School, undergraduate & advanced physics
437 entries · 1–20 in Dictionary. Search: all. Topic: all. Level: all. Letter: all. Titles first, then explanatory matches.
The apparent magnitude an object would have at a standard distance of 10 parsecs.
It compares intrinsic optical brightness without the direct effect of differing distances.
Also known as: magnitude M
An uncertainty stated in the same unit as the measured quantity, usually written as value ± uncertainty.
For sums and differences, absolute uncertainties are commonly added for a worst-case estimate.
The lower limit of thermodynamic temperature, defined as 0 K.
It does not imply that every quantum system has zero energy because zero-point energy can remain.
Also known as: 0 K
The rate of change of velocity: a = dv/dt.
An object can accelerate at constant speed when its direction changes, as in circular motion.
A dopant that accepts a valence electron, leaving a mobile hole and tending to create p-type material.
Ionised acceptors are fixed negative ions; the holes carry current through the valence band.
Also known as: acceptor dopant
Adjustment of the eye lens's shape and power to focus objects at different distances on the retina.
For a near object the ciliary muscles contract and the lens becomes more strongly curved.
Also known as: visual accommodation
The closeness of a measured result to the accepted or true value.
A tightly clustered set can be precise but inaccurate if every reading has the same bias.
For a medium, Z = ρc relates sound pressure to particle motion and helps determine reflection at boundaries.
A larger difference in impedance produces a stronger echo, not necessarily stronger transmission.
Also known as: sound impedance
The time integral of the Lagrangian, S = ∫L dt, evaluated along a possible path of the system.
The physical path makes the action stationary, which need not mean it is an absolute minimum.
Also known as: action functional
The mean decay rate of a radioactive sample, A = −dN/dt = λN, measured in becquerels.
Activity is not the same as the number of undecayed nuclei or the radiation dose received.
Also known as: decay rate
A process in which no heat is transferred between the system and surroundings.
Adiabatic does not mean constant temperature; rapid gas expansion usually cools the gas.
Transverse MHD disturbance propagated along a magnetic field by magnetic tension and plasma inertia.
The ideal shear mode travels at the Alfvén speed in a simple uniform plasma. Compressibility, geometry and kinetic effects add other modes and corrections.
Also known as: magnetic-tension wave
Misidentification of high-frequency signal content as lower-frequency content because sampling is too slow.
Increasing display smoothness after sampling cannot recover information already aliased.
Also known as: frequency folding
Emission of a helium-4 nucleus, reducing nucleon number by 4 and proton number by 2.
Alpha radiation is strongly ionising but has a short range in matter.
Also known as: α decay
Current that changes direction periodically, commonly with a sinusoidal waveform.
The quoted mains voltage is an rms value, not its peak voltage.
Also known as: AC · a.c.
For steady currents, the circulation of magnetic field around a closed path equals μ₀ times the enclosed current.
The Ampère–Maxwell form adds displacement current for time-varying electric fields.
Also known as: Ampere circuital law
The maximum displacement of an oscillating quantity from its equilibrium value.
Wave intensity is commonly proportional to amplitude squared, so doubling amplitude can quadruple intensity.
Also known as: wave amplitude · A
A device that samples an analogue input and represents each sample by a finite digital code.
Resolution is limited by bit depth, while accuracy can also be limited by reference error, noise and nonlinearity.
Also known as: ADC · A/D converter
The phase change per unit time of a sinusoidal oscillation, ω = 2πf.
Its unit is rad s⁻¹; it is not the same numerical value as frequency in hertz.
Also known as: ω · angular speed of oscillation
The time integral of torque, equal to the change in angular momentum.
A longer application time can produce the same angular-momentum change with a smaller average torque.
Also known as: torque impulse
A growing reference from school fundamentals through undergraduate and selected advanced physics. No finite collection contains every specialised equation or theory; use topic and level filters, and always check assumptions.
Definitions use original wording, and each reference links to source material. Study labels are guides, not exam-board checklists. Worked check questions are educational examples, not an exam question bank.
Equations use SI quantities unless stated otherwise. Numerical values retain the units, precision and conditions of their source. Diagrams are explanatory models; their captions identify simplifications.