20 ≈ 1.67 m/s (forward).

1) Work Done by a Force

A box is pushed with a constant force of 50 N over 6.0 m along a level floor.

Work: W = F × d = 50 × 6.0 = 300 J.

2) Power from Energy per Time

An electric motor lifts 800 J of energy every 4 s.

Power: P = E ÷ t = 800 ÷ 4 = 200 W.

3) Gravitational Potential Energy

A 2.5 kg book is raised by 1.2 m.

ΔE_p: = m × g × Δh = 2.5 × 9.8 × 1.2 ≈ 29 J.

4) Kinetic Energy

A 1,000 kg car travels at 12 m/s.

E_k: = 1/2 × 1000 × 12^2 = 72,000 J.

5) Efficiency

A device takes 2,000 J of electrical energy and delivers 1,400 J of useful output.

Efficiency: = (1400 ÷ 2000) × 100% = 70%.

A 4400 N load sits on an 0.50 m² plate. P = 4400 / 0.50 = 8800 Pa (8.8 kPa).

Water of depth 20 m: P = 1000 × 10 × 20 = 2.0 × 10⁵ Pa (200 kPa). On a 0.50 m² gate, F = P × A = 1.0 × 10⁵ N.

Process Summary

• Melting: Solid → Liquid
• Boiling / Evaporation: Liquid → Gas
• Condensation: Gas → Liquid
• Freezing: Liquid → Solid

Summary by State

• Solid: Molecules form a 3D lattice; vibrate about fixed positions. When heated, they gain kinetic energy and may melt or sublime.
• Liquid: Molecules stay in contact but move freely; forces of attraction are weaker than in solids, allowing flow but maintaining volume.
• Gas: Forces between molecules are negligible; particles move freely and collide with each other and container walls, producing gas pressure.

Temperature Scales

• Celsius scale: 0 °C = melting point of ice, 100 °C = boiling point of water.
• Fahrenheit scale: 32 °F = melting point, 212 °F = boiling point.
• Kelvin scale: 273 K = freezing point, 373 K = boiling point.

T(K) = T(°C) + 273

Absolute zero = 0 K or −273 °C → particles have minimum kinetic energy.

• Gas pressure results from collisions between gas particles and container walls.
• Each collision exerts a force on the wall.
• Increasing the number of particles or their temperature increases collision frequency → higher pressure.

Summary of Effect (Constant Volume)

• ↑ Temperature → ↑ Molecular speed → ↑ Pressure
• ↓ Temperature → ↓ Molecular speed → ↓ Pressure

Explanation

Reducing the volume of a gas pushes particles closer together, increasing the rate of collisions with the container walls. This increases pressure. Doubling the volume halves the pressure, and vice versa.

• Gas pressure is due to collisions of particles with container walls.
• Each collision exerts a force on the wall.
• More particles or higher temperature → more frequent collisions → higher pressure.
• Pressure = Force / Area.

Examples

• Metal washer fitting: Engineers heat a metal washer before fitting it on a steel rod. Heating causes expansion, allowing it to fit; on cooling, it contracts tightly.
• Overhead wires sag: On hot days, wires expand and sag; on cold days, they contract and become taut.
• Iron tyre on a wheel: Heating expands the iron ring so it can be placed on the wooden wheel. Cooling makes it contract, gripping the wheel firmly.

Measurements Required

• Mass of aluminium block (m)
• Initial temperature (θ₁) and final temperature (θ₂)
• Voltage (V), Current (I) and Time (t) for which heat is supplied

Typical Values & Observation

Water has a very high specific heat capacity:

• Takes a long time to heat up and to cool down.
• Requires large amount of energy → expensive for heating.
• Helps stabilise climate and body temperature in living organisms.

Stages of Heating a Solid

• A–B: Temperature of the solid rises as particles vibrate faster with increased kinetic energy.
• B–C: Temperature remains constant while particles overcome forces of attraction — the solid melts.
• C–D: Temperature rises again as liquid particles gain more kinetic energy.
• D–E: Temperature remains constant; energy breaks remaining forces between particles — the liquid boils.
• E–F: Gas particles move freely, far apart, with rapidly increasing kinetic energy.

