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Flashcards · 7408 · Topic 2
Particles and radiation flashcards
20 cards on Topic 2, subtopics 3.2.1 to 3.2.2. Pick a subtopic or study them all.
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3.2.1 Particles
- Specific charge: definition and unit
- Charge ÷ mass, in C kg⁻¹. For an ion use the net charge and mass ≈ A × 1.67 × 10⁻²⁷ kg.
- Range and nature of the strong nuclear force
- Acts between nucleons. Attractive up to about 3 fm, repulsive below about 0.5 fm (which stops the nucleus collapsing).
- Why was the neutrino proposed?
- Beta particles are emitted with a range of energies, so to conserve energy another particle had to carry the rest.
- Annihilation
- Particle + antiparticle → two photons (two, to conserve momentum). Minimum energy of each photon = the rest energy of one particle.
- Pair production
- One photon → particle + antiparticle. Minimum photon energy = 2 × the rest energy of the particle; any extra becomes kinetic energy.
- Exchange particles for the electromagnetic, weak and strong (nucleon) interactions
- Electromagnetic: virtual photon. Weak: W⁺ or W⁻. Strong between nucleons: pion.
- Hadron vs lepton
- Hadrons feel the strong interaction; leptons do not.
- Baryon vs meson
- Baryon: three quarks (baryon number B = ±1). Meson: a quark and an antiquark (B = 0).
- Charge Q, baryon number B and strangeness S of the u, d and s quarks
- u: Q = +⅔, B = +⅓, S = 0. d: Q = −⅓, B = +⅓, S = 0. s: Q = −⅓, B = +⅓, S = −1. Antiquarks: reverse every sign.
- Quark change in β⁻ decay and in β⁺ decay
- β⁻: d → u (n → p + e⁻ + ν̄ₑ). β⁺: u → d (p → n + e⁺ + νₑ).
- Which quantities are conserved in particle interactions, and which can change in the weak interaction?
- Charge, baryon number, and the lepton numbers Lₑ and L_μ (separately) are always conserved, as are energy and momentum. Strangeness is conserved in the strong interaction but can change by 0 or ±1 in the weak interaction.
- How are strange particles produced, and how do they decay?
- Produced in pairs by the strong interaction; they decay by the weak interaction.
3.2.2 Electromagnetic radiation and quantum phenomena
- Photoelectric equation, threshold frequency and stopping potential
- hf = ϕ + Ek(max), where ϕ is the work function. Threshold frequency f₀ = ϕ/h. Stopping potential: eVs = Ek(max).
- Photoelectric effect: what intensity changes vs what frequency changes
- Intensity sets the number of photoelectrons per second; frequency sets Ek(max). Below the threshold frequency there is no emission at any intensity.
- Light of photon energy 3.0 eV falls on a metal with work function 2.3 eV. Find Ek(max) and the stopping potential.
- Ek(max) = 3.0 − 2.3 = 0.7 eV, so the stopping potential is 0.7 V.
- Excitation vs ionisation
- Excitation: an electron moves up to a higher energy level. Ionisation: an electron is removed from the atom.
- How does a fluorescent tube produce visible light?
- Electrons collide with mercury atoms and excite them. The mercury atoms de-excite, emitting UV photons. The phosphor coating absorbs the UV and emits visible light.
- Energy of a photon emitted when an electron falls between two levels
- hf = E₁ − E₂ (convert the levels to joules before using h). Line spectra are evidence for discrete energy levels.
- de Broglie wavelength, and the evidence for wave-particle duality
- λ = h/mv. Electron diffraction shows particles behaving as waves; the photoelectric effect shows light behaving as particles.
- Electron diffraction: effect of increasing the electrons' speed
- Greater momentum → shorter de Broglie wavelength → less diffraction, so the rings are smaller.