Physics flashcards

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

Kinematic equation without time

v² = u² + 2as relates final and initial velocities, acceleration, and displacement when time is unknown. It applies only for constant acceleration along a straight line.

Newton's second law

The net force on an object equals its mass times its acceleration, ΣF = ma. Acceleration is in the same direction as the net force and inversely proportional to mass.

Work–energy theorem

The net work done on an object equals its change in kinetic energy, W_net = ΔKE. Work is positive when the force has a component along the displacement.

Conservation of momentum

If the net external force on a system is zero, total linear momentum stays constant. This holds in collisions and explosions, including when kinetic energy is not conserved.

Centripetal acceleration

For uniform circular motion, acceleration has magnitude a_c = v²/r and points toward the centre. It changes the direction of velocity, not its magnitude.

Parallel-axis theorem

The moment of inertia about any axis equals that about a parallel axis through the centre of mass plus md², where d is the separation of the axes.

Orbital speed

For a circular orbit of radius r around a body of mass M, the speed is v = √(GM/r). Larger orbits correspond to slower orbital speeds.

Coulomb's law

The electrostatic force between two point charges is F = kq₁q₂/r², directed along the line joining them. Like charges repel and opposite charges attract.

Electric potential of a point charge

At distance r from a point charge q, the potential is V = kq/r, a scalar. Potential difference between two points equals the work per unit charge moved between them.

Capacitor energy

A charged capacitor stores energy U = ½CV² = Q²/(2C). For parallel plates, capacitance is C = ε₀A/d, increasing with plate area and decreasing with separation.

Kirchhoff's loop rule

Around any closed loop in a circuit, the algebraic sum of potential differences is zero. This expresses energy conservation for charges moving round the loop.

Magnetic force on moving charge

A charge q moving with speed v in field B feels F = qvB sinθ, perpendicular to both v and B. A charge moving parallel to B feels no force.

Faraday's law of induction

The induced emf in a circuit equals the negative rate of change of magnetic flux through it, emf = −N dΦ/dt. The minus sign reflects Lenz's law.

Wave speed relation

For any periodic wave, speed equals frequency times wavelength, v = fλ. Frequency is set by the source; speed is set by the medium.

Doppler effect

The observed frequency of a wave changes when source and observer move relative to each other. Approach raises the observed frequency; recession lowers it.

Snell's law

At a boundary between media, n₁sinθ₁ = n₂sinθ₂, where n is the refractive index and angles are measured from the normal. Light bends toward the normal entering a denser medium.

Total internal reflection

Occurs when light travels from a denser to a less dense medium at an angle beyond the critical angle, where sinθ_c = n₂/n₁. No light is refracted out.

Thin lens equation

1/f = 1/d_o + 1/d_i relates focal length, object distance, and image distance. Magnification is m = −d_i/d_o; negative m indicates an inverted image.

Double-slit interference

Bright fringes occur where d sinθ = mλ and dark fringes where d sinθ = (m + ½)λ. Fringe spacing on a distant screen is λL/d.

Ideal gas law

pV = nRT relates pressure, volume, amount in moles, and absolute temperature, with R = 8.31 J/(mol·K). It assumes point molecules with no intermolecular forces.

First law of thermodynamics

ΔU = Q − W, where Q is heat added to the system and W is work done by the system. Internal energy changes by heating or by work.

Carnot efficiency

The maximum efficiency of a heat engine between reservoirs at absolute temperatures T_H and T_C is 1 − T_C/T_H. No real engine between these reservoirs can exceed it.

Root-mean-square speed

For a gas, v_rms = √(3RT/M), where M is molar mass. At a given temperature, lighter molecules move faster on average than heavier ones.

Latent heat

During a phase change at constant temperature, heat transferred is Q = mL, where L is the specific latent heat. Temperature stays constant while the phase changes.

Photoelectric equation

Maximum photoelectron kinetic energy is K_max = hf − φ, where φ is the work function. Emission occurs only above the threshold frequency f₀ = φ/h.

de Broglie wavelength

A particle with momentum p has an associated wavelength λ = h/p. This wave nature is measurable for electrons and other low-mass particles.

Bohr energy levels

Hydrogen energy levels are E_n = −13.6 eV/n². A photon emitted or absorbed on a transition has energy equal to the difference between levels.

Half-life

The time for half the nuclei in a sample to decay. After n half-lives the remaining fraction is (1/2)ⁿ, and activity falls by the same factor.

Time dilation

A clock moving relative to an observer runs slow by factor γ = 1/√(1 − v²/c²). The dilated interval is Δt = γΔt₀, where Δt₀ is proper time.

SI base units

The seven SI base units are metre, kilogram, second, ampere, kelvin, mole, and candela. Derived units such as newton and joule combine these.

Significant figures in multiplication

When multiplying or dividing, the result carries the same number of significant figures as the input with the fewest. For addition, keep the fewest decimal places.

Propagating uncertainties in powers

If a quantity is raised to power n, its fractional uncertainty is multiplied by |n|. For products, fractional uncertainties of the factors add.