Key Notes

• A pure substance shows sharp melting (B–C) and boiling (D–E) points.
• An impure substance melts at a lower and boils at a higher temperature range.

Explanation

• Only molecules with the highest kinetic energy escape from the surface.
• Energy is used to overcome intermolecular attraction, not to raise temperature.
• Hence, the liquid cools down after evaporation — evaporation causes cooling.

• Making salt in salt pans by evaporating seawater.
• Drying clothes faster on a hot or windy day.
• Cooling of tea and perspiration from skin.

• Metals contain free (mobile) electrons that move through the metal lattice.
• When heated at one end, these electrons gain kinetic energy and move faster.
• They collide with other electrons and positive ions in cooler regions, transferring energy.
• Additionally, metal ions vibrate more vigorously and pass on their vibrations to neighbouring ions.
• Thus, heat is transferred efficiently from the hot end to the cold end through both free electrons and ion vibrations.

• Non-metals do not have free electrons.
• Heat is transferred only by vibration of atoms within the lattice.
• This process is much slower and less efficient than in metals.
• Hence, non-metals are poor conductors (thermal insulators).

Gases have very few molecules, spaced far apart. Energy transfer occurs only by collisions and diffusion during random molecular motion. Hence, conduction in gases is very weak.

Key Features

• Occurs only in liquids and gases.
• Heat is transferred by circulating currents within the fluid.
• Hotter, less dense regions rise; cooler, denser regions sink, creating a convection current.

Convection Currents

Convection currents efficiently transfer thermal energy from hotter to cooler parts of a fluid. This is why stirring is not required when heating liquids; heat spreads naturally by convection.

During the day, the land heats up faster than the sea. The air above the land becomes hot and less dense, so it rises. Cooler air from above the sea moves in to take its place, forming a sea breeze — air moving from sea to land.

At night, the land cools faster than the sea. The air above the warmer sea becomes hotter and less dense, so it rises. Cooler air from the land moves out to take its place, forming a land breeze — air moving from land to sea.

Key Features

• Occurs through infra-red radiation.
• Can take place even in a vacuum.
• All objects emit and absorb infra-red radiation.
• Hotter objects emit more radiation per second.

• Part of the electromagnetic spectrum.
• Every object emits infra-red radiation — hotter objects emit more.
• Very hot objects also emit visible light along with IR radiation.
• Infra-red radiation travels at the speed of light and does not need particles.

Demonstration — Absorbers and Emitters

Two beakers are painted differently — one shiny silver, the other dull black. Both are filled with equal hot water. The black beaker cools faster, proving it is a better emitter and absorber of infra-red radiation.

• In hot countries, houses are painted white to reflect more radiation.
• White blinds or shutters reduce radiation absorption.
• Thick walls prevent excessive conduction of heat inside buildings.

• Surface temperature — higher temperature → greater emission.
• Surface area — larger area → faster radiation.
• Colour and texture — dark and dull → better emitters.

Explanation

• The base of the metal pan is heated by a flame.
• Conduction transfers heat from the flame through the metal base.
• The water inside the pan is heated by convection currents.
• The hot water rises and the cooler water sinks, ensuring even heating throughout.

Explanation

• Heaters are placed near the floor since warm air rises.
• As air near the heater warms up, it expands and becomes less dense.
• The warm air rises, and cooler, denser air moves in to take its place.
• This sets up a convection current that circulates warm air throughout the room.

Explanation

• Heat from the fire spreads in three ways:
• Radiation: Infra-red rays heat nearby objects and people directly.
• Convection: Hot air rises, carrying heat upwards and warming surroundings.
• Conduction: The metal grate and nearby surfaces conduct heat from the fire.

Explanation

• Hot water from the engine flows through thin metal tubes of the radiator.
• Conduction transfers heat from the hot water to the metal walls.
• Convection carries the heat away through the moving air around the tubes.
• Radiation helps transfer some heat directly to the surroundings.
• This prevents the engine from overheating.

• Raising temperature → increases average kinetic energy → particles move faster.
• At a flat part of a heating curve (melting/boiling), energy is used to overcome intermolecular bonds, so temperature stays constant.
• Cooling curves: the reverse — energy released as bonds form (exothermic plateaus).

