Modern Physics Applications (AP Physics 2): Particles, Materials, and Devices

Standard Model

Best-tested framework describing fundamental particles and three forces (electromagnetic, weak, strong); it does not include a complete, experimentally confirmed theory of gravity.

Fundamental particle

A particle treated as indivisible (as far as current evidence shows); examples in the Standard Model include quarks and leptons.

Quark

A fundamental matter particle that combines with other quarks to form hadrons (such as protons and neutrons) via the strong force.

Lepton

A fundamental matter particle that does not feel the strong force; includes electrons and neutrinos.

Hadron

A composite particle made of quarks bound together by the strong interaction.

Baryon

A hadron made of three quarks (or three antiquarks); protons and neutrons are baryons.

Meson

A hadron made of a quark–antiquark pair.

Antiparticle

A partner to a particle with the same mass but opposite charge (and other opposite quantum numbers), e.g., positron is the antiparticle of the electron.

Annihilation

Process in which a particle and its antiparticle meet and convert their mass energy into other forms (often photons), while conserving total energy.

Mass–energy equivalence (E = mc^2)

Relationship showing mass is a form of energy: energy E equals mass m times the speed of light squared c^2.

Boson (force carrier)

Particle exchanged in interactions to mediate forces (e.g., photon for electromagnetic, gluon for strong, W/Z for weak).

Photon

Boson that mediates the electromagnetic force; also a quantum (packet) of light.

Gluon

Boson that mediates the strong force that binds quarks inside hadrons.

W and Z bosons

Bosons that mediate the weak interaction, responsible for processes like beta decay where particle types can change.

Conservation of electric charge

Rule that total electric charge before a reaction/decay equals total electric charge after.

Baryon number conservation

Rule that total baryon number is conserved; baryons count as +1 and antibaryons as −1.

Lepton number conservation

Rule that total lepton number is conserved; electrons and electron-neutrinos count as +1 and their antiparticles as −1.

Beta minus (β−) decay

Weak decay in which a neutron becomes a proton while emitting an electron and an electron antineutrino: n → p + e− + ν̅e.

Neutrino

Electrically neutral lepton that interacts very weakly with matter; included in decay equations to satisfy conservation of energy and lepton number.

Electron-volt (eV)

Energy unit commonly used in atomic/particle physics; 1 eV = 1.60 × 10^−19 J.

Semiconductor

Material with conductivity between a conductor and an insulator; conductivity is tunable and explained using quantum energy bands.

Band gap (Eg)

Forbidden energy range between the valence and conduction bands; its size helps determine whether a material is a conductor, semiconductor, or insulator.

Hole

A vacancy left in the valence band when an electron is excited; behaves like a positive charge carrier in semiconductors.

Doping

Deliberately adding small amounts of impurities to a semiconductor to increase and control the number of charge carriers.

p–n junction

Interface between p-type and n-type semiconductor regions; forms a depletion region and is the basis of diodes, LEDs, photodiodes, and solar cells.