Key definitions

A

Accuracy

Accuracy is how close a result sits to the true value. — Uncertainty and error

Amp

One amp is one coulomb of charge per second, which makes the coulomb an amp second, and rearranging gives ΔQ = IΔt, the form most calculations use. — Current, charge and the direction problem

Area under the force-displacement graph

The area under the force-displacement graph is the work done, for any shape of force. — Work, energy and power

Average kinetic energy per molecule

The average kinetic energy per molecule is proportional to absolute temperature, and to nothing else: not the gas, not the pressure, just T. — Molecular kinetic theory

C

Couple

A couple is a pair of equal and opposite coplanar forces whose lines of action do not coincide, the grip of two hands on a steering wheel. — Moments and equilibrium

Critical mass

The critical mass is the minimum amount of fissile material in which, on average, one neutron per fission is retained to fission again. — Fission and fusion

D

Damping

Damping is the loss of energy to resistive forces, air resistance, internal friction, and its signature is an amplitude that decays, exponentially for the common case, while the period barely changes. — SHM systems: pendulums and springs

Diffraction grating

A diffraction grating is a plate ruled with hundreds of slits per millimetre. — Diffraction gratings

Drag

Drag is the resistive force from moving through a fluid, air or water, and lift is the component of the fluid's force perpendicular to the flow, the force that holds aircraft up. — Drag and terminal speed

E

Electric current

Electric current is the rate of flow of charge: — Current, charge and the direction problem

Electronvolt

One electronvolt is the energy gained by an electron accelerated through a pd of one volt: 1 eV = 1.60 × 10⁻¹⁹ J. — Collisions of electrons with atoms

F

Force field

A force field is a region in which a body experiences a non-contact force. — The field concept

H

Hadrons

Hadrons are the particles subject to the strong interaction. — Classification of particles

Half-life

The half-life is the time for N to halve. — Radioactive decay and half-life

I

Isotopes

Isotopes are atoms with the same Z but different A: the same element, the same chemistry, a different neutron count and so a different nuclear character. — Constituents of the atom

L

Laser

A laser is the convenient source because its light is coherent and effectively monochromatic; the classic alternative is one lamp behind a single slit, split into two. — Interference and Young's double slit

Leptons

Leptons are not: the electron, the muon, and their neutrinos (electron-type and muon-type are the only flavours AQA tests), plus the antiparticles of all of them. — Classification of particles

Limit of proportionality

The limit of proportionality is where the graph stops being straight. — Density and Hooke's law

M

Mass

Mass is the amount of matter in an object: a scalar, measured in kilograms, the same wherever the object goes. — Mass and weight

O

Order of magnitude

An order of magnitude is a power of ten. — Estimation and orders of magnitude

P

Pair production

Pair production is the reverse: a single sufficiently energetic photon converts into a particle-antiparticle pair, typically while passing a nucleus, which takes the recoil that momentum conservation demands. — Antimatter and photons

Potential energy

Potential energy is maximal at the extremes and zero at the centre, growing as the square of displacement; kinetic energy is its mirror, all at the centre, none at the ends; the total stays constant. — SHM systems: pendulums and springs

Power

Power is the rate of doing work, equivalently the rate of energy transfer: — Work, energy and power

Precision

Precision is how tightly repeated readings cluster, and it says nothing about being right. — Uncertainty and error

R

Radian

The radian is the natural unit: one radian is the angle whose arc length equals the radius, so θ = s/r and a full circle is 2π radians. — Circular motion

Repeatability

Repeatability means the same experimenter with the same equipment gets consistent results; reproducibility means a different experimenter, or different equipment, still agrees, which is the stronger claim. — Uncertainty and error

Resistance

Resistance is defined as the ratio of the pd across a component to the current through it: — Current, charge and the direction problem

Resolution

Resolution is the smallest change an instrument can display, one millimetre for an ordinary ruler. — Uncertainty and error

S

Semiconductor diode

A semiconductor diode is a one-way valve. — Current-voltage characteristics

Simple harmonic motion

Simple harmonic motion is oscillation with one strict property: the acceleration is proportional to the displacement from equilibrium and directed against it, a ∝ −x. — Simple harmonic motion

Step-index optical fibre

A step-index optical fibre is a thin glass core wrapped in cladding, a layer of glass with a slightly lower refractive index. — Refraction and total internal reflection

T

Thermistor

A thermistor is a semiconductor component whose resistance falls steeply as it warms; only this negative temperature coefficient (ntc) type is considered. — Resistivity and superconductivity

U

Uncertainty

The uncertainty is the interval the true value probably sits inside, written after a ± sign. — Uncertainty and error

W

Weak interaction

The weak interaction is the only force that can change a particle's type, turning a neutron into a proton or the reverse, and its exchange particles are the charged W⁺ and W⁻ bosons. — Particle interactions and exchange particles

Weight

Weight is a force: the gravitational pull on that mass, a vector pointing toward the centre of the planet, measured in newtons. — Mass and weight

Work

In physics, work is done on an object when a force moves it: work is energy transferred by a force acting through a displacement, — Work, energy and power