OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Note~395 words · 2 min

Flux, and why it's the thing that matters Induction needs a changing magnetic flux. Not a strong field, a changing one. Faraday's law Three ways to make dΦ/dt non-zero, and every generator uses one of them. Lenz's law: reading the minus sig

Physics

Physics · L4 · Electromagnetic Induction

@openstem

Note~262 words · 1 min

One curve, two independent problems The trick is refusing to treat it as a curved-path problem. It's two straight-line problems running at the same time, and they don't talk to each other. Split the launch velocity The three formulas Worked

Physics

Physics · L3 · Projectile Motion: Range, Height, and Time of Flight

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Note~234 words · 1 min

Torque is a twist A force applied off-centre from a pivot doesn't just push, it turns. Doors, wrenches and see-saws all run on this. Three dials, all of them matter Worked: 150 N on a wrench 0.25 m from the bolt. Rotational equilibrium Work

Physics

Physics · L3 · Torque & Rotational Equilibrium

@openstem

Note~387 words · 2 min

What makes motion simple harmonic One condition: the restoring force pulling the object back is proportional to how far it's been displaced. A spring obeying Hooke's law does this exactly. A pendulum does it approximately, and only for smal

Physics

Physics · L3 · Simple Harmonic Motion: Springs & Pendulums

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Note~320 words · 2 min

Putting a number on the push or pull You know the rule already: like charges repel, opposites attract. Coulomb's law says exactly how hard. Inverse square, again Same mathematical shape as Newton's law of gravitation. Double the distance an

Physics

Physics · L3 · Electric Fields & Coulomb's Law

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Note~355 words · 2 min

This force ignores charges that sit still An electric force acts on a charge whether it's parked or racing. A magnetic force is fussier: no motion, no force, however strong the field. Direction: the right-hand rule F comes out perpendicular

Physics

Physics · L3 · Magnetic Force on Moving Charges

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Note~231 words · 1 min

How much heat does a temperature change cost? Specific heat is a material property It's the energy to lift 1 kg by 1°C. Water is the outlier. Worked: heating a pot of water 2 kg of water from 20°C to 80°C. Calorimetry is conservation of ene

Physics

Physics · L3 · Specific Heat & Calorimetry

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Note~394 words · 2 min

Two shapes, two behaviours A lens is curved transparent material that bends light on purpose. The curvature means rays hitting different parts get bent by different amounts, and that's what forms an image instead of just letting light throu

Physics

Physics · L3 · Lenses & Optical Instruments

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Note~232 words · 1 min

The passing siren Why motion shifts the pitch Moving away stretches the same crests out behind it, lowering the pitch. The formula Minus while approaching. Flip to plus while receding: Worked, with a 700 Hz siren at 30 m/s and sound at 343

Physics

Physics · L3 · The Doppler Effect: Why a Passing Siren Changes Pitch

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Note~379 words · 2 min

Equal, opposite, and on two different objects Push on something and it pushes back on you exactly as hard. That's the law. The part people miss is the last clause. Same force, wildly different result A bullet and a recoiling rifle feel forc

Physics

Physics · L3 · Newton's Third Law

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Note~342 words · 2 min

Five quantities, four equations Straight line, constant acceleration. Five letters describe the whole motion. Each equation leaves one letter out. That's the feature, not a flaw. Choosing the right equation Three worked examples The braking

Physics

Physics · L3 · 1D Kinematics: The SUVAT Equations

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Note~270 words · 1 min

Potential is energy per charge A charge in an electric field has potential energy, just like a mass has gravitational potential energy. Divide out the charge and you get potential, better known as voltage. Capacitors Two conductive plates w

Physics

Physics · L3 · Electric Potential & Capacitors

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Note~347 words · 2 min

Current is moving charge, so a wire feels the force too A moving charge in a magnetic field feels a push. A current is an enormous number of moving charges. So the whole wire gets pushed, hard enough to move it. For direction: point your fi

Physics

Physics · L3 · Magnetic Force on Current-Carrying Wires

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Note~269 words · 1 min

First law: energy bookkeeping Nothing is created or destroyed. The first law is conservation of energy with heat and work spelled out. Second law: heat picks a direction The first law alone would happily allow a cold object to warm a hot on

Physics

Physics · L3 · The Laws of Thermodynamics & Heat Engines

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Note~288 words · 1 min

Continuity: the fluid has nowhere else to go Incompressible fluid in a pipe can't pile up or vanish partway along. So the same volume must cross every section every second. That's why a hose sprays further when you squeeze the end with your

