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Physics flashcards

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Topic 1: Forces and motion

SI units of speed, acceleration, force and gravitational field strength: Speed: metre/second (m/s). Acceleration: metre/second² (m/s²). Force: newton (N). Gravitational field strength: newton/kilogram (N/kg).
Unit of the moment of a force: Newton metre (N m). (Paper 2 only)
Unit of momentum: Kilogram metre/second (kg m/s). (Paper 2 only)
Average speed: Average speed = distance moved ÷ time taken.
What does a distance–time graph show?: Gradient = speed. A horizontal line means stationary; a curve means the speed is changing.
Acceleration equation: Acceleration = change in velocity ÷ time taken: a = (v − u)/t, in m/s².
What do the gradient and the area of a velocity–time graph show?: Gradient = acceleration. Area under the graph = distance travelled. (Do not mix them up.)
A car starts from rest and accelerates at 2.0 m/s² over 25 m. Final speed?: v² = u² + 2as = 0 + 2 × 2.0 × 25 = 100, so v = 10 m/s.
Scalars and vectors: Scalars have magnitude only (distance, speed, mass). Vectors have magnitude and direction (displacement, velocity, force, momentum).
Resultant force of forces along a line: Add them, taking direction into account. 4000 N forward and 1500 N backward gives 2500 N forward.
Force, mass and acceleration; weight: F = m × a. Weight = mass × gravitational field strength, W = m × g.
Newton's first law: An object stays at rest or moves at constant velocity unless a resultant force acts on it.
Stopping distance, and what increases each part: Stopping distance = thinking distance + braking distance. Thinking: speed, tiredness, alcohol, drugs, distraction. Braking: speed, poor brakes, worn tyres, wet or icy roads.
Why does braking distance rise so sharply with speed?: Kinetic energy is proportional to v², so doubling the speed quadruples the braking distance.
Explain terminal velocity in four steps: Weight makes the object accelerate. As speed rises, air resistance increases. When air resistance equals weight, the resultant force is zero. Acceleration is zero, so it falls at constant velocity.
A skydiver at terminal velocity opens her parachute. What happens?: Air resistance suddenly increases above her weight, so the resultant force is upward and she decelerates. Air resistance falls until it equals weight again: a new, lower terminal velocity.
Hooke's law and elastic behaviour: F = kx: extension is proportional to force up to the limit of proportionality. Elastic behaviour: the object returns to its original shape when the force is removed. Beyond the elastic limit the deformation is permanent.
Momentum: Momentum = mass × velocity, p = mv. It is a vector: give opposite directions opposite signs. (Paper 2 only)
A 1200 kg car slows from 25 m/s to 15 m/s in 5.0 s. Average force?: Force = change in momentum ÷ time, F = (mv − mu)/t = 1200 × 10 ÷ 5.0 = 2400 N. (Paper 2 only)
How do crumple zones, airbags and seatbelts reduce injury?: They increase the time over which the momentum changes. Force is the rate of change of momentum, so the force on the passenger is smaller. (Paper 2 only)
Conservation of momentum: In a closed system, the total momentum before a collision or explosion equals the total momentum after. (Paper 2 only)
Newton's third law: For every force there is an equal and opposite force, acting on a different body. (Paper 2 only)
Moments: a 2.5 N weight is 40 cm from the pivot. What weight 20 cm on the other side balances it?: Moment = force × perpendicular distance from the pivot. In equilibrium, clockwise = anticlockwise: 2.5 × 40 = 100 N cm, so W = 100 ÷ 20 = 5.0 N. (Paper 2 only)

