Physics — UK A-Level
Comprehensive UK A-Level Physics flashcards (AQA/OCR/Edexcel) covering measurements and uncertainties, particles and radiation, waves, mechanics and materials, electricity, further mechanics, thermal physics, fields, and nuclear physics.
Ämne: Fysik · Nivå: Gymnasium (16–19) · 409 kort
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- The seven SI base units are: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.
- Derived units are combinations of base units. Examples: the newton (N = kg·m·s⁻²), the joule (J = kg·m²·s⁻² = N·m), the watt (W = J·s⁻¹), the pascal (Pa = N·m⁻²), and the coulomb (C = A·s).
- SI prefixes (large): deca (da, 10¹), hecto (h, 10²), kilo (k, 10³), mega (M, 10⁶), giga (G, 10⁹), tera (T, 10¹²), peta (P, 10¹⁵).
- SI prefixes (small): deci (d, 10⁻¹), centi (c, 10⁻²), milli (m, 10⁻³), micro (μ, 10⁻⁶), nano (n, 10⁻⁹), pico (p, 10⁻¹²), femto (f, 10⁻¹⁵).
- A random error causes readings to scatter unpredictably about the true value (e.g. judging the exact moment to start a stopwatch). It can be reduced by repeating measurements and averaging.
- A systematic error shifts all readings by the same amount or in the same direction (e.g. a zero error on an instrument). Averaging does not remove it; it requires recalibration or a corrected method.
- Precision describes how close repeated measurements are to one another (small spread). Accuracy describes how close a measurement is to the true value. A measurement can be precise but inaccurate.
- Fractional uncertainty = absolute uncertainty ÷ measured value. Percentage uncertainty = fractional uncertainty × 100%.
- When quantities are multiplied or divided, add their percentage (fractional) uncertainties. When raising to a power n, multiply the percentage uncertainty by n.
- When quantities are added or subtracted, add their absolute uncertainties (never the percentage uncertainties).
- An order-of-magnitude estimate gives a value to the nearest power of ten. For example, the diameter of a human hair is of order 10⁻⁴ m, and the height of an adult is of order 10⁰ m (1 m).
- The atom consists of a tiny dense positive nucleus (protons and neutrons) surrounded by orbiting electrons. The nucleus contains almost all the mass but occupies a tiny fraction of the atomic volume.
- Relative charges: proton +1, electron −1, neutron 0. Relative masses: proton ≈ 1, neutron ≈ 1, electron ≈ 1/1836 (about 0.0005).
- Specific charge is the charge-to-mass ratio (Q/m) measured in C·kg⁻¹. The electron has the largest specific charge of any particle (≈ 1.76 × 10¹¹ C·kg⁻¹) because of its very small mass.
- In the notation ᴬᴢX, A is the nucleon (mass) number = number of protons + neutrons, and Z is the proton (atomic) number = number of protons. Number of neutrons = A − Z.
- Isotopes are atoms of the same element (same proton number Z) with different numbers of neutrons (different nucleon number A). They have identical chemical properties but different masses.
- Every particle has a corresponding antiparticle with the same mass and rest energy but opposite charge (and opposite quantum numbers). Examples: electron/positron, proton/antiproton, neutron/antineutron, neutrino/antineutrino.
- In annihilation, a particle meets its antiparticle and their mass is converted into energy as two (or more) photons. In pair production, a single photon converts into a particle–antiparticle pair (requiring a minimum photon energy of 2× the rest energy).
- The four fundamental forces are: the strong nuclear force, the weak nuclear force, the electromagnetic force, and the gravitational force. Forces are transmitted by exchange particles (gauge bosons).
- Exchange particles: the electromagnetic force is carried by the virtual photon (γ), the weak interaction by the W⁺, W⁻ and Z⁰ bosons, the strong force between quarks by gluons, and gravity (hypothetically) by the graviton.
- Hadrons are particles that feel the strong nuclear force. They are subdivided into baryons (three quarks, e.g. protons and neutrons) and mesons (a quark–antiquark pair, e.g. pions and kaons).
- Leptons are fundamental particles that do not feel the strong force. They include the electron, the muon, the tau, and their associated neutrinos (plus antiparticles). Leptons have lepton number +1 (antileptons −1).
- The three quarks studied at A-Level are up (u, charge +2/3), down (d, charge −1/3) and strange (s, charge −1/3). The strange quark has strangeness −1.
- Quark composition: a proton is (uud) giving charge +1; a neutron is (udd) giving charge 0. An antiproton is (ūūd̄) and an antineutron is (ūd̄d̄).
