Bagrut Physics 5 Units flashcards

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

Uniform circular motion acceleration

In uniform circular motion the speed is constant but the velocity direction changes, producing a centripetal acceleration a = v²/r = ω²r directed toward the center, with period T = 2πr/v.

Impulse-momentum theorem

The impulse of the net force equals the change in momentum: J = FΔt = Δp = mv_f − mv_i. Extending the collision time with the same impulse reduces the average force, as in airbags.

Conservation of mechanical energy

When only conservative forces do work, KE + U is constant: ½mv₁² + mgh₁ = ½mv₂² + mgh₂. Friction converts mechanical energy to internal energy, so it is not conserved.

Moment of inertia

Moment of inertia I = Σmr² measures resistance to angular acceleration about a chosen axis. It depends on mass distribution: I = ½MR² for a solid disk and MR² for a hoop about the central axis.

Kepler's third law

For circular or elliptical orbits, T² = (4π²/GM)·r³, where M is the central mass and r the semi-major axis. The constant depends on the central body, not on the satellite's mass.

Orbital speed

For a circular orbit of radius r around mass M, gravity supplies the centripetal force: GMm/r² = mv²/r, so v = √(GM/r). Larger orbits have slower orbital speeds.

Escape velocity

The minimum launch speed to escape a body's gravity with zero total energy: v_esc = √(2GM/R). For Earth it is about 11.2 km/s, independent of the projectile's mass.

Geostationary satellite conditions

A geostationary satellite orbits in the equatorial plane, in the direction of Earth's rotation, with a period of exactly 24 h. Its radius is about 4.2×10⁷ m from Earth's center.

Electric field of a point charge

E = kq/r², directed radially outward for positive q and inward for negative q, with k ≈ 9×10⁹ N·m²/C². The field is force per unit positive test charge, E = F/q₀.

Resistors in parallel

Parallel resistors share the same voltage, and the currents add: 1/R_eq = 1/R₁ + 1/R₂ + …. The equivalent resistance is less than the smallest individual resistance.

Capacitor energy and series rule

Stored energy is U = ½CV² = Q²/(2C). Capacitors in series carry the same charge Q, and the equivalent capacitance satisfies 1/C_eq = Σ1/C, which is smaller than any single capacitance.

Kirchhoff's loop rule

Around any closed loop, the algebraic sum of potential changes is zero: ΣΔV = 0. This expresses energy conservation per unit charge; voltage rises across sources and drops across resistors.

Lenz's law

The induced current flows in the direction whose magnetic field opposes the change in flux that produced it. This is the meaning of the minus sign in Faraday's law ε = −NΔΦ/Δt.

Magnetic flux

Φ = BA·cosθ, where θ is the angle between B and the area's normal vector. Flux is measured in webers (1 Wb = 1 T·m²) and changes when B, A, or orientation changes.

Charged particle in magnetic field

A charge moving perpendicular to B follows a circle of radius r = mv/(qB) because the magnetic force qvB is always perpendicular to v. The field does no work and the speed stays constant.

Motional emf

A conducting rod of length L moving at speed v perpendicular to a uniform field B develops emf ε = BLv. If the rod slides on rails forming a closed circuit, a current flows and Lenz's law determines its direction.

Simple harmonic motion conditions

SHM requires a restoring force proportional to displacement and opposite to it: F = −kx. Then a = −ω²x with ω = √(k/m), and x oscillates sinusoidally with constant amplitude and period.

Standing waves on a string

A string fixed at both ends supports standing waves with nodes at the ends: f_n = nv/(2L), where v = √(F/μ). The lowest frequency n = 1 is the fundamental, and all harmonics are present.

Doppler effect in sound

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

Beat frequency

Two sound waves of slightly different frequencies f₁ and f₂ superpose to give loudness varying at f_beat = |f₁ − f₂|. The beat frequency equals the difference of the two frequencies.

Snell's law

At a refracting interface, n₁sinθ₁ = n₂sinθ₂, with angles measured from the normal and n = c/v. Light bends toward the normal when entering a medium with a higher refractive index.

Total internal reflection

When light goes from a denser to a less dense medium and the incidence angle exceeds the critical angle sinθ_c = n₂/n₁, no light refracts out and all of it reflects internally. It requires n₁ > n₂.

Double-slit fringe spacing

For two slits separated by d, illuminated by wavelength λ, the bright-fringe spacing on a screen at distance L (L ≫ d) is Δy = λL/d. Larger λ or smaller d widens the pattern.

Thin lens equation

1/f = 1/d_o + 1/d_i, with magnification m = −d_i/d_o. Converging lenses have f > 0; a positive d_i gives a real inverted image, and a negative d_i gives a virtual upright image.

First law of thermodynamics

ΔU = Q − W, where Q is heat added to the system and W is work done by the system. In an isothermal ideal-gas process ΔU = 0; in an adiabatic process Q = 0 and ΔU = −W.

Root-mean-square molecular speed

v_rms = √(3RT/M) = √(3kT/m) for an ideal gas, where M is molar mass. It rises with temperature and is lower for heavier molecules at the same temperature.

Carnot efficiency limit

No heat engine operating between reservoirs at T_H and T_C can exceed η_max = 1 − T_C/T_H, with temperatures in kelvin. Equality holds only for a reversible cycle.

Photoelectric stopping potential

The stopping potential V_stop is the reverse voltage that just stops the fastest photoelectrons: eV_stop = K_max = hf − W. It depends on frequency, not on light intensity.

de Broglie wavelength

Every particle has wavelength λ = h/p = h/(mv), where h = 6.63×10⁻³⁴ J·s. It is observable for electrons and other microscopic particles and explains electron diffraction.

Radioactive half-life

The half-life T½ = ln2/λ is the time for half the nuclei to decay: N = N₀e^(−λt). After n half-lives, the fraction remaining is (½)ⁿ, independent of the initial amount.

Bohr hydrogen energy levels

Hydrogen levels are E_n = −13.6 eV/n². A photon is emitted when an electron drops to a lower level, with hf = E_i − E_f; the series ending at n = 2 is the Balmer (visible) series.

Combining uncertainties in products

For multiplication or division, relative uncertainties add: Δ(ab)/ab = Δa/a + Δb/b. For powers y = xⁿ, the relative uncertainty is |n| times that of x.

Systematic versus random error

Systematic error shifts all measurements in the same direction (e.g., a zero offset) and is not reduced by averaging. Random error scatters readings and is reduced by averaging repeated measurements.