Photoelectric effect, radioactivity, nuclear fission/fusion, quarks, leptons, fundamental forces · 48 terms
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A sub-atomic negatively-charged particle that orbits the nucleus.
The energy gained or lost by an electron moving through a potential difference of 1 V. 1 eV = 1.6×10⁻¹⁹ J.
A tiny unit used when dealing with atomic-scale energies — much more convenient than joules.
1 eV = 1.6×10⁻¹⁹ J. Know the conversion.
The emission of electrons from the surface of a metal when it is heated sufficiently.
Heat gives electrons enough energy to escape the metal surface — like boiling water into steam.
Used in cathode ray tubes and X-ray tubes.
Streams of fast electrons travelling from the cathode to the anode in an evacuated tube.
The emission of electrons from a metal surface when electromagnetic radiation of sufficient frequency falls on it.
Shine UV on zinc: electrons fly off instantly. This proved light comes in "packets" (photons).
Key points: instantaneous emission, threshold frequency exists, intensity doesn't help if f < f₀, Einstein explained it with photons.
A discrete "packet" of electromagnetic energy. E = hf.
Light behaves like a stream of particles (photons) as well as a wave.
E = hf = hc/λ. h = 6.63×10⁻³⁴ J s (Planck's constant)
The minimum frequency of radiation required to cause photoemission from a particular metal surface.
Below this frequency, no electrons are emitted no matter how intense the light — it's like a key that doesn't fit.
Each metal has a unique f₀. hf₀ = work function (φ)
The minimum energy required to remove an electron from the surface of a particular metal.
φ = hf₀. Einstein's equation: hf = φ + ½mv²_max
A fixed, discrete energy state that an electron can occupy in an atom.
Electrons can only exist at certain "rungs" of an energy ladder — they can jump up (absorb photon) or fall down (emit photon).
ΔE = hf when electron moves between levels.
Bright lines on a dark background produced when electrons in excited atoms fall to lower energy levels, emitting photons of specific frequencies.
Each element has a unique emission spectrum — a "fingerprint" of light.
High-frequency EM radiation produced when high-speed electrons are rapidly decelerated upon striking a metal target.
X-rays can penetrate soft tissue but are absorbed by bone — used in medical imaging.
The number of protons in the nucleus of an atom — defines which element it is.
Z = number of protons = number of electrons in neutral atom
The total number of protons and neutrons in the nucleus.
A = Z + N (N = neutron number)
Atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons).
Carbon-12 and Carbon-14 are isotopes — both have 6 protons, but C-14 has 2 extra neutrons.
Same Z, different A. Same chemical properties, different nuclear properties.
The spontaneous disintegration of the nucleus of certain atoms with the emission of one or more types of radiation.
The nucleus is unstable — it randomly decays to become more stable.
Spontaneous means it cannot be affected by chemical or physical conditions.
Fast-moving helium nuclei (²₄He) emitted from the nucleus of a radioactive atom.
Stopped by a sheet of paper or a few cm of air. Highly ionising.
Symbol: ⁴₂α or ²₄He. Atomic number decreases by 2, mass number by 4.
Fast-moving electrons emitted from the nucleus when a neutron converts to a proton.
Stopped by a few mm of aluminium. Moderately ionising.
Atomic number increases by 1, mass number unchanged.
High-frequency electromagnetic radiation emitted from the nucleus after alpha or beta decay.
Requires thick lead or concrete to stop. Least ionising per unit path.
No change to atomic or mass number — just energy released.
The number of nuclei decaying per second in a radioactive sample.
1 Bq = 1 decay per second. A = λN
The SI unit of radioactive activity. 1 Bq = 1 nuclear disintegration per second.
The time taken for half of the radioactive nuclei in a sample to decay.
After one half-life, 50% remains. After two, 25%. After ten, less than 0.1%.
N = N₀(½)^(t/T½). Each element has a fixed, unique half-life.
The activity of a radioactive sample is directly proportional to the number of undecayed nuclei remaining.
A = λN, N = N₀e^(-λt)
The process by which an atom gains or loses electrons, forming a charged ion.