- Quantities conserved in ALL particle interactions: charge, baryon number, and lepton number (each lepton family), and energy/momentum. Strangeness is conserved in strong and electromagnetic interactions but can change by 0 or ±1 in weak interactions.
- In β⁻ decay, a neutron turns into a proton, emitting an electron and an electron antineutrino: n → p + e⁻ + ν̄ₑ. At the quark level a down quark changes into an up quark, mediated by the W⁻ boson.
- In β⁺ decay, a proton turns into a neutron, emitting a positron and an electron neutrino: p → n + e⁺ + νₑ. At the quark level an up quark changes into a down quark, mediated by the W⁺ boson.
- The neutrino was postulated by Wolfgang Pauli in 1930 to account for the missing energy and momentum in beta decay. The continuous energy spectrum of beta particles implied a third, almost undetectable particle carried away the rest of the energy.
- The photoelectric effect is the emission of electrons from a metal surface when electromagnetic radiation above a threshold frequency shines on it. It provides evidence for the particle (photon) nature of light.
- The energy of a photon is E = hf = hc/λ, where h is the Planck constant (6.63 × 10⁻³⁴ J·s), f is frequency, c is the speed of light and λ is wavelength.
- The work function φ is the minimum energy needed to release an electron from a metal surface. Einstein's photoelectric equation: hf = φ + Eₖ(max), where Eₖ(max) is the maximum kinetic energy of the emitted electrons.
- The threshold frequency f₀ is the minimum frequency of light that will cause photoemission, given by f₀ = φ/h. Below this frequency no electrons are emitted, however intense the light.
- The electronvolt (eV) is the energy gained by an electron accelerated through a potential difference of 1 volt: 1 eV = 1.60 × 10⁻¹⁹ J. It is a convenient energy unit for particles.
- Excitation is when an electron in an atom absorbs energy and moves to a higher energy level. Ionisation is when an electron gains enough energy to be completely removed from the atom.
- When an excited electron falls from a higher energy level E₂ to a lower level E₁, a photon is emitted with energy hf = E₂ − E₁. Because energy levels are discrete, only specific photon energies (line spectra) are produced.
- A line emission spectrum (bright lines on a dark background) is direct evidence that electrons occupy discrete energy levels within atoms. Each element has a unique spectral fingerprint.
- In a fluorescent tube, accelerated electrons collide with mercury atoms, exciting/ionising them. The mercury atoms de-excite emitting UV photons, which are absorbed by the phosphor coating, which then emits visible light.
- Wave-particle duality: light and matter exhibit both wave-like properties (diffraction, interference) and particle-like properties (the photoelectric effect, photons). The behaviour observed depends on the experiment.
- The de Broglie wavelength of a particle is λ = h/p = h/(mv), where p is momentum. It shows that all moving particles have an associated wavelength, confirmed by electron diffraction.
- Electron diffraction provides evidence for the wave nature of matter: a beam of electrons passed through a thin graphite film produces a diffraction ring pattern, just as waves do. Faster electrons (higher momentum) give a smaller diffraction angle.
- A progressive wave transfers energy from one place to another without transferring matter. Its key properties are amplitude, wavelength, frequency, period, and speed.
- The wave equation is c = fλ, where c is wave speed, f is frequency and λ is wavelength. Frequency f = 1/T, where T is the time period.
- In a transverse wave the oscillations are perpendicular to the direction of energy transfer (e.g. electromagnetic waves, waves on a string). In a longitudinal wave the oscillations are parallel to the direction of energy transfer (e.g. sound).
- Phase difference is the difference in the stage of oscillation between two points or two waves, measured in radians or degrees. A full cycle is 2π radians (360°); points in phase differ by 0 (or multiples of 2π).
- Polarisation restricts the oscillations of a transverse wave to a single plane. Only transverse waves can be polarised, so polarisation is evidence that light is a transverse wave. Longitudinal waves cannot be polarised.
- The principle of superposition states that when two or more waves meet, the resultant displacement at any point is the vector sum of the individual displacements of the waves at that point.
- Constructive interference occurs when waves meet in phase (path difference = nλ), producing a larger amplitude. Destructive interference occurs when waves meet in antiphase (path difference = (n + ½)λ), producing a smaller or zero amplitude.
- Two sources are coherent if they have a constant phase difference and the same frequency. Coherence is essential to produce a stable, observable interference pattern.
- A stationary (standing) wave is formed when two progressive waves of the same frequency and amplitude travel in opposite directions and superpose. No net energy is transferred along it.
- On a stationary wave, nodes are points of zero amplitude (where the waves always cancel) and antinodes are points of maximum amplitude. The distance between adjacent nodes (or adjacent antinodes) is λ/2.