Physics — Core Revision Notes

1. Forces & Motion

A scalar quantity has magnitude only (e.g. speed, distance, mass, temperature). A vector quantity has both magnitude and direction (e.g. velocity, displacement, acceleration, force, momentum). Speed = distance / time; Velocity = displacement / time (with direction). Acceleration = change in velocity / time = (v − u) / t. On a distance-time graph: gradient = speed. On a velocity-time graph: gradient = acceleration; area under graph = displacement. Newton's First Law: an object remains at rest or moves at constant velocity unless acted on by a resultant force. Newton's Second Law: F = ma (resultant force = mass × acceleration). Newton's Third Law: every action has an equal and opposite reaction on the other object.

SUVAT equations of motion (uniformly accelerated)

• v = u + at (final velocity = initial velocity + acceleration × time)
• s = ½(u + v)t (displacement = average velocity × time)
• s = ut + ½at² (displacement using initial velocity and acceleration)
• v² = u² + 2as (useful when time is not given)
• Where: s = displacement (m), u = initial velocity (m/s), v = final velocity (m/s), a = acceleration (m/s²), t = time (s)
• List your known variables first — then choose the equation that has only one unknown

Velocity-time graph: the gradient gives acceleration (a = Δv/Δt); the total area under the curve gives displacement. The green tangent line shows how to find instantaneous acceleration at a point.

2. Momentum & Energy

Momentum p = mv (kg m/s). The principle of conservation of momentum: in a closed system, total momentum before a collision = total momentum after. Impulse = change in momentum = F × t (Newton-seconds). In elastic collisions, kinetic energy is conserved. In inelastic collisions, kinetic energy is NOT conserved (converted to heat, sound, deformation). Work done W = F × d × cos θ (for force at angle θ to direction of motion). W = Fd when force is parallel to motion. Kinetic energy Eₖ = ½mv². Gravitational potential energy Ep = mgh. Power P = W/t = energy/time (watts). Efficiency = useful output energy ÷ total input energy (× 100%). Energy is ALWAYS conserved overall — it is converted to other forms, not destroyed.

Energy stores and transfers

• Mechanical (kinetic + potential), thermal, chemical, electrical, nuclear, light, sound
• Energy is transferred by: mechanical work, electrical work, heating, waves (radiation, conduction, convection)
• A car braking: kinetic → thermal (brakes heat up) — energy is not lost, just transferred to a less useful form
• Efficiency = (useful output / total input) × 100% — always less than 100% due to heat losses
• Sankey diagrams: width of arrows represents amount of energy — thick = large amount
• Specific heat capacity: energy needed to raise 1 kg by 1°C. Q = mcΔT

3. Waves

A wave is a periodic disturbance that transfers energy from place to place without transferring matter. Transverse waves: oscillation is perpendicular to the direction of wave travel (light, water waves, all EM waves). Longitudinal waves: oscillation is parallel to the direction of travel — compressions and rarefactions (sound, seismic P-waves). Key quantities: Amplitude (A) = maximum displacement from equilibrium (in metres). Wavelength (λ) = distance between two successive identical points (e.g. crest to crest, in metres). Frequency (f) = number of complete waves per second (hertz, Hz). Period (T) = time for one complete wave = 1/f (seconds). Wave speed v = f × λ (m/s). All electromagnetic waves travel at the speed of light (3 × 10⁸ m/s) in a vacuum.

The electromagnetic spectrum (low to high frequency)

• Radio waves: wavelength > 0.1m — broadcasting, telecommunications; pass through most materials
• Microwaves: 1mm–0.1m — cooking (absorbed by water molecules), satellite communications
• Infrared: 700nm–1mm — thermal imaging, TV remotes, heating; emitted by all warm objects
• Visible light: 400nm–700nm — the narrow band detectable by human eyes
• Ultraviolet: 10nm–400nm — causes sunburn and skin cancer; detected by fluorescent materials
• X-rays: 0.01nm–10nm — medical imaging (absorbed by bone, transmitted by soft tissue)
• Gamma rays: < 0.01nm — most energetic; cancer treatment; sterilising equipment; nuclear decay
• All EM waves travel at 3×10⁸ m/s in vacuum; all are transverse; all can travel through a vacuum

Snell's Law: when light moves from one medium to another, it refracts (bends). n₁ sin θ₁ = n₂ sin θ₂. Light bends TOWARDS the normal when entering a denser medium, and AWAY from the normal when entering a less dense medium.

4. Electricity & Circuits

Current (I) is the rate of flow of electric charge: I = Q/t (amperes). Voltage/potential difference (V) is the energy transferred per unit charge: V = W/Q (volts). Resistance (R) opposes current flow: V = IR (Ohm's Law, valid when temperature is constant). In series circuits: current is the same throughout; voltages add up; total resistance = R₁ + R₂ + R₃. In parallel circuits: voltage is the same across each branch; currents add up; 1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂ + ... — total resistance is LESS than the smallest individual resistance. Power P = IV = I²R = V²/R (watts). Electrical energy E = VIt = Pt. Charge Q = It. A fuse protects a circuit by melting when current exceeds a safe level. An MCB (miniature circuit breaker) can be reset.

Component characteristics (I-V graphs)

• Resistor (Ohmic): straight line through origin — constant resistance at constant temperature
• Filament bulb: curve flattening — resistance INCREASES as temperature increases (more vibration impedes electrons)
• Diode: near-zero current in reverse bias; sudden large current in forward bias above ~0.6V
• Thermistor: resistance DECREASES as temperature increases — used in temperature sensors
• LDR (light-dependent resistor): resistance DECREASES as light intensity increases — used in light sensors
• Voltmeter: connected in PARALLEL (high resistance — does not affect circuit)
• Ammeter: connected in SERIES (low resistance — minimal voltage drop)

5. Thermal Physics

Temperature measures how hot something is (in °C or K). Thermal energy (heat) flows from higher temperature to lower temperature. The three methods of heat transfer: Conduction — vibrating particles pass energy to neighbours; occurs mainly in solids; metals are good conductors because free electrons transfer energy rapidly. Convection — heated fluid (liquid or gas) becomes less dense and rises, cooler fluid sinks; creates convection currents; cannot occur in solids or a vacuum. Radiation (infrared) — emitted by all objects above absolute zero; no medium needed; transfers at the speed of light; dark matt surfaces emit and absorb better than shiny light surfaces. The kinetic particle model explains: solids (particles vibrate in fixed positions), liquids (particles move past each other), gases (particles move rapidly and randomly with large spaces between them).

6. Radioactivity

Radioactive decay is the spontaneous emission of radiation from an unstable nucleus. Alpha (α) particles: 2 protons + 2 neutrons (helium nucleus); charge +2; low penetrating power (stopped by paper); most ionising. Beta (β⁻) particles: a neutron → proton + electron; the electron is emitted; stopped by a few mm of aluminium; moderate ionisation. Gamma (γ) rays: high-frequency electromagnetic radiation from nucleus; no charge, no mass; highly penetrating (reduced by thick lead/concrete); least ionising. Half-life (t₁/₂) is the time for HALF the radioactive nuclei in a sample to decay — it is constant and cannot be changed by temperature, pressure, or chemical state. Uses of radiation: medical tracers (γ, short half-life), cancer treatment (γ, focused beams), sterilising equipment (γ), smoke detectors (α). Nuclear fission: heavy nucleus (U-235) splits into two smaller nuclei + neutrons + large energy release. Chain reaction occurs when neutrons released trigger further fissions.