Topic 2: Electricity

Units of charge and current: Charge: coulomb (C). Current: ampere (A). 1 A = 1 C/s.
Units of voltage and resistance: Voltage: volt (V); 1 V = 1 J/C. Resistance: ohm (Ω).
Units of energy and power: Energy: joule (J). Power: watt (W); 1 W = 1 J/s. Time in seconds (s).
a.c. vs d.c.: Mains is alternating current (a.c.), reversing direction periodically. Cells and batteries supply direct current (d.c.), flowing one way only.
How do a fuse and a circuit breaker protect a circuit?: Both cut off the current if it becomes too large. A fuse melts; a circuit breaker switches off and can be reset.
Earthing vs double insulation: Earthing: a metal case is connected to earth, so a fault sends a large current to earth and the fuse blows. Double insulation: the case is non-conducting, so it cannot become live.
Why does a current in a resistor heat it, and where is this useful?: The moving electrons collide with the ions of the resistor, transferring energy so its temperature rises. Used in heaters, kettles, toasters and irons.
A kettle draws 10 A from 230 V. Power, and the best fuse from 3 A, 5 A and 13 A?: P = IV = 10 × 230 = 2300 W. The fuse must be just above the normal current: 13 A.
The same kettle (10 A, 230 V) runs for 3 minutes. Energy transferred?: E = IVt = 10 × 230 × 180 = 414 000 J (414 kJ).
Series circuit rules: The current is the same everywhere; the voltage is shared between the components. Resistances add.
Parallel circuit rules, and why house lighting is parallel: The voltage is the same across every branch; the current divides between branches. Each lamp gets full mains voltage and one failing does not switch off the others.
A 6.0 V battery drives 0.50 A through a resistor. Resistance?: V = IR, so R = 6.0 ÷ 0.50 = 12 Ω.
Current–voltage graphs: resistor, filament lamp, diode: Resistor (constant temperature): straight line through the origin. Filament lamp: curve, as resistance rises with temperature. Diode: conducts one way only; almost no current in reverse.
LDR and thermistor: LDR: resistance falls as light intensity increases. Thermistor: resistance falls as temperature increases.
Current and charge. 15 C passes a point in 5.0 s: current?: Current is the rate of flow of charge: Q = It, so I = 15 ÷ 5.0 = 3.0 A.
What is the current in a metal, and what happens at a junction?: A flow of electrons. Current is conserved at a junction: current in = current out.
Voltage in terms of energy and charge: Voltage is the energy transferred per unit charge: E = QV. 1 V = 1 J/C.
4 Ω and 8 Ω resistors in series across 6 V. Current, and voltage across the 8 Ω?: Total R = 12 Ω, so I = 6 ÷ 12 = 0.5 A. V = IR = 0.5 × 8 = 4 V (and 2 V across the 4 Ω).
Conductors and insulators: Conductors (metals) allow charge to flow; insulators (plastics) do not. (Paper 2 only)
How does rubbing two insulators charge them?: Electrons are transferred. The surface that loses electrons becomes positive; the one that gains electrons becomes negative. Charge is not created. (Paper 2 only)
Forces between charges: Like charges repel; unlike charges attract. (Paper 2 only)
Uses and dangers of electrostatic charge: Uses: photocopiers, inkjet printers. Danger: sparks while fuelling aircraft or tankers, so fuelling equipment is designed to dissipate static. (Paper 2 only)

