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OpenStem
@openstem · Joined Jul 2026
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Flashcards7 cards
What is the Coulomb barrier problem in stellar fusion, and how does quantum tunnelling resolve it?1 / 7
Classically, two positively charged nuclei need enough kinetic energy to overcome their mutual electrostatic repulsion before the short-range strong force can bind them — but typical thermal energies in stellar cores (~1 keV) are far below the MeV-scale Coulomb barrier. Quantum tunnelling allows a nonzero probability of fusion even when the classical kinetic energy is insufficient, with the tunnelling probability given by the Gamow factor — exponentially sensitive to energy