PAU Physics (Andalusia) key terms

Force per unit mass experienced by a small test mass placed at a point in space; a vector pointing toward the source mass, measured in N/kg or equivalently m/s².

Every pair of point masses attracts each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres, directed along the joining line.

The proportionality constant in Newton's law of gravitation, approximately 6.67 × 10⁻¹¹ N·m²/kg²; it converts masses and squared distances into a force and is universal for all matter.

Work done per unit mass by the gravitational field in bringing a small test mass from infinity to a point; a scalar, always negative near a mass, measured in J/kg.

A surface on which the gravitational potential has the same value everywhere; no work is done moving along it, and field lines cross it at right angles.

Minimum launch speed needed for a projectile to move from a point to infinity with no further propulsion, equal to the square root of twice the gravitational potential magnitude at that point.

Speed a satellite needs to maintain a circular orbit of given radius; it equals the square root of the gravitational parameter divided by the orbital radius and decreases as the radius grows.

For satellites of the same central body, the square of the orbital period is proportional to the cube of the semi-major axis; the constant of proportionality depends only on the central mass.

A circular equatorial orbit in which the satellite's period equals one sidereal day, so it appears fixed above one point on the equator; its radius is roughly 42,000 km from Earth's centre.

The net inward force that keeps a body moving on a curved path; for circular motion it equals mass times speed squared divided by radius and points toward the centre.

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation, acting along the line joining them; attractive for opposite signs.

Force per unit positive charge at a point in an electric field; a vector measured in N/C or V/m, pointing away from positive charge and toward negative charge.

Imaginary curves whose tangent at any point gives the direction of the electric field; their density indicates field magnitude, and they begin on positive and end on negative charge.

Work done per unit positive charge by an external agent in bringing a test charge from infinity to a point in an electric field; a scalar measured in volts, J/C.

The difference in electric potential between two points; it equals the work done per unit charge moving between them and is measured in volts.

Energy stored in a system of charges because of their positions; for two point charges it equals the product of the charges times Coulomb's constant divided by their separation.

The constant ε₀, about 8.85 × 10⁻¹² F/m, appearing in Coulomb's law and in the relation between electric field and surface charge density in a vacuum.

The total electric flux through any closed surface equals the enclosed net charge divided by the permittivity; useful for finding fields of highly symmetric charge distributions such as spheres and lines.

Charge stored per unit potential difference across a conductor or capacitor, measured in farads; for parallel plates it grows with plate area and shrinks with separation.

An insulating material placed between capacitor plates; its molecules polarise and reduce the internal field, multiplying capacitance by the material's relative permittivity.

A vector describing the strength and direction of a magnetic field, defined through the force on a moving charge or current; measured in teslas, one tesla being one newton per ampere-metre.

The force on a charge moving in electric and magnetic fields; the magnetic part is perpendicular to both velocity and field, with magnitude qvB sin θ.

A mnemonic giving the direction of the force on a current-carrying conductor in a magnetic field: thumb for force, first finger for field, second finger for current.

The product of magnetic flux density and the area perpendicular to it; a measure of the number of field lines passing through a surface, measured in webers.

An electromotive force is induced in a circuit whenever the magnetic flux through it changes; its magnitude equals the rate of change of flux linkage.

The induced current always flows in a direction whose magnetic effect opposes the change in flux that produced it, a consequence of energy conservation.

The production of an electromotive force and current in a conductor by a changing magnetic flux, as when a magnet moves near a coil or a coil rotates in a field.

A device in which a coil rotating in a magnetic field experiences continuously changing flux, producing a sinusoidal emf whose peak equals the product of flux density, area, turns and angular speed.

The property of a circuit whereby a changing current induces an emf opposing that change; defined as flux linkage per unit current and measured in henries.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed opposite to it; the defining relation gives a sinusoidal displacement-time graph with constant period.

The maximum displacement of an oscillator from its equilibrium position; for a wave it is the maximum disturbance of the medium, related to the energy carried.

The distance between two consecutive points in phase on a wave, such as crest to crest; it equals wave speed divided by frequency.

The number of complete oscillations or wave cycles passing a point per second, measured in hertz; it is the reciprocal of the period.

The time taken for one complete oscillation or for one wavelength to pass a fixed point; it is the reciprocal of frequency and is measured in seconds.

The distance a wave disturbance travels per unit time; for a periodic wave it equals frequency multiplied by wavelength and depends on the medium.

The ratio of light speed in vacuum to its speed in a medium; it determines how much a ray bends at an interface through Snell's law and is always at least one.

At a boundary between two media, the sine of the angle of incidence divided by the sine of the angle of refraction equals the ratio of the wave speeds, or of the refractive indices, of the two media.

When light travelling in a denser medium strikes a boundary beyond the critical angle, it is entirely reflected back; no refracted ray emerges into the less dense medium.

The spreading of waves when they pass an obstacle or aperture, becoming pronounced when the gap is comparable in size to the wavelength; it explains fringe patterns from narrow slits.

The superposition of two or more coherent waves, producing reinforcement where they arrive in phase and cancellation where they arrive in antiphase; observed as bright and dark fringes.

Two wave sources with a constant phase difference and the same frequency, a condition needed to produce a stable interference pattern rather than a rapidly shifting one.

The change in observed frequency when a source, observer or reflecting surface moves relative to the medium; approaching motion raises the frequency and receding motion lowers it.

A large amplitude response occurring when a system is driven at a frequency close to its natural frequency; damping reduces the peak and broadens the response curve.

The emission of electrons from a metal surface when light of sufficiently high frequency strikes it; the maximum kinetic energy depends on frequency, not on intensity.

The minimum energy needed to remove an electron from the surface of a particular metal; it equals Planck's constant times the threshold frequency for photoemission.

The lowest light frequency that can eject electrons from a given metal surface, equal to the work function divided by Planck's constant; below it no emission occurs whatever the intensity.

A quantum of electromagnetic radiation carrying energy equal to Planck's constant times frequency and momentum equal to energy divided by light speed.

The fundamental constant h, about 6.63 × 10⁻³⁴ J·s, linking a photon's energy to its frequency and a particle's momentum to its de Broglie wavelength.

The wavelength associated with a moving particle, equal to Planck's constant divided by its momentum; it becomes measurable for electrons and other light particles.

The difference between the total mass of a nucleus's separate nucleons and the mass of the bound nucleus; it corresponds to the binding energy holding the nucleus together.

The energy required to separate a nucleus completely into its constituent protons and neutrons; it equals the mass defect multiplied by the square of the speed of light.

The relation stating that mass and energy are interchangeable, with an energy equivalent equal to mass times the speed of light squared; it underlies nuclear energy calculations.

The spontaneous transformation of an unstable nucleus into a more stable one, emitting alpha particles, beta particles or gamma radiation; the rate falls exponentially with time.

The time taken for half of the undecayed nuclei in a radioactive sample, or half the activity, to disappear; it is independent of the initial amount.

The number of nuclear decays occurring per unit time in a sample, measured in becquerels; it decreases exponentially with the same decay constant as the number of nuclei.

The slowing of a clock measured by an observer moving relative to it; the dilated interval equals the proper interval multiplied by the Lorentz factor.

The quantity equal to one divided by the square root of one minus speed squared over light speed squared; it grows from one toward infinity as speed approaches light speed.

The shortening of an object's measured length along its direction of motion when observed from a frame moving relative to it; the proper length is the longest measured value.

The time interval between two events measured in the frame where the events occur at the same place; it is the shortest interval any observer can record between them.

The energy a body possesses simply because it has mass, equal to rest mass times light speed squared; it is released only in processes that change the particle's mass.