Key ideas — A-level physics

Key ideas

The one or two sentences worth carrying out of every topic, one lesson at a time, straight from the notes. A fast last-minute skim, or a way to check a topic has actually stuck.

Jump to: Measurements · Mechanics · Materials · Waves · Quantum phenomena · Particles · Electricity · Periodic motion · Thermal physics · Gravitational fields · Electric fields · Capacitance · Magnetic fields · Nuclear physics

Measurements

SI units and prefixes

Ten prefixes. Learn them all.
The data sheet does not print them.

Uncertainty and error

Averaging fixes random error.
It does nothing to systematic error.

Estimation and orders of magnitude

Estimate before you calculate.
The calculator's answer must land near it.

Mechanics

Scalars and vectors

Vectors add tip to tail.
Only their components add as numbers.

Moments and equilibrium

Moments balance, not forces.
Choose the pivot that kills the unknown.

Motion graphs and the SUVAT equations

Gradient and area read every graph.
SUVAT needs constant acceleration.

Projectile motion

Across: constant velocity.
Down: constant acceleration. Time joins them.

Newton's laws and the resultant force

Forces change motion, never sustain it.
Third-law pairs live on different bodies.

Mass and weight

Mass travels with you.
Weight belongs to where you are.

Drag and terminal speed

Drag grows with speed.
Terminal speed is where it catches the weight.

Momentum and impulse

No external force: momentum is conserved.
Kinetic energy is the one you must check.

Work, energy and power

Only force along the motion works.
Power is how fast the joules move.

Conservation of energy

Energy is never used up.
It is moved, and you can always audit the move.

Materials

Density and Hooke's law

Straight line: use ½FΔL.
Curved: the energy is still the area.

Stress, strain and the Young modulus

k describes the object.
E describes the material.

Waves

Progressive waves

The wave moves on.
The particles only oscillate.

Longitudinal, transverse and polarisation

Only transverse waves polarise.
Polarised light is the proof light is transverse.

Stationary waves

Nothing travels along the string.
Only certain wavelengths are allowed.

Refraction and total internal reflection

TIR needs both: into lower n,
and past the critical angle.

Diffraction and the single slit

Every gap diffracts.
A gap near λ diffracts most.

Interference and Young's double slit

Bright: paths differ by nλ.
Dark: paths differ by (n + ½)λ.

Diffraction gratings

d sin θ = nλ.
sin θ can never pass 1, and that caps n.

Quantum phenomena

The photoelectric effect

Frequency decides if electrons leave, and how fast.
Brightness only decides how many.

Collisions of electrons with atoms

Atoms accept exact gaps, or nothing.
The tube is that rule, run as a chain.

Energy levels and photon emission

Discrete lines, discrete gaps, discrete levels.
One transition makes one photon.

Wave-particle duality

Everything carries both behaviours.
Momentum decides which one you see.

Particles

Constituents of the atom

Z is the identity, A is the headcount.
Specific charge audits both at once.

Stable and unstable nuclei

The strong force holds the nucleus, out to 3 fm.
Every decay balances A and Z exactly.

Antimatter and photons

Same mass, opposite charge, every particle.
Annihilation and pair production trade at the rest-energy price.

Particle interactions and exchange particles

Forces are carried, not conjured.
Only the weak interaction changes what a particle is.

Classification of particles

Hadrons feel the strong force; leptons are fundamental.
Strangeness is born in pairs and broken only weakly.

Quarks and antiquarks

Baryons are three quarks; mesons are a pair.
The neutron's decay is one d becoming u.

Conservation laws

Audit charge, baryon and lepton numbers, and strangeness.
Energy and momentum always get a vote.

Electricity

Current, charge and the direction problem

Current: plus to minus, by convention.
Electrons: the other way, slowly.

Current-voltage characteristics

R is the ratio V over I.
Never the gradient of a curve.

Resistivity and superconductivity

R belongs to the wire.
ρ belongs to the material.

Circuits and Kirchhoff's laws

Charge is conserved at every junction.
Energy is conserved round every loop.

Potential dividers

Same current, so shares follow resistance.
Change a resistance, move the share.

EMF and internal resistance

The emf is what the cell promises.
The terminal pd is what you get, minus Ir.

