DZI Physics and Astronomy flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Newton's second law

The net force on an object equals the rate of change of its momentum; for constant mass this reduces to F = ma.

Kepler's third law

The square of an orbital period is proportional to the cube of the semimajor axis, with the constant of proportionality set by the total mass.

Escape speed

The minimum speed needed to leave a gravitational field without further propulsion; it equals the square root of 2GM/r.

Angular momentum conservation

When the net external torque on a system is zero, its angular momentum remains constant, linking changes in rotation rate to changes in moment of inertia.

First law of thermodynamics

The change in internal energy equals heat added to the system minus work done by the system, expressing energy conservation for thermal processes.

Carnot efficiency

The maximum efficiency of a heat engine operating between two reservoirs equals one minus the ratio of the colder to the hotter absolute temperature.

Entropy

A state function whose change for a reversible process is the integral of heat divided by temperature; it never decreases in an isolated system.

Equipartition theorem

Each quadratic degree of freedom of a system in thermal equilibrium carries an average energy of one half kT.

Doppler effect

The observed frequency of a wave shifts upward when source and observer approach and downward when they recede, depending on relative motion.

Snell's law

The product of refractive index and sine of the angle from the normal is constant across an interface between two transparent media.

Total internal reflection

Light traveling from a denser to a less dense medium is completely reflected when the incidence angle exceeds the critical angle.

Thin-lens equation

The reciprocal of focal length equals the sum of the reciprocals of object and image distances, with sign conventions distinguishing real and virtual images.

Gauss's law

The electric flux through a closed surface equals the enclosed charge divided by the permittivity of free space, useful for symmetric charge distributions.

Faraday's law

A changing magnetic flux through a loop induces an electromotive force proportional to the rate of change of that flux.

Lenz's law

The direction of an induced current opposes the change in magnetic flux that produced it, consistent with energy conservation.

Capacitance

The ratio of stored charge to potential difference for a conductor arrangement; for parallel plates it grows with area and shrinks with separation.

Photoelectric effect

Electrons are ejected from a metal when light exceeds a threshold frequency; maximum kinetic energy depends on frequency, not intensity.

de Broglie wavelength

A particle's wavelength equals Planck's constant divided by its momentum, showing wave behavior for matter at small scales.

Heisenberg uncertainty principle

Certain pairs of observables, such as position and momentum, cannot both be known precisely; their uncertainties multiply to at least half the reduced Planck constant.

Pauli exclusion principle

No two fermions in the same system can occupy identical quantum states, which structures electron shells and degeneracy pressure.

Time dilation

A clock moving relative to an observer ticks more slowly as measured by that observer, with the factor depending on relative speed.

Mass–energy equivalence

Mass and energy are interchangeable through the relation E = mc², so a small mass defect corresponds to a large released energy.

Half-life

The time for half the nuclei in a radioactive sample to decay; it relates to the decay constant by the natural logarithm of two.

Relativistic velocity addition

Combining velocities at high speed requires a formula that keeps the result below the speed of light, unlike simple arithmetic addition.

Stefan–Boltzmann law

The total power radiated per unit area by a blackbody scales with the fourth power of its absolute temperature.

Hydrostatic equilibrium

In a stable star, the outward pressure gradient exactly balances the inward pull of gravity at every radius.

Distance modulus

The difference between apparent and absolute magnitude equals five times the base-ten logarithm of distance in parsecs divided by ten.

Chandrasekhar limit

The maximum mass a white dwarf can support against gravity by electron degeneracy pressure, approximately 1.4 solar masses.

Hubble–Lemaître law

The recession speed of a distant galaxy is proportional to its distance, with the Hubble constant as the proportionality factor.

Cosmological redshift

Stretching of photon wavelengths by the expansion of space, expressed as one plus redshift equaling the ratio of scale factors.

Critical density

The density for which the universe is spatially flat, equal to three times the square of the Hubble constant divided by eight pi G.

Standard error of the mean

The standard deviation of a sample divided by the square root of the sample size, quantifying uncertainty in an estimated average.