OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Note~351 words · 2 min

Moving charge is the source of B Electrostatics starts with charge. Magnetostatics starts with charge in motion. Two laws describe it, at different levels of effort. The Biot-Savart law Integrate along the whole current path. It's Coulomb's

Physics

Physics · L4 · Magnetism: the Biot–Savart law and Ampere's law

@openstem

Note~286 words · 1 min

Two conservation laws are enough However tangled the circuit, charge is conserved at every junction and energy is conserved around every loop. Those two facts solve all of it. Series and parallel RC charging Put a resistor, a capacitor and

Physics

Physics · L4 · DC circuits: Kirchhoff's laws and RC transients

@openstem

Note~350 words · 2 min

The bridge this note builds PV = nRT is a lab result. Kinetic theory derives it from molecules bouncing around, and in doing so tells you what temperature actually is. Pressure is just a lot of small collisions Model molecules as points bou

Physics

Physics · L4 · Kinetic theory of gases

@openstem

Note~345 words · 2 min

One law, three centuries of orbits Gravitational potential energy Circular orbits Gravity supplies precisely the centripetal force the circle demands. Set them equal: Escape velocity Set total mechanical energy to zero: just enough kinetic

Physics

Physics · L4 · Gravitation and orbital mechanics

@openstem

Note~342 words · 2 min

Separate the load from the shape A thick steel cable stretches less than a thin one under the same pull. That's geometry, not material. Stress and strain divide the geometry out so you can compare materials directly. Hooke's law for materia

Physics

Physics · L4 · Elasticity: stress, strain, and Young's modulus

@openstem

Note~360 words · 2 min

Sound is a pressure wave, and it needs stuff Longitudinal compressions and rarefactions travelling through a medium. No medium, no sound. Speed is set by the balance of stiffness and inertia. Decibels Your ear handles a range of about 10¹³

Physics

Physics · L4 · Sound waves, intensity, and the Doppler effect

@openstem

Note~382 words · 2 min

Three mechanisms, and how to tell them apart Heat is energy in transit down a temperature gradient. It gets there three ways, and most real situations run all three at once. Conduction: Fourier's law Convection Radiation Specific heat and l

Physics

Physics · L4 · Heat transfer: conduction, convection, and radiation

@openstem

Note~345 words · 2 min

What a capacitor stores Not charge, exactly: the plates hold +Q and −Q, so the net charge is zero. What's stored is energy, in the electric field across the gap. One farad is enormous. Real capacitors run from picofarads in radio circuits t

Physics

Physics · L4 · Capacitance and dielectrics

@openstem

Note~313 words · 2 min

State space and Dirac notation A complete orthonormal basis {|n⟩} obeys the resolution of the identity: Any state can be expanded as |ψ⟩ = Σₙ cₙ|n⟩ with cₙ = ⟨n|ψ⟩, and Σₙ |cₙ|² = 1 (normalisation). Observables and the spectral theorem Obse

Physics

Physics · L5 · Quantum Mechanics: Hilbert Space and the Schrödinger Equation

@openstem

Note~272 words · 1 min

The canonical ensemble in one line Put a system in contact with a big reservoir at temperature T. Count the reservoir's accessible states microcanonically, take the thermodynamic limit, and the Boltzmann weight falls out. Z is the partition

Physics

Physics · L5 · Statistical Mechanics: Canonical Ensemble and Thermodynamics

@openstem

Note~296 words · 1 min

Gravity isn't a force here In GR, spacetime is a four-dimensional manifold carrying a metric tensor g_μν. Mass and energy curve it. Free particles then move as straight as they can through the curved geometry, and you call the result gravit

Physics

Physics · L5 · General Relativity: Metric, Geodesics, and the Line Element

@openstem

Note~478 words · 2 min

Bloch's theorem A conduction electron in a crystal sees a potential that repeats: U(r + a) = U(r) for every lattice vector a. That periodicity forces a specific form on the eigenstates. Where the gap comes from Both give the same answer fro

Physics

Physics · L5 · Condensed Matter Physics: Band Theory & Semiconductors

@openstem

Note~488 words · 2 min

The whole particle content Two families of spin-½ matter fermions, three generations each, plus the bosons that carry the forces. Only the first generation makes up anything you'll ever hold. Quarks and confinement Pull two quarks apart and

Physics

Physics · L5 · The Standard Model: Fermions, Bosons, and Hadrons

@openstem

Note~454 words · 2 min

What chaos actually means Three conditions together: aperiodic long-term behaviour, bounded trajectories, and sensitive dependence on initial conditions. Lorenz found this by rounding a number in a toy weather model and getting completely d