Topic 3: Waves

Unit of frequency: Hertz (Hz): one wave (oscillation) per second.
Units of wavelength, wave speed and period: Wavelength: metre (m). Wave speed: metre/second (m/s). Period: second (s).
Unit of the angles of incidence, reflection and refraction: Degrees (°), measured from the normal.
Transverse vs longitudinal waves: Transverse: oscillations perpendicular to the direction of travel (light, water waves). Longitudinal: oscillations parallel to the direction of travel (sound).
What do waves transfer?: Energy and information, without transferring matter.
Amplitude and wavefront: Amplitude: the maximum displacement from the equilibrium position. Wavefront: a line joining points on a wave that are in phase.
Wavelength, frequency and period: Wavelength: distance between successive identical points. Frequency: number of waves per second. Period: time for one full oscillation.
A 500 Hz sound wave travels at 340 m/s. Wavelength?: v = fλ, so λ = v/f = 340 ÷ 500 = 0.68 m.
A wave has a period of 0.02 s. Frequency?: f = 1/T = 1 ÷ 0.02 = 50 Hz.
The Doppler effect: Source moving towards you: wavefronts bunch up, wavelength shortens, observed frequency rises. Moving away: wavelength lengthens, frequency falls. A siren sounds higher approaching and lower receding.
Electromagnetic spectrum in order of decreasing wavelength: Radio, microwave, infrared, visible, ultraviolet, x-ray, gamma. Frequency increases along this order.
What do all electromagnetic waves have in common in free space?: They all travel at the same speed (3.0 × 10⁸ m/s). Only frequency and wavelength differ.
Colours of visible light in order of increasing frequency: Red, orange, yellow, green, blue, indigo, violet.
Uses of radio waves, microwaves and infrared: Radio: broadcasting. Microwaves: cooking, communications. Infrared: thermal imaging, remote controls.
Uses of visible light, ultraviolet, x-rays and gamma rays: Visible: photography, optical fibres. Ultraviolet: fluorescent lamps, sterilising. X-rays: medical imaging. Gamma: sterilisation, medical treatment.
Harmful effects of excessive exposure to electromagnetic waves: Microwaves: internal heating of body tissue. Infrared: skin burns. Ultraviolet: damage to surface cells, blindness. Gamma: cancer, mutation.
Law of reflection: Angle of incidence = angle of reflection, both measured from the normal.
Light enters glass from air at an angle. What happens?: It refracts: it slows down and bends towards the normal, so the angle of refraction is smaller than the angle of incidence.
A ray in air meets glass at i = 40° and refracts to r = 25°. Refractive index?: n = sin i / sin r = sin 40° ÷ sin 25° = 0.643 ÷ 0.423 = 1.52.
Critical angle of a material with n = 1.5: sin c = 1/n = 1 ÷ 1.5 = 0.667, so c = 41.8°.
Total internal reflection: when it happens, and two uses: Light travels from a denser to a less dense medium and meets the boundary above the critical angle. Uses: optical fibres; prisms in periscopes and binoculars.
Sound waves: type, and two wave behaviours: Sound waves are longitudinal. They can be reflected and refracted.
Range of human hearing: 20 Hz to 20 000 Hz. (Paper 2 only)
What sets the pitch and the loudness of a sound?: Pitch: frequency (higher frequency, higher pitch). Loudness: amplitude (greater amplitude, louder). (Paper 2 only)
How can a sound wave be displayed and its frequency investigated?: A microphone connected to an oscilloscope displays the waveform; the frequency can be found from the trace. (Paper 2 only)