• Black/dull → better absorber & emitter of IR.
• White/shiny → poorer absorber & emitter; better reflector.
• No medium needed → IR works through a vacuum (space).

Key Ideas

• Waves transmit energy — not material particles.
• Examples: water ripples, sound waves, light, and radio waves.
• Waves can be transverse or longitudinal.

• Transverse wave: particle motion ⟂ to wave direction (e.g., light, water ripples).
• Longitudinal wave: particle motion ∥ to wave direction (e.g., sound).
• Longitudinal waves show compressions (high pressure) and rarefactions (low pressure).

Shorter wavelengths → higher frequency → more energy (e.g., UV > visible > IR). Longer wavelengths (e.g., radio) spread more by diffraction.

• The image is virtual because the reflected rays only appear to meet behind the mirror — they don’t actually intersect.
• The image is the same size as the object and at the same distance behind the mirror as the object is in front.
• It is laterally inverted — the right side of the object appears on the left of its image.

• Enter denser medium → ray slows and bends towards the normal.
• Exit to rarer medium → ray speeds up and bends away from the normal.
• Angle of emergence = angle of incidence (rays are parallel before entry and after exit).

Key Ideas

• Critical angle (c) — angle of incidence for which the refracted ray emerges along the boundary.
• Total internal reflection — occurs when the ray is in the denser medium and i > c.
• At i = c, the refracted angle is 90° (along the boundary).

• Conditions for TIR in fibre: light must be in the denser core; incidence angle > critical angle; no refraction out of the core.
• Medical endoscope: bundles of fibres carry illumination in and image out for internal viewing.
• Telecommunications: light/IR pulses confined by TIR carry huge data rates with low loss.

• Focus a distant object (like the Sun) onto a paper screen.
• Move the paper until a sharp image appears.
• The distance between the lens centre and the image = focal length.

• Ray 1 — from top of object through the centre of lens (passes straight).
• Ray 2 — from top of object through the focus then travels parallel to the axis after refraction.
• Ray 3 — from top of object parallel to axis, then passes through the focus behind lens.
• Any two rays are enough to locate the image position:contentReference[oaicite:2]{index=2}.

• White light is a combination of several colours, each having different frequency and wavelength.
• When white light enters a prism, each colour is refracted by a different amount.
• Red light (longest wavelength) bends the least.
• Violet light (shortest wavelength) bends the most.
• Thus, a spectrum of colours appears on the other side of the prism.

• Each colour travels at a different speed in glass.
• Violet light slows more than red light.
• Hence, violet deviates most, and red deviates least:contentReference[oaicite:2]{index=2}.

• Explains formation of rainbows — sunlight dispersed by raindrops.
• Used in spectrometers to study the wavelengths of light from different sources.

• All electromagnetic waves travel at a speed of 3.0 × 10⁸ m/s in a vacuum.
• They can travel through vacuum — no medium required.
• They are all transverse waves.
• The energy they transfer depends on their wavelength.
• Shorter wavelength → higher frequency → higher energy.

• Analogue signal — continuous signal representing a physical quantity (e.g. sound wave).
• Digital signal — discrete pulses that represent binary data.
• Advantages of digital: higher data rates, easier error correction, and longer transmission distance:contentReference[oaicite:2]{index=2}.

• Mobile phones and Wi-Fi use microwaves — can pass through walls with small antennas.
• Bluetooth uses radio waves — passes through walls but weakens slightly.
• Optical fibres use visible or infra-red light — glass is transparent to these, allowing high-speed data transfer.

