Praxis Physics 5265 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 its mass times acceleration: ΣF = ma. Acceleration is in the same direction as the net force and inversely proportional to mass.

Centripetal acceleration

For an object moving in a circle of radius r at speed v, the acceleration points toward the center with magnitude a_c = v²/r; the net inward force provides it.

Conservation of momentum

In an isolated system with no external net force, total momentum Σmv remains constant. This holds in all collisions, elastic or inelastic.

Work-energy theorem

The net work done on an object equals its change in kinetic energy: W_net = ΔK = ½mv_f² − ½mv_i². Work is force times displacement along the force.

Torque

Torque is the rotational effect of a force: τ = rF sinθ, where θ is the angle between the position vector and force. Units are newton-meters.

Coulomb's law

The electric force between two point charges is F = kq₁q₂/r², with k ≈ 8.99×10⁹ N·m²/C². Like charges repel; opposite charges attract.

Electric field

Electric field E = F/q₀ is the force per unit positive test charge. Field lines point away from positive charges and toward negative charges.

Capacitance

Capacitance C = Q/V is the charge stored per unit potential difference. For a parallel-plate capacitor, C = ε₀A/d; stored energy is U = ½CV².

Resistors in parallel

For resistors connected in parallel, the reciprocal of equivalent resistance equals the sum of reciprocals: 1/R_eq = 1/R₁ + 1/R₂ + …; voltage is the same across each.

Kirchhoff's loop rule

Around any closed circuit loop, the algebraic sum of potential changes is zero: ΣΔV = 0. This expresses conservation of energy in a circuit.

Faraday's law of induction

The induced emf in a coil equals the negative rate of change of magnetic flux through it: ε = −N ΔΦ_B/Δt. Lenz's law gives the negative sign.

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.

Standing waves on a string

A string fixed at both ends supports standing waves with wavelengths λ_n = 2L/n and frequencies f_n = nv/(2L), where n = 1, 2, 3, ….

Doppler effect

The observed frequency shifts when source and observer move relative to each other: f' = f(v ± v_o)/(v ∓ v_s). Approach raises frequency; recession lowers it.

Snell's law

At an interface between media, n₁ sinθ₁ = n₂ sinθ₂, where n is the index of refraction and angles are measured from the normal.

Total internal reflection

When light travels from a higher-index to a lower-index medium and the incident angle exceeds the critical angle θ_c = sin⁻¹(n₂/n₁), all light is reflected.

Thin lens equation

For a thin lens, 1/f = 1/d_o + 1/d_i, and magnification m = −d_i/d_o. A positive image distance means a real image on the opposite side.

Double-slit interference

Light through two slits separated by d produces bright fringes where d sinθ = mλ (m = 0, ±1, …) and dark fringes where d sinθ = (m + ½)λ.

Ideal gas law

PV = nRT = Nk_BT relates pressure, volume, amount, and absolute temperature. R ≈ 8.31 J/(mol·K) and k_B ≈ 1.38×10⁻²³ J/K.

Root-mean-square speed

The rms speed of gas molecules is v_rms = √(3RT/M) = √(3k_BT/m), where M is molar mass. It increases with temperature and decreases with molecular mass.

First law of thermodynamics

The change in internal energy equals heat added to the system minus work done by the system: ΔU = Q − W. It is a statement of energy conservation.

Carnot efficiency

The maximum efficiency of a heat engine operating between hot and cold reservoirs is e = 1 − T_c/T_h, with temperatures in kelvin.

Latent heat

The heat required to change phase at constant temperature is Q = mL, where L is the latent heat of fusion or vaporization. No temperature change occurs during the phase change.

Second law of thermodynamics

The total entropy of an isolated system never decreases: ΔS_universe ≥ 0. Heat flows spontaneously from hot to cold, and no engine is 100% efficient.

Photon energy

A photon's energy is E = hf = hc/λ, where h ≈ 6.63×10⁻³⁴ J·s. Shorter wavelength means higher photon energy.

Photoelectric effect

Electrons are ejected from a metal when light exceeds a threshold frequency. Maximum kinetic energy is K_max = hf − φ, where φ is the work function.

de Broglie wavelength

Every particle has a wavelength λ = h/p = h/(mv). It is significant for low-mass particles such as electrons and negligible for macroscopic objects.

Bohr energy levels

In the Bohr model of hydrogen, E_n = −13.6 eV/n². A photon is emitted when an electron drops to a lower level, with energy equal to the level difference.

Radioactive half-life

The half-life t₁/₂ = ln2/λ is the time for half the radioactive nuclei to decay. The number remaining follows N = N₀e^(−λt).

Heisenberg uncertainty principle

Position and momentum cannot both be known precisely: ΔxΔp ≥ ħ/2, where ħ = h/(2π). The same limit applies to energy and time.

Significant figures

In multiplication and division, the result carries the same number of significant figures as the factor with the fewest. In addition, keep the fewest decimal places.

Accuracy versus precision

Accuracy is closeness to the accepted value; precision is repeatability among measurements. A systematic error reduces accuracy; random error reduces precision.