Topic 4: Energy resources and energy transfers

Unit of energy and work done: Joule (J).
Unit of power: Watt (W). 1 W = 1 J/s.
Units in kinetic energy = ½ × mass × speed²: Mass in kilograms (kg), speed in metre/second (m/s), energy in joules (J).
Energy stores: Kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic.
Energy transfer pathways: Mechanically, electrically, by heating, by radiation. "Heat" and "light" are pathways, not stores.
Conservation and dissipation of energy: Energy cannot be created or destroyed, only transferred between stores. Dissipated energy is not destroyed: it goes to the thermal store of the surroundings, spread out and no longer useful.
Efficiency. A motor gets 500 J of electrical energy and gives 350 J of kinetic energy.: Efficiency = (useful energy output ÷ total energy output) × 100% = 350 ÷ 500 × 100 = 70%.
What does a Sankey diagram show?: Energy transfers, with each arrow's width proportional to the amount of energy, so the split between useful and wasted energy is visible.
Conduction: Vibrating particles pass energy to neighbours; in metals, free electrons carry it far faster. Mainly in solids.
Convection: Heated fluid expands, becomes less dense and rises; cooler fluid sinks to replace it. Fluids only.
Radiation: Infrared electromagnetic waves; needs no medium. Can happen anywhere, including a vacuum.
Which surfaces are the best emitters, absorbers and reflectors of radiation?: Matt black: best emitter and best absorber. Shiny silver: best reflector and poorest emitter. A good absorber is also a good emitter.
How do loft insulation, cavity walls and shiny foil behind radiators reduce energy transfer?: Insulation traps air (a poor conductor) and stops convection currents. Shiny foil reflects radiation back into the room.
Work done: Work done = force × distance moved in the direction of the force. Work done = energy transferred.
Gravitational potential energy and kinetic energy equations: GPE = mass × g × height (mgh). KE = ½ × mass × speed² (½mv²).
A crane lifts 200 kg through 15 m in 10 s (g = 9.8 N/kg). Work done and power?: W = mgh = 200 × 9.8 × 15 = 29 400 J. P = W/t = 29 400 ÷ 10 = 2940 W.
Power: The rate of transfer of energy, or the rate of doing work: power = work done ÷ time taken.
An object falls with no air resistance. How are its GPE and KE linked?: By conservation of energy, the GPE lost equals the KE gained (the work done by gravity).
A car doubles its speed. What happens to its kinetic energy?: It quadruples, because KE depends on v².
Renewable and non-renewable energy resources: Renewable: solar, wind, hydroelectric, tidal, wave, geothermal, biomass. Non-renewable: coal, oil, gas, nuclear. (Paper 2 only)
Energy transfers in a fossil-fuel power station: Chemical energy in the fuel → thermal energy (burning heats water to steam) → kinetic energy of the turbine → electrical energy from the generator. (Paper 2 only)
Wind and hydroelectric generation: Moving air or falling water turns a turbine directly, which drives a generator: kinetic energy → electrical energy. (Paper 2 only)
Solar cells vs solar heating systems: Solar cells transfer light directly to electrical energy. Solar heating systems use the Sun's radiation to heat water. (Paper 2 only)
Nuclear power: one advantage and two disadvantages: High output with no CO₂ during generation (non-renewable but not a fossil fuel). It produces long-lived radioactive waste and has high decommissioning costs. (Paper 2 only)
Factors for evaluating an energy resource: Reliability, environmental impact, cost (setup and running) and power output. E.g. wind is clean but unreliable; gas is reliable but emits CO₂. (Paper 2 only)

Topic 5: Solids, liquids and gases

Units of density and of pressure: Density: kilogram/metre³ (kg/m³). Pressure: pascal (Pa); 1 Pa = 1 N/m².
Units of temperature: Degree Celsius (°C) and kelvin (K). A change of 1 K equals a change of 1 °C.
Unit of specific heat capacity: Joules per kilogram degree Celsius (J/kg °C). (Paper 2 only)
Density equation: Density = mass ÷ volume, ρ = m/V.
How do you find the density of an irregular solid?: Measure its mass on a balance. Find its volume by displacement: the rise in water level in a measuring cylinder. ρ = m/V.
Pressure equation: Pressure = force ÷ area, p = F/A.
In which direction does pressure act in a liquid or gas at rest?: Equally in all directions at a given point.
Pressure difference between the surface and 5.0 m deep in water (ρ = 1000 kg/m³, g = 9.8 N/kg): p = h × ρ × g = 5.0 × 1000 × 9.8 = 49 000 Pa.
Why is a dam built thicker at the base?: Pressure in a liquid depends only on depth and density (not the container's shape or the volume), so it is greatest at the bottom.
Particle arrangement and motion in solids, liquids and gases: Solid: regular, close-packed, vibrate about fixed positions. Liquid: close, irregular, slide past one another. Gas: far apart, random, fast in all directions. (Paper 2 only)
What can heating a substance do to its stored energy?: Raise its temperature, or change its state. (Paper 2 only)
Why does the temperature stay constant during a change of state?: The energy supplied breaks the forces between particles instead of increasing their kinetic energy. On a temperature–time graph this is a flat section. (Paper 2 only)
Specific heat capacity: The energy needed to raise the temperature of 1 kg of a substance by 1 °C. ΔQ = m × c × ΔT. (Paper 2 only)
Energy to heat 2.0 kg of water (c = 4200 J/kg °C) from 20 °C to 100 °C?: ΔQ = mcΔT = 2.0 × 4200 × 80 = 672 000 J. (Paper 2 only)
Do particles expand when a solid is heated?: No. They vibrate more vigorously and take up more space; the particles themselves do not expand. (Paper 2 only)
What causes gas pressure?: Molecules in random motion collide with the container walls and exert a force on them.
Absolute zero: 0 K (−273 °C): the temperature at which molecules have minimum kinetic energy (not zero energy).
Kelvin temperature and molecular energy: T (K) = θ (°C) + 273. The kelvin temperature is directly proportional to the average kinetic energy of the molecules: doubling T doubles the average kinetic energy.
Gas at 2.0 × 10⁵ Pa and 0.30 m³ is compressed to 0.10 m³ at constant temperature. New pressure?: p₁V₁ = p₂V₂ (fixed mass, constant temperature): p₂ = 2.0 × 10⁵ × 0.30 ÷ 0.10 = 6.0 × 10⁵ Pa.
A fixed volume of gas at 1.0 × 10⁵ Pa is heated from 27 °C to 127 °C. New pressure?: p₁/T₁ = p₂/T₂ with T in kelvin: 300 K and 400 K. p₂ = 1.0 × 10⁵ × 400 ÷ 300 = 1.3 × 10⁵ Pa.
Particle explanation: reducing volume vs raising temperature: Smaller volume (constant T): molecules hit the walls more frequently, so pressure rises. Higher temperature (constant V): molecules move faster, hitting the walls more frequently and harder.