Alpha particles cause intense ionisation in air — they knock electrons off atoms.
The splitting of a large nucleus (e.g. uranium-235) into two smaller nuclei of similar size, with the release of neutrons and large amounts of energy.
A neutron hits U-235: it splits, releases 2-3 more neutrons, which can trigger a chain reaction.
Chain reaction requires critical mass. Energy from E = Δmc².
The joining of two light nuclei to form a heavier nucleus, releasing enormous energy.
The sun's energy comes from hydrogen nuclei fusing to form helium. Clean but extremely difficult to control on Earth.
Requires enormous temperatures (millions °C). More energy per kg than fission.
The minimum mass of fissile material required to sustain a nuclear chain reaction.
A material (usually graphite or heavy water) in a nuclear reactor used to slow down fast neutrons so they can cause further fission reactions.
Fast neutrons fly past U-235 without causing fission — slowing them down makes them far more effective.
Rods made of neutron-absorbing material (e.g. boron) inserted into a nuclear reactor to control the rate of the chain reaction.
Push control rods in more → fewer neutrons → reaction slows. Pull out → speeds up.
The forced change of one element into another by altering the number of protons in the nucleus.
The four basic forces of nature: Gravitational, Electromagnetic, Weak Nuclear, Strong Nuclear.
GEMS: Gravity, Electromagnetic, weak (muon), Strong.
The weakest fundamental force — attracts all bodies with mass. Infinite range. Acts on all particles with mass.
Acts on all charged particles. Second strongest. Infinite range. Responsible for chemical bonding and light.
Responsible for beta decay — very short range (smaller than a nucleus). Second weakest.
Acts on all particles with mass. Range ≈ 10⁻¹⁸ m.
Binds protons and neutrons in the nucleus — the strongest force. Short range (≈ size of large nucleus).
Acts on hadrons. Range ≈ 10⁻¹⁵ m. Overcomes electromagnetic repulsion between protons.
Matter composed of antiparticles — identical to normal particles but with opposite charge.
A positron is the antiparticle of an electron — same mass, opposite charge.
When a particle meets its antiparticle: annihilation, releasing energy as gamma rays.
A particle identical to its counterpart in mass and spin but with opposite charge and quantum numbers.
The creation of a particle and its antiparticle simultaneously from electromagnetic energy (a gamma photon) near a nucleus.
Energy required: E = 2m₀c² minimum.
When a particle and its antiparticle meet and destroy each other, converting their mass entirely into two gamma photons.
PET scans in medicine use positron annihilation to create images.
Two gamma photons produced, each with energy = m₀c² minimum.
An accelerator that accelerates charged particles in a straight line using large potential differences.
An accelerator that uses magnets to curve charged particles into a circular path while electric fields accelerate them.
The Large Hadron Collider at CERN is a circular accelerator — 27 km circumference.
The family of all particles that have mass — includes leptons and hadrons.
Fundamental fermions that do not experience the strong nuclear force. Examples: electron, neutrino, muon.
6 leptons: electron, muon, tau, and their associated neutrinos.
Composite particles made of quarks — experience all four forces including the strong force.
Hadrons = baryons + mesons.
Hadrons made of three quarks. Examples: proton (uud), neutron (udd).
Proton: 2 up quarks + 1 down quark. Neutron: 1 up + 2 down.
Hadrons made of one quark and one antiquark. Examples: pion (ud̄), kaon (us̄).
Pion: up + anti-down quark. Short-lived.
Fundamental particles with fractional charge that combine to form hadrons. Six "flavours": up, down, strange, charm, top, bottom.
Quarks cannot exist alone — always found in combinations (hadrons).
Up quark: charge +2/3. Down quark: charge −1/3. Proton = uud = +2/3 +2/3 −1/3 = +1. Know this.
A lepton with zero charge and very tiny mass, produced in beta decay.
Billions of neutrinos pass through you every second from the sun — they interact so weakly they barely notice matter exists.
High-energy particles (mostly protons) originating from outside the solar system, mostly decaying in the upper atmosphere.