Core Properties

• Caused by vibrations of the source.
• Longitudinal — particles oscillate along direction of wave. :contentReference[oaicite:6]{index=6}
• Requires a medium; cannot travel through vacuum. :contentReference[oaicite:7]{index=7}
• Travels through solids, liquids, gases (fastest in solids, slowest in gases). :contentReference[oaicite:8]{index=8}

• Start stopwatch at the flash and stop at the bang. Distance / time = speed. :contentReference[oaicite:15]{index=15}
• Improve accuracy: increase distance, repeat trials, use precise timer. :contentReference[oaicite:16]{index=16}

• Ultrasound — medical imaging (prenatal scans), flaw detection in metals, sonar. :contentReference[oaicite:17]{index=17}
• Infrasound — not in syllabus; very low-frequency sound. :contentReference[oaicite:18]{index=18}

• An echo is a reflection of sound. Hard, smooth surfaces reflect; soft, rough surfaces absorb/scatter. :contentReference[oaicite:19]{index=19}
• Reduce unwanted echoes: soft coverings (absorb) or uneven walls (scatter). :contentReference[oaicite:20]{index=20}

Core Properties

• Produced by vibrating sources.
• Longitudinal — particles vibrate in the same direction as wave travel.
• Requires a medium; cannot travel through a vacuum.
• Travels fastest in solids and slowest in gases.

• Use the flash–bang method: start timer at the flash and stop at the bang.
• Speed = Distance / Time.
• Improve accuracy by increasing distance and repeating trials.

• An echo is a reflection of sound from hard surfaces.
• Soft or uneven surfaces absorb or scatter sound to reduce echoes.
• distance = (speed × time delay) / 2

• Direction of vibration is perpendicular to the direction of propagation.
• Electromagnetic radiation: oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel (e.g., visible, radio, microwaves, X-rays, gamma).
• Surface water waves: particles move in circular orbits; produces the up-and-down motion seen at the surface.
• Seismic S-waves (secondary): transverse waves in Earth’s interior; do not travel through liquids.

• All EM waves travel at the same high speed in a vacuum: 3.0 × 108 m/s.
• In air, they travel at approximately the same speed as in vacuum.

• Communication is by microwaves and radio waves.
• Some satellite phones use low-orbit satellites.
• Some satellite phones and direct broadcast TV use geostationary satellites.

• Magnetic substances — examples: iron, steel, nickel, cobalt.
• Ferromagnetism — in iron, steel, nickel, cobalt.
• Magnetic field — direction is the force on a N pole at that point.

• Attract other magnets and unmagnetised magnetic substances.
• Attract iron, steel, nickel, cobalt.
• Ends are poles (north & south) of equal strength.
• A freely suspended magnet points north–south.
• Like poles repel; unlike poles attract.

• Arrows show direction N → S outside the magnet.
• Field is strongest where lines are most concentrated (at poles).
• The direction at a point is the force on a north pole there.

• Two kinds of charge: positive and negative — like charges repel, unlike attract.
• Charge is measured in coulombs (C).
• Electric field direction at a point is the direction of force on a positive test charge. :contentReference[oaicite:0]{index=0}

• Single positive charge: field lines are radially outward.
• Single negative charge: field lines are radially inward.
• Unlike charges: lines go from + to –; like charges: lines spread apart with a region of weak field between them.
• Parallel plates: uniform field from the positive plate to negative plate.

• Rubbing insulators transfers electrons → objects become charged (electrons move; protons don’t).
• Use a gold-leaf electroscope to detect charge (by contact or induction). :contentReference[oaicite:1]{index=1}

• e.m.f. (E): work done per unit charge supplied by a source, E = W / Q.
• p.d. (V): work done per unit charge across a component, V = W / Q.
• Measured in volts (V) using a voltmeter in parallel. :contentReference[oaicite:3]{index=3}

• Ohmic resistor: straight line through origin (constant R).
• Filament lamp: curve flattening (hotter → R increases → non-ohmic).
• Diode: conducts in forward bias; almost no current in reverse. :contentReference[oaicite:5]{index=5}

• Diode — used for rectification.
• Centre-zero galvanometer — detects tiny currents and polarity.

• Current: same at every point → I = I₁ = I₂ = I₃.
• Potential difference: shares add → V = V₁ + V₂ + ….
• Combined resistance: adds → R = R₁ + R₂ + ….
• Sources in series: e.m.f.s add algebraically (mind polarity) — e.g. three 12 V cells → 36 V.

• Voltage: same across each branch = supply e.m.f.
• Current: splits & recombines (junction rule). Example set: I₁ = I₂ + I₃, I₆ = I₄ + I₅ → I₂ = I₄, I₃ = I₅.
• Resistance: 1/R = 1/R₁ + 1/R₂ (+ …).
• Advantages (lamps): same brightness; one failure doesn’t black out others.

• Thermistor: temperature ↑ → resistance ↓ → its share of p.d. ↓ → p.d. across the fixed series resistor ↑ (output ↑).
• LDR: light level ↑ → resistance ↓ → its share of p.d. ↓ → p.d. across series resistor ↑ (output ↑).

• Action: conducts in forward direction (very low R), blocks in reverse (very high R).
• Rectifiers: half-wave (single diode) and bridge (full-wave) to convert a.c. → d.c.

• High voltage contact can be fatal; small currents through the body can kill.
• Low-voltage circuits can still overheat and cause fires if a fault allows excessive current.
• Short circuit = unintended low-resistance path → very large current → rapid overheating and potential fire.

• Damaged insulation on cables exposes live conductors.
• Overloading sockets or extension leads increases current → overheating.
• Damp/wet conditions provide unwanted conducting paths and increase shock risk.
• Overheated cables can melt insulation, exposing bare wires and risking short circuits.

• Operate exposed switches using an insulated pull cord/tool where damp or heat is present.
• Use switches with an insulating cover and, where possible, locate them outside the damp/hot area.

• Automatically cut off the supply when current exceeds a preset limit. Reset after the fault is cleared.
• Provide fast protection against overheating and wiring damage; similar purpose to fuses.

• Earthing metal cases provides a low-resistance path to ground if a live conductor touches the case, reducing shock risk.
• Three-pin plug: live, neutral and earth. The earth pin is longer so the appliance is earthed first.
• Double-insulated appliances have non-conducting outer casings and do not require an earth; a correctly rated fuse still protects the cable/appliance.
• Switch in live: ensure switches break the live conductor so the appliance is truly isolated when off.

• The conductor must move across magnetic field lines or the magnetic field around it must change.
• No current or e.m.f. is induced if the conductor is stationary or moves parallel to field lines.
• Induction occurs only in conductors, not in insulators such as nylon or plastic.

• Moving wire in magnetic field: move a wire up and down between magnet poles → galvanometer shows deflection in opposite directions for opposite motions.
• Moving magnet in coil: push magnet into solenoid → current flows one way; pull it out → current reverses; stationary magnet → no current.

• Move the conductor or magnet faster.
• Use a stronger magnet or increase coil turns.
• Reverse the motion or flip the magnet’s poles to reverse current direction.

The direction of an induced current is always such that it opposes the change that caused it. For example, when a magnet approaches a coil, the coil becomes an electromagnet whose near face develops the same pole as the approaching magnet, thus repelling it.

• Faraday’s Law: The magnitude of induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
• Lenz’s Law: Direction of induced current opposes the flux change producing it.
• Fleming’s Right-Hand Rule: Predicts direction of motion, field, and induced current.

• Consists of an armature coil, slip rings, carbon brushes, and a magnet.
• When the coil rotates between magnet poles, it cuts magnetic field lines, inducing an e.m.f. according to Faraday’s Law.
• As the coil rotates, the direction of induced e.m.f. reverses every half turn, producing an alternating current.
• Slip rings maintain continuous connection between the coil and the external circuit as it spins.

• The induced e.m.f. reverses direction every half-turn → output current is alternating.
• The time for one full revolution of the coil corresponds to one complete a.c. cycle.
• Increasing speed of rotation → increases frequency and amplitude of the induced e.m.f.

• The direction of the magnetic field depends on the direction of the current.
• Reversing the current reverses the direction of the field lines.
• Strength of the field decreases as distance from the wire increases.

• Field is stronger inside the solenoid and weaker outside.
• Field lines are nearly parallel and evenly spaced inside → uniform field.
• Field lines spread outward at the ends; the end where lines emerge is the north pole, and where they enter is the south pole.
• Strength of the magnetic field decreases with distance from the solenoid.

• Used in electric motors.
• Used to create electromagnets for lifting scrap metal or in relays.
• Used in cathode ray tubes and magnetic resonance (MRI) scanners.
• Used in instruments where magnetic deflection is needed (e.g., galvanometers).

• Force increases with higher current.
• Force increases with a stronger magnetic field.
• Force is greatest when the wire is perpendicular to the magnetic field lines.
• Force is zero when the wire is parallel to the magnetic field lines.

• Reverse the current direction in the wire.
• Reverse the magnetic field direction.

• The beam bends according to Fleming’s Left-Hand Rule.
• Increasing the magnetic field strength increases the amount of deflection.
• Using an electromagnet instead of a permanent magnet produces the same effect.

• Increase the number of turns in the coil.
• Increase the current through the coil.
• Use a stronger magnet to increase field strength.

• A rectangular armature coil of insulated wire is placed between two magnetic poles.
• The coil is connected to the power supply through carbon brushes and a split-ring commutator.

• Speed of rotation is controlled by changing the current magnitude.
• Direction of rotation is reversed by reversing the current direction.

• It has two coils of insulated wire wound around a soft iron core.
• The coil connected to the alternating voltage supply is the primary coil.
• The coil connected to the output is the secondary coil.
• The two coils are not electrically connected — they are linked magnetically through the iron core.

• Heating of coils due to current (minimized by using low-resistance copper wire).
• Eddy current loss in the iron core (reduced by using laminated cores).
• Magnetisation and vibration losses causing sound and heat.

• Step-up transformers are used at power stations to raise voltage for efficient long-distance transmission.
• Step-down transformers are used at distribution points and in devices (e.g., chargers, adaptors) to lower voltage for safe use.

• Reduces current in cables for the same power transfer.
• Less current → less heating → lower energy loss in cables.
• Results in higher efficiency of the power grid.

• A transformer does not change the frequency of the current.
• Both step-up and step-down transformers work only with alternating current.

• Central mass: The nucleus contains protons and neutrons.
• Protons are positively charged and neutrons are neutral.
• The orbiting electrons are negatively charged.
• Overall, the atom is electrically neutral because the number of protons equals the number of electrons.

• A very thin sheet of gold foil was placed in the path of alpha particles.
• A movable detector was used to record scattering at different angles.

• Most alpha particles passed straight through with little or no deflection.
• A small number were deflected at large angles.
• A very few were reflected directly back toward the source.

• Loss of electrons → formation of positive ions (cations).
• Gain of electrons → formation of negative ions (anions).

• The nucleus contains protons and neutrons.
• Electrons orbit the nucleus in energy levels or shells.
• The atom is electrically neutral because the number of protons equals the number of electrons.

• Strong nuclear force binds protons and neutrons in the nucleus.
• Electrostatic attraction exists between positive protons and negative electrons.

• Occurs in nuclear reactors.
• Isotopes used: Uranium-235 and Plutonium-239.
• When struck by a neutron, the nucleus splits into smaller nuclei and releases more neutrons.
• This creates a chain reaction.

• Fusion occurs naturally in the Sun and other stars.
• Deuterium and tritium nuclei combine to form helium and release energy.
• To overcome electrostatic repulsion, the nuclei must move extremely fast.
• Requires very high temperature and pressure.

• Comes from both natural and artificial sources.
• Varies from place to place depending on soil, altitude, and building materials.
• Measured before experiments to allow correction of readings from radioactive samples.

• Radon gas in the air.
• Rocks and building materials that contain radioactive isotopes.
• Food and drink containing trace radioactive substances (e.g., potassium-40).
• Cosmic rays from space and the Sun.
• Nuclear waste and fallout from past nuclear testing.

• Radioactive changes are spontaneous and random — they cannot be predicted or controlled.
• Isotopes may be radioactive due to:
  • Excess neutrons in the nucleus, or
  • A nucleus too heavy to remain stable.
• During α-decay or β-decay, the nucleus transforms into that of a different element.
• Decay increases nuclear stability by reducing excess neutrons and energy.

• The nucleus loses two protons and two neutrons (an alpha particle).
• Mass number decreases by 4; atomic number decreases by 2.
• A new element is formed that is two places lower in the Periodic Table.

• A neutron changes into a proton and emits an electron (beta particle).
• Mass number remains the same; atomic number increases by 1.
• The nucleus of the new atom has one more proton and one less neutron.