Physics

Physics · L3 · Fluid Dynamics: Continuity & Bernoulli's Principle

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Note~366 words · 2 min

Run the relationship backwards So far, currents made magnetic fields. Induction goes the other way: a changing magnetic field creates a current in a nearby loop, with no battery anywhere. Lenz's law: it fights back That minus sign is a whol

Physics

Physics · L3 · Electromagnetic Induction: Faraday's & Lenz's Laws

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Note~239 words · 1 min

A wave that stops travelling They alternate along the string, spaced half a wavelength apart. Below is the superposition that builds them. Drag the controls and watch two waves add: where they cancel you get a node, where they reinforce you

Physics

Physics · L3 · Standing Waves & Resonance

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Note~451 words · 2 min

What actually travels Energy moves. Matter doesn't. Each bit of the medium oscillates about its own spot and hands the disturbance on. The quantities, and what each is for Superposition: waves just add Two waves in the same place don't coll

Physics

Physics · L4 · Wave mechanics: interference, diffraction, and standing waves

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Note~361 words · 2 min

Four laws, in one breath First law: bookkeeping The four standard processes Second law: entropy, and why heat has a direction Entropy counted properly Boltzmann's formula says entropy is really a count of how many microscopic arrangements p

Physics

Physics · L4 · Thermodynamics: the four laws

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Note~341 words · 2 min

The shift you have to make Classical mechanics tells you where a particle is. Quantum mechanics tells you the probability of finding it there. That's the whole change, and everything else follows from it. Wave-particle duality de Broglie's

Physics

Physics · L4 · Introduction to quantum mechanics

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Note~373 words · 2 min

Two postulates, and everything else follows The second one is the troublemaker. Hold onto it and time and length have to give way instead. The Lorentz factor runs the whole show At everyday speeds γ sits so close to 1 that nothing looks unu

Physics

Physics · L4 · Special relativity: time dilation, length contraction, and E=mc²

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Note~356 words · 2 min

Torque: force, but for spinning Force changes how fast something moves in a line. Torque changes how fast it spins. Where and at what angle you push matters as much as how hard. Moment of inertia: rotational mass I plays the role mass plays

Physics

Physics · L4 · Rotational Dynamics, Torque & Angular Momentum

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Note~412 words · 2 min

Two equations run most of fluid flow Use them in that order, every time. Geometry fixes the speed, then Bernoulli converts that speed into a pressure. Continuity That's pure mass conservation. It says nothing about pressure or viscosity and

Physics

Physics · L4 · Fluid Mechanics: Bernoulli's equation, continuity, and viscous flow

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Note~336 words · 2 min

What makes motion simple harmonic One condition: the restoring force is proportional to displacement and points back toward equilibrium. F = −kx. Everything else follows. Frequency comes from the system, not the swing Pull the pendulum furt

Physics

Physics · L4 · Oscillations: SHM, Damping, and Resonance

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Note~356 words · 2 min

Why AC needs more than Ohm's law An AC source pushes a voltage that swings sinusoidally. A resistor doesn't care. A capacitor and an inductor care a great deal, and they care in opposite directions. That φ is the whole complication: current

Physics

Physics · L4 · AC Circuits: Impedance and RLC Resonance

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Note~406 words · 2 min

The ray picture, and when it's enough Treat light as straight lines that bend at a lens and bounce off a mirror. That's all you need whenever the optics are much bigger than a wavelength. Slits and gratings are where this breaks down and wa

Physics

Physics · L4 · Geometric Optics: Lenses, Mirrors, and Ray Diagrams

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Note~428 words · 2 min

Entropy, defined properly "Heat flows hot to cold" is the slogan. Here's the number behind it. The Clausius inequality Equality only for a reversible cycle. Apply it to an isolated system and you get the second law as a number. Friction, mi

Physics

Physics · L4 · Entropy, heat engines, and the Carnot cycle

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Note~499 words · 2 min

Three ways a nucleus sheds trouble An unstable nucleus has either too much energy or the wrong neutron-to-proton ratio. It emits something to fix that. Half-life A single nucleus decays at random. A trillion of them decay on a curve you can

Physics

Physics · L4 · Nuclear physics: radioactivity, decay, and binding energy

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Note~371 words · 2 min

Four equations, one prediction nobody asked for Maxwell gathered everything known about electricity and magnetism into four statements, fixed an inconsistency in one of them, and out fell a wave travelling at the speed of light. 1. Gauss's

Physics

Physics · L4 · Maxwell's Equations & Electromagnetic Waves

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Note~459 words · 2 min

Two consequences of light being transverse Light's electric field oscillates across the direction of travel, not along it. Ask which way it points and you get polarization. Ask what happens when many slits interfere and you get diffraction

Physics

Physics · L4 · Wave Optics: Polarization & Diffraction Gratings

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