Physics

Physics · L5 · Nonlinear Dynamics and Chaos

@openstem

Note~379 words · 2 min

Why bother with a tensor In vector form, Maxwell is four equations about two fields, and you have to check Lorentz covariance one equation at a time. Special relativity is telling you there's a better object: one rank-2 tensor holding both

Physics

Physics · L5 · Covariant Electrodynamics: The Electromagnetic Field Tensor

@openstem

Note~410 words · 2 min

The order parameter Pick a quantity that's zero in the disordered high-symmetry phase and non-zero once the symmetry breaks. That's your order parameter, and it organises the whole subject. Two kinds of transition Critical exponents Near a

Physics

Physics · L5 · Phase Transitions and Critical Phenomena

@openstem

Note~450 words · 2 min

Fields first, particles second Ordinary quantum mechanics fixes the particle number and gives you a wavefunction on configuration space. QFT inverts that. The fundamental objects are field operators φ̂(x) at every spacetime point, and parti

Physics

Physics · L5 · Quantum Field Theory: Fields, Propagators, and Feynman Diagrams

@openstem

Note~414 words · 2 min

One assumption, one metric The cosmological principle: on large enough scales the universe is homogeneous (same everywhere) and isotropic (same in every direction). That single assumption forces the metric into one form. Everything dynamic

Physics

Physics · L5 · Cosmology: the Friedmann Equations and the Expanding Universe

@openstem

Note~381 words · 2 min

Two properties, not one Cooper pairing Cooper's insight: in a Fermi sea, an arbitrarily weak net attraction between two electrons near the Fermi surface binds them. Not a strong attraction. Any attraction at all. Individually the electrons

Physics

Physics · L5 · Superconductivity: BCS Theory and the Meissner Effect

@openstem

Note~402 words · 2 min

Qubits Entanglement A joint state that cannot be written as a product of two single-qubit states. The canonical example: Bell's theorem Einstein, Podolsky and Rosen argued in 1935 that quantum mechanics must be incomplete, and that pre-exis

Physics

Physics · L5 · Quantum Information: Entanglement and Bell's Theorem

@openstem

Note~502 words · 3 min

Global to local The Standard Model is built by one repeated move: take a global symmetry of a free theory and demand it hold locally, with a different transformation α(x) at every point. A free Lagrangian doesn't survive that demand, and fi

Physics

Physics · L5 · Gauge Theories and the Higgs Mechanism

@openstem

Note~397 words · 2 min

Ionised gas is not automatically a plasma A candle flame has free charges in it. What makes something a plasma is collective behaviour: each particle's motion is set by the self-consistent field of the whole ensemble, not by occasional clos

Physics

Physics · L5 · Plasma Physics: Debye Shielding and Collective Behaviour

@openstem

Note~377 words · 2 min

Sum over histories Canonical quantisation gives you operators. Schrödinger gives you a wave. Feynman gives you a third route: add up every path the particle could take, each weighted by a pure phase built from its classical action. Where th

Physics

Physics · L5 · The Path Integral Formulation of Quantum Mechanics

@openstem

Note~600 words · 3 min

Stars shouldn't burn, classically The Sun's core sits near 10⁷ K, giving thermal energies around 1 keV. The Coulomb barrier between two protons is measured in MeV. Classically, nothing fuses. It happens anyway, by tunnelling. The Gamow peak

Physics

Physics · L5 · Nuclear Astrophysics: How Stars Build the Elements

@openstem

Note~440 words · 2 min

Each field mode is a harmonic oscillator Quantise the electromagnetic field and every independent mode (fixed ω, wavevector, polarisation) turns out to be mathematically identical to one quantum harmonic oscillator. Coherent states: as clas

Physics

Physics · L5 · Quantum Optics: Coherent States and Cavity QED

@openstem

Plot1 plot · 1 param

Physics

Physics · L2 · Distance–Time Graph (Steady Speed)

@openstem

Plot1 plot · 1 param

Physics

Physics · L2 · Current vs Voltage (Fixed Resistance)

@openstem

Plot1 plot · 2 params

Physics

Simple Harmonic Motion: y = A·sin(ω·x)

@openstem

Plot1 plot · 2 params

Physics

Damped Oscillation: y = e^(−k·x)·sin(b·x)

@openstem

Plot1 plot · 1 param

Physics

Projectile Trajectory: y = x·tan(a) − x²/40

@openstem