Balearic PAU Physics flashcards

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

Gravitational field strength

Force per unit mass at a point, g = G·M/r², directed toward the source mass. Near Earth's surface its magnitude is about 9.8 N/kg.

Gravitational potential

Work per unit mass to bring a mass from infinity to a point: V = −G·M/r. It is always negative and zero at infinity.

Escape velocity

Minimum launch speed for a projectile to escape a body's gravity without further propulsion: v_e = √(2GM/R). It does not depend on the projectile's mass.

Kepler's third law

For circular orbits, the square of the period is proportional to the cube of the orbital radius: T² = (4π²/GM)·r³, where M is the central mass.

Electric field of a point charge

E = k·q/r² with k = 1/(4πε₀) ≈ 9×10⁹ N·m²/C². The vector points away from positive charge and toward negative charge.

Gauss's law

The electric flux through any closed surface equals the enclosed charge divided by ε₀. Choosing a symmetric surface makes the field easy to calculate.

Magnetic force on a charge

F = q·v×B, of magnitude qvB·sinθ. Because the force is perpendicular to velocity, it changes direction but does no work on the charge.

Faraday–Lenz law

An induced emf equals the negative rate of change of magnetic flux: ε = −dΦ/dt. The minus sign means the induced current opposes the flux change.

Simple harmonic motion

Oscillation where acceleration is proportional and opposite to displacement: a = −ω²x. Displacement follows x = A·sin(ωt + φ) with ω = 2π/T.

Wave speed relation

For any periodic wave, speed equals wavelength times frequency: v = λ·f = λ/T. The medium sets v, while the source sets the frequency.

Standing waves on a string

For a string fixed at both ends, allowed wavelengths are λ_n = 2L/n and frequencies f_n = n·v/(2L), where n is a positive integer.

Doppler effect for sound

Observed frequency shifts when source and observer move relative to each other: f' = f·(v ± v_o)/(v ∓ v_r). Approach raises the frequency, recession lowers it.

Total internal reflection

Occurs when light travels from a denser to a less dense medium beyond the critical angle, given by sinθ_c = n₂/n₁. No refracted ray leaves the boundary.

Thin lens equation

1/f = 1/s + 1/s', relating focal length, object distance and image distance. Magnification is m = −s'/s under the usual Cartesian sign convention.

Young's double-slit fringes

Bright fringes appear where d·sinθ = nλ; on a distant screen their spacing is Δy = λ·D/d, where D is the slit-to-screen distance.

Diffraction grating equation

Principal maxima satisfy d·sinθ = nλ, where d is the slit spacing and n the order. Smaller spacing produces wider angular separation between orders.

Lorentz factor

γ = 1/√(1 − v²/c²), always at least 1. It quantifies time dilation, length contraction and relativistic energy increase at speed v.

Time dilation

A clock moving relative to an observer runs slow: Δt = γ·Δτ, where Δτ is the proper time measured in the clock's rest frame.

Relativistic energy–momentum relation

E² = (p·c)² + (m·c²)². For a photon, mass is zero, so E = p·c = h·f; for a particle at rest, E = m·c².

Relativistic velocity addition

Velocities combine as u = (u' + v)/(1 + u'·v/c²). Unlike the Galilean sum, the result never exceeds the speed of light.

Photon energy

The energy of a photon is E = h·f = h·c/λ, with h = 6.63×10⁻³⁴ J·s. Shorter wavelength means higher energy per photon.

Photoelectric equation

Maximum kinetic energy of emitted electrons is K_max = h·f − W₀, where W₀ is the work function. Emission requires f above the threshold f₀ = W₀/h.

de Broglie wavelength

Every particle has an associated wavelength λ = h/p = h/(m·v). It is measurable for electrons and explains electron diffraction patterns.

Hydrogen energy levels

In the Bohr model E_n = −13.6 eV/n². A transition emits or absorbs a photon of energy h·f equal to the difference between two levels.

Heisenberg uncertainty principle

Position and momentum cannot both be known precisely: Δx·Δp ≥ ħ/2, where ħ = h/(2π). This is a fundamental property of nature, not an instrument limitation.

Mass defect and binding energy

A nucleus has less mass than its separate nucleons; the difference Δm corresponds to binding energy ΔE = Δm·c², with 1 u equivalent to 931.5 MeV.

Alpha decay

A nucleus emits a helium-4 nucleus, reducing its nucleon number A by 4 and its atomic number Z by 2. The daughter is a different element.

Beta-minus decay

A neutron converts into a proton, emitting an electron and an antineutrino. The nucleon number A stays the same while Z increases by one.

Radioactive decay law

The number of undecayed nuclei falls exponentially: N = N₀·e^(−λt), where λ is the decay constant. Activity A = λ·N follows the same law.

Half-life

Time for half the radioactive nuclei in a sample to decay: T½ = ln2/λ ≈ 0.693/λ. After n half-lives, a fraction (1/2)^n remains.

Nuclear fission

A heavy nucleus splits into lighter fragments after absorbing a neutron, releasing energy because the fragments have higher binding energy per nucleon.

Nuclear fusion

Light nuclei combine to form a heavier nucleus, releasing energy because the product is more tightly bound. It powers stars but requires very high temperature.