• Occurs after α or β decay, when the nucleus remains in an excited state with extra energy.
• The nucleus emits a gamma ray — a wave of high-frequency electromagnetic radiation.
• No change in atomic number or mass number.
• Gamma emission helps the nucleus lose excess energy and become more stable.

• A beta emitter is placed on one side of a sheet and a detector on the opposite side.
• If the sheet becomes thicker → fewer beta particles pass through → detected activity decreases.
• This signals the rollers to reduce pressure to maintain correct thickness.
• Beta radiation is ideal because it can penetrate paper or thin aluminium, but not thick metal.

• Gamma rays have high penetrating power and can destroy living cells.
• In radiotherapy, focused gamma beams kill cancer cells without surgery.
• In diagnosis, radioactive tracers injected into the body emit gamma rays that are detected using a gamma camera.
• Cancerous cells absorb more tracer due to their higher metabolic rate, forming a brighter image.

• It can kill or change the nature of body cells (mutation).
• Exposure may cause immediate effects such as tissue burns or sickness.
• Long-term exposure can lead to cancer and genetic damage to reproductive cells.

• Wear protective clothing such as gloves, aprons, or lead coats.
• Keep sources as far away as possible from the body — use tongs or remote tools.
• Limit exposure time to the shortest possible duration.
• Keep radioactive materials in lead-lined containers clearly labelled with the hazard symbol.

• Store all radioactive substances in thick lead containers.
• Ensure containers are clearly labelled with the radiation hazard symbol.
• Keep them in locked and restricted areas away from public access.

• Isotopes — atoms of the same element having equal numbers of protons but different numbers of neutrons.
• Nuclide notation — used to represent isotopes clearly, with the mass number (A) at the top left and atomic number (Z) at the bottom left of the chemical symbol.
• Example: ¹⁴₆C (carbon-14) → mass number 14, atomic number 6.
• Nuclear energy is the energy stored in the nucleus of an atom, released in nuclear reactions such as fission or fusion.

• Emphasis on contamination control — avoid allowing radioactive material to come into contact with skin or clothing.
• When handling sources, use tongs and wear protective gloves.
• Store all radioactive sources in lead-lined containers labelled with the hazard symbol.
• Minimise exposure using the TDS principle — Time, Distance, and Shielding.

• The axis of the Earth is tilted at an angle of about 66° to its orbital plane.
• The Earth completes one full rotation in approximately 24 hours.
• This rotation causes the cycle of day and night.

• As the Earth orbits the Sun, each hemisphere alternately tilts toward or away from the Sun.
• The hemisphere tilted toward the Sun experiences summer (direct solar energy).
• The hemisphere tilted away experiences winter (indirect solar energy).
• Countries near the equator remain hotter throughout the year.

• The Earth's axis tilt slowly shifts between 22° and 25° over about 41,000 years.
• Greater tilt → more extreme seasons (hotter summers, colder winters).
• Smaller tilt → milder seasons (warmer winters, cooler summers).

• One star — the Sun.
• Eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
• Asteroids in the asteroid belt.
• Dwarf planets such as Pluto.
• Moons orbiting the planets.
• Comets and other smaller Solar System bodies.

• The four planets nearest the Sun (Mercury, Venus, Earth, Mars) are rocky and small — the terrestrial planets.
• The four outer planets (Jupiter, Saturn, Uranus, Neptune) are large and gaseous — the gas giants.

• Gravitational field strength depends on a planet’s mass and decreases with distance from its centre.
• The Sun’s gravitational field keeps planets in orbit.
• Planets travel fastest when closest to the Sun because their kinetic energy is highest and their gravitational potential energy is lowest.
• Total energy is conserved: KE + PE = constant.

• Galaxies are made of billions of stars.
• Other stars in the Milky Way are far more distant from Earth than the Sun.
• Astronomical distances are often measured in light-years (1 ly ≈ 9.5 × 1015 m).

• When core hydrogen is mostly used up, stars leave the main sequence.
• Lower-mass stars (up to ~1.5× Sun) expand into red giants.
• More massive stars (≈1.5–>3× Sun and above) become red supergiants.
• In this phase, fusion shifts to shells around the core (hydrogen-shell fusion).

• The Sun radiates most of its energy in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum.