GRE Physics Subject Test key terms

A body's resistance to angular acceleration about a chosen axis; it equals the sum of each mass element times the square of its distance from that axis, so it depends on the axis chosen.

The rotational effect of a force about a pivot, equal to the cross product of the position vector from the pivot to the force's point of application with the force vector. It changes angular momentum.

The rotational analog of linear momentum, equal to the cross product of position and linear momentum for a particle, or moment of inertia times angular velocity for a rigid body. It is conserved when net external torque vanishes.

The acceleration of a body moving on a curved path, directed toward the center of curvature, with magnitude equal to speed squared divided by radius of curvature.

Motion along a circle at constant speed. The speed is constant but velocity changes direction continuously, so a centripetal acceleration always points toward the center.

The effective inertial mass of a two-body system, equal to the product of the two masses divided by their sum. It converts the relative-motion problem into an equivalent one-body problem.

For bound orbits, the square of the orbital period is proportional to the cube of the semi-major axis, with the constant depending on the total mass of the system.

The electric flux through any closed surface equals the enclosed charge divided by the permittivity of free space. It gives the field of highly symmetric charge distributions easily.

Potential energy per unit charge at a point, defined so that its negative gradient gives the electric field. Only differences between points are physically measurable.

The line integral of magnetic field around a closed loop equals the permeability of free space times the enclosed current. It determines magnetic fields of symmetric current distributions.

A changing magnetic flux through a loop induces an electromotive force equal to the negative rate of change of that flux. The minus sign expresses Lenz's law.

An induced current flows in the direction that opposes the change in magnetic flux that produced it. This follows from energy conservation and gives the sign in Faraday's law.

A term proportional to the rate of change of electric flux, added to the real current in Ampere's law so that charge conservation holds and changing electric fields can produce magnetic fields.

The cross product of electric and magnetic fields divided by permeability, giving the directional energy flux density of an electromagnetic wave. Its time average gives wave intensity.

A self-propagating oscillation of electric and magnetic fields, perpendicular to each other and to the direction of travel, moving in vacuum at the speed of light.

The orientation of the electric field oscillation of a transverse wave. It can be linear, circular, or elliptical, and is altered by reflection, scattering, and passage through anisotropic media.

The intensity transmitted through a linear polarizer equals the incident intensity times the square of the cosine of the angle between the incident polarization and the polarizer's transmission axis.

The angle of incidence at which light reflected from a dielectric surface is fully polarized perpendicular to the plane of incidence; its tangent equals the ratio of the two refractive indices.

Interference between light reflected from the two surfaces of a thin layer. Whether a given wavelength is reinforced or cancelled depends on film thickness, refractive indices, and phase shifts on reflection.

A periodic array of slits or rulings that disperses light into sharp maxima at angles where path differences between adjacent elements equal whole numbers of wavelengths.

A thermodynamic change with no heat exchange with the surroundings. Work done comes entirely from internal energy, and the pressure-volume relation follows a power law with the adiabatic exponent.

The change in internal energy of a system equals heat added to it minus work done by it. It is the statement of energy conservation for thermodynamic processes.

A state function measuring the number of accessible microscopic configurations, or equivalently the reversible heat transfer divided by temperature. It never decreases for an isolated system.

The probability distribution of speeds or energies of particles in a classical gas at equilibrium, peaking at a most probable speed and having a long high-energy tail.

In classical thermal equilibrium each quadratic degree of freedom carries an average energy of one half times Boltzmann's constant times temperature. It fails when quantum level spacings are large.

The maximum possible efficiency of a heat engine operating between two reservoirs, equal to one minus the ratio of the colder to the hotter absolute temperature. No real engine exceeds it.

Certain pairs of observables, such as position and momentum, cannot both have arbitrarily precise values; the product of their uncertainties is at least half the reduced Planck constant.

The operator formed by subtracting the product of two operators in one order from the product in the other. If it vanishes the observables commute and can be measured simultaneously.

A state for which measuring a given observable yields one definite value with certainty. The corresponding wavefunction satisfies the operator's eigenvalue equation.

The complex-valued function describing a quantum system, whose squared magnitude gives probability density. It evolves according to the Schrodinger equation and must be normalizable.

The penetration of a particle through a potential barrier even when its energy is below the barrier height, giving a nonzero transmission probability that falls roughly exponentially with barrier width.

Small splittings of atomic spectral lines caused by spin-orbit coupling and relativistic corrections to electron motion, scaling roughly with the fourth power of atomic number.

Conditions on quantum-number changes that determine whether a transition is allowed. For electric dipole radiation in hydrogen, the orbital angular momentum quantum number must change by exactly one.

The slowing of a clock's measured rate as observed in a frame in which it moves, given by the Lorentz factor. It is symmetric between inertial frames and confirmed by muon lifetimes and atomic clocks.

The linear coordinate transformation between inertial frames in relative motion that leaves the speed of light invariant, mixing space and time coordinates through the Lorentz factor.

The quantity formed from the squares of the time separation times the speed of light squared minus the spatial separation squared. All inertial observers agree on its value and sign.

Momentum defined as the Lorentz factor times rest mass times velocity, so it grows without bound as speed approaches the speed of light and reduces to the classical expression at low speed.

The uncertainty in an averaged result, equal to the sample standard deviation divided by the square root of the number of measurements. It shrinks as more data are averaged.

The rule for combining uncertainties of measured quantities into the uncertainty of a calculated result, adding independent contributions in quadrature after weighting by partial derivatives.

A goodness-of-fit statistic summing squared differences between observed and expected values divided by expected values. Comparing it with its degrees of freedom tests whether a model fits.

The ejection of electrons from a surface by light above a threshold frequency. The maximum electron energy depends on frequency, not intensity, which supports the photon picture.

The scattering of photons by electrons in which the photon loses energy and its wavelength increases by an amount depending on the scattering angle, confirming photon momentum.