Periodic motion

Circular motion

Circular motion is accelerated motion, toward the centre.
Something real must supply the force.

Simple harmonic motion

Pushed back in proportion to how far you've gone.
The size of the swing never changes the time.

SHM systems: pendulums and springs

Springs count m and k; pendulums count l and g.
The energy see-saws; the total holds still.

Forced vibrations and resonance

Every system has a frequency it wants.
Damping decides how badly it wants it.

Thermal physics

Thermal energy transfer and specific heat capacity

Internal energy: random KE plus PE, summed over particles.
State changes spend energy on PE, so T stands still.

Ideal gases and the gas laws

Kelvin in every gas law, no exceptions.
Moles ride with R; molecules ride with k.

Molecular kinetic theory

Pressure is bombardment, derived from momentum.
Temperature is average kinetic energy, in kelvin.

Gravitational fields

The field concept

A field is a region that exerts force without touch.
Gravity and electrostatics share grammar; only charge can repel.

Newton's law of gravitation

Every mass pulls every other, as the inverse square.
Field strength is force per unit mass, from the centre.

Gravitational potential

Potential is the work per kilogram to arrive from infinity, and it is always negative.
Gradient gives g from V; area gives ΔV from g.

Orbits and satellites

Gravity is the centripetal force; T² grows as r³.
Geostationary: 24 hours, equatorial, one fixed ring.

Electric fields

Coulomb's law and electric field strength

Coulomb's law is the inverse square with charge in the seats.
Uniform field: E = V/d, and right-angle entry means a parabola.

Electric potential

Potential is work per coulomb from infinity; the sign follows the charge.
Gradient gives E from V; area gives ΔV from E.

Comparing electric and gravitational fields

One inverse-square shape, two casts of characters.
Electricity wins by 10³⁶; neutrality hands the universe to gravity.

Capacitance

Capacitors and energy stored

Capacitance is charge parked per volt; the dielectric raises it.
Energy is the triangle: ½QV, growing as the voltage squared.

Charging and discharging

Discharge: Q, V and I fall along one shared curve.
Charging: Q and V climb while the current starts big and dies.

The time constant and exponential decay

RC is the clock: 37% left after one, half gone every 0.69.
Log the data and the exponential stands up straight.

Magnetic fields

Magnetic flux density and the force on a wire

The magnetic force acts on current, at right angles to everything.
One tesla: one newton per amp per metre of crossing wire.

Force on a moving charge

F = BQv steers and never works: circles at constant speed.
Radius mv/BQ: momentum written as curvature.

Magnetic flux and flux linkage

Flux is field through area; linkage multiplies by the turns.
The cosine's angle runs to the normal, never the coil face.

Electromagnetic induction: Faraday and Lenz

Emf is the rate of change of flux linkage; direction opposes.
Spin a coil and out comes BANω times a sine.

Alternating currents

Quoted ac is rms: the dc-equivalent for power, 0.707 of the peak.
On the scope, squares become volts and seconds; f comes from 1/T.

Transformers

Turns set the voltage ratio; power only ever passes through.
The grid starves I²R: high volts, small current, tiny loss.

Nuclear physics

Rutherford scattering and the nuclear atom

Most alphas missed: the atom is nearly all empty space.
The rare rebounds mark a nucleus that is tiny, massive and positive.

Radioactive decay and half-life

One nucleus is a coin toss; a mole of them is a clock.
Every half-life keeps the same fraction, so the count halves forever and never reaches zero.

Nuclear radius and density

R = R₀A^(1/3): nuclear volume simply counts nucleons.
So every nucleus, light or heavy, shares one density near 2 × 10¹⁷ kg m⁻³.

Mass-energy and binding energy

A bound nucleus weighs less than its parts: the gap is the binding energy.
Divide by A and iron-56 tops the curve near 8.8 MeV per nucleon.

Fission and fusion

Both roads lead to iron: heavy nuclei split, light nuclei fuse.
Sum the masses before and after; the missing u, times 931.5, is the MeV set free.

Nuclear reactors and safety

Moderator slows them, control rods count them, coolant carries the prize away.
Safety is half-life arithmetic: shield it, cool it, store it for long enough.

Each links to its full lesson. See also command words and the revision checklist.