Topic 6: Magnetism and electromagnetism

Units of current, voltage and power used in electromagnetism: Current: ampere (A). Voltage: volt (V). Power: watt (W).
Express 1 watt in terms of amperes and volts: 1 W = 1 A × 1 V, since power = current × voltage.
Units in the ideal transformer relation Vp × Ip = Vs × Is: Voltages in volts (V), currents in amperes (A); each side is a power in watts (W).
What do magnets attract and repel?: Magnets repel and attract other magnets (like poles repel, unlike attract), and attract magnetic substances.
Magnetically hard vs soft materials: Hard (steel): keeps its magnetism; used for permanent magnets. Soft (iron): loses it easily; used for electromagnet cores.
Magnetic field lines: Run from north to south outside the magnet, never cross, and are closer together where the field is stronger.
Induced magnetism: Magnetism produced in a magnetic material placed in a magnetic field. It is temporary: it appears only in the field.
How can a uniform magnetic field be produced?: With two permanent magnets, opposite poles facing each other: the field between them is uniform.
Magnetic field of a current-carrying wire: Circular field lines around the wire. Right-hand grip rule: thumb along the current, fingers curl in the field direction.
Field patterns of a straight wire, a flat coil and a solenoid: Straight wire: concentric circles. Flat coil: like a short bar magnet. Solenoid: like a bar magnet, strong and nearly uniform inside. (Paper 2 only)
How is an electromagnet made, and made stronger?: A coil of wire (solenoid) carrying a current, usually around a soft iron core. Stronger with more current, more turns, or a soft iron core. (Paper 2 only)
Fleming's left-hand rule: For the force on a current-carrying wire in a magnetic field: first finger = field, second finger = current, thumb = motion (force).
When is the force on a current-carrying wire in a field zero, and how is it reversed?: Zero when the wire is parallel to the field (maximum when perpendicular). Reversing either the current or the field reverses the force; reversing both leaves it unchanged.
How can the force on a current-carrying wire be increased?: Increase the current or use a stronger magnetic field.
What does the split-ring commutator in a d.c. motor do?: It reverses the current every half turn, so the force on each side keeps acting in the same rotational sense and the coil keeps turning.
How does a loudspeaker work?: An alternating current in a coil in a magnetic field produces a changing force (motor effect), making the cone vibrate and produce sound.
Force on a charged particle moving in a magnetic field: It experiences a force, unless it moves parallel to the field. (Paper 2 only)
When is a voltage induced in a conductor or coil?: When it moves through a magnetic field (cuts field lines), or when the magnetic field through it changes.
Three ways to increase an induced voltage: Move faster, use a stronger magnet, use more turns on the coil.
How is electricity generated by rotation?: By rotating a magnet inside a coil, or rotating a coil in a magnetic field; the changing field induces an alternating voltage.
Lenz's law, and why it must be true: The induced current opposes the change producing it. If it helped the motion, energy would be created from nothing, breaking conservation of energy.
Structure of a transformer, and why it needs a.c.: Two coils (primary and secondary) on a soft iron core. a.c. makes a continuously changing field in the core, inducing a voltage in the secondary; steady d.c. induces none. (Paper 2 only)
A transformer has 100 primary turns, 500 secondary turns and 12 V input. Output voltage?: Vp/Vs = Np/Ns: 12/Vs = 100/500, so Vs = 60 V (step-up). (Paper 2 only)
Why are step-up transformers used for transmission?: For the same power, a higher voltage means a lower current. Cable losses are I²R, so halving the current quarters the loss. Step-down transformers then lower the voltage for consumers. (Paper 2 only)
Power in a 100% efficient transformer: Input power = output power: Vp × Ip = Vs × Is. (Paper 2 only)

Topic 7: Radioactivity and particles

Unit of activity: The becquerel (Bq): one decay per second.
Units for half-life: Any unit of time: seconds (s), minutes (min) or hours (h), chosen to suit the isotope.
Which unit is used for how far radiation travels through air?: Centimetres (cm). For example, alpha particles travel only a few cm in air.
Atomic number and mass number: Atomic number: number of protons. Mass number: protons + neutrons. Nuclide notation shows the mass number above and the atomic number below the symbol.
Isotopes: Atoms with the same number of protons but different numbers of neutrons: identical chemistry, different nuclear stability.
Alpha radiation: A helium nucleus, charge +2. Stopped by paper. Strongly ionising.
Beta radiation: A fast electron, charge −1. Stopped by about 3 mm of aluminium. Moderately ionising.
Gamma radiation: An electromagnetic wave, no charge. Reduced by thick lead. Weakly ionising.
Effect on mass number and atomic number of α, β, γ and neutron emission: α: mass −4, atomic −2. β: mass unchanged, atomic +1 (a neutron becomes a proton + electron). γ: no change. Neutron: mass −1, atomic unchanged.
How is ionising radiation detected?: With photographic film (it darkens) or a Geiger–Müller detector (clicks or counts).
Sources of background radiation: Rocks (notably radon gas), cosmic rays, food, and medical sources. Subtract it from measured count rates before half-life calculations.
Half-life: The time for half the undecayed nuclei in a sample to decay, or for the activity (count rate) to halve. Decay is random, so it is an average; it is not affected by temperature or pressure.
A source has activity 800 Bq and a half-life of 2 hours. Activity after 6 hours?: 6 ÷ 2 = 3 half-lives: 800 → 400 → 200 → 100 Bq.
Monitoring foil thickness: Beta. Alpha would be completely absorbed and gamma would pass almost entirely through, so neither would respond to changes in thickness; beta is partly absorbed, so the count rate varies as the foil thickness changes.
Smoke detector: Alpha. It strongly ionises the air in the detector but is safely stopped within the device itself.
Medical tracer: Gamma, with a short half-life. Gamma penetrates the body so it can be detected externally, and a short half-life means activity falls quickly, limiting the patient's overall exposure.
Contamination vs irradiation: Contamination: radioactive material gets onto or into an object or person, so the risk continues after the source is removed. Irradiation: exposure to an external source; the risk stops when the source is removed.
Dangers of ionising radiation and how to reduce exposure: Mutation and damage to cells and tissue; radioactive waste must be disposed of safely. Reduce exposure by shielding (lead, concrete), distance (tongs) and limiting time.
Fission of uranium-235: A U-235 nucleus absorbs a neutron, becomes unstable and splits into two daughter nuclei, releasing energy and two or three neutrons.
Chain reaction: Neutrons released by one fission cause further fissions, which release more neutrons, and so on.
Roles of control rods, the moderator and shielding in a reactor: Control rods absorb neutrons to control the rate of fission. The moderator slows neutrons so they can cause fission. Thick shielding absorbs radiation escaping from the core.
Nuclear fusion: Small (light) nuclei join to form a larger nucleus, with a loss of mass and a release of energy. Fusion is the energy source of stars.
Why is fusion hard to achieve on Earth?: Both nuclei are positively charged and repel. Overcoming this electrostatic repulsion needs extremely high temperature and pressure, which are hard to contain.
Fission vs fusion: Fission splits a large nucleus into smaller ones; fusion joins small nuclei into a larger one. Both release energy.

Topic 8: Astrophysics

Unit of gravitational field strength: Newton/kilogram (N/kg).
Units in orbital speed = 2π × orbital radius ÷ time period: Orbital radius in metres (m), time period in seconds (s), orbital speed in metre/second (m/s).
Units in weight = mass × gravitational field strength: Weight in newtons (N), mass in kilograms (kg), g in N/kg.
The universe, galaxies and our solar system: The universe is a large collection of billions of galaxies; a galaxy is a large collection of billions of stars. Our solar system is in the Milky Way galaxy.
Is gravitational field strength the same everywhere?: No. g varies with location, and is different on other planets and on the Moon.
What causes moons, planets, artificial satellites and comets to orbit?: The gravitational force (gravity provides the centripetal force).
How do the orbits of planets, moons and comets differ?: Planets orbit the Sun in nearly circular ellipses. Moons orbit planets. Comets orbit the Sun in highly elliptical orbits.
Where is a comet fastest, and why?: At its closest approach to the Sun. As it approaches, gravitational potential energy is converted into kinetic energy.
A satellite orbits at radius 8000 km with a period of 7200 s. Orbital speed?: v = 2πr/T = 2π × 8 000 000 ÷ 7200 ≈ 6980 m/s.
How are stars classified by colour?: Colour relates to surface temperature: hotter stars appear blue/white, cooler stars red.
Evolution of a star like the Sun: Nebula → (protostar) → main sequence star → red giant → (planetary nebula) → white dwarf.
Evolution of a star much more massive than the Sun: Main sequence → red supergiant → supernova → neutron star or black hole. Mass determines the fate.
Why is a main sequence star stable?: The inward gravitational force balances the outward pressure from the energy released by fusion of hydrogen into helium.
Absolute magnitude: How bright a star would appear from a standard distance, so stars can be compared fairly. (Apparent brightness also depends on distance.) (Paper 2 only)
Hertzsprung–Russell (HR) diagram: A plot of stars by temperature against luminosity (or absolute magnitude), showing patterns such as the main sequence band. (Paper 2 only)
Red-shift: Light from a galaxy moving away is shifted towards longer wavelengths (Doppler effect). (Paper 2 only)
Two red-shift observations, and what they show: Almost all galaxies are red-shifted (receding), and more distant galaxies show greater red-shift (receding faster). So the universe is expanding. (Paper 2 only)
Argument for the Big Bang from an expanding universe: If the universe is expanding, it was smaller, denser and hotter in the past. Space itself expands, so any galaxy sees the same pattern: Earth is not at the centre. (Paper 2 only)
Cosmic microwave background radiation (CMBR): Microwave radiation found uniformly in all directions: the cooled remnant of radiation from the hot early universe. Only the Big Bang model explains it. (Paper 2 only)
Red-shift equation. A 500 nm line is observed at 500.5 nm. Galaxy's speed?: Δλ/λ₀ = v/c, so v = c × Δλ/λ₀ = 3.0 × 10⁸ × 0.5 ÷ 500 = 3.0 × 10⁵ m/s. (Paper 2 only)

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Written for Pearson Edexcel International GCSE in Physics (4PH1), syllabus 4PH1 (Issue 4).

Cards by Marlbridge Academic Team; 9 restate definitions from the 4PH1 revision notes. This deck was reviewed by Hassan on 2026-11-05. Report an error.

All 4PH1 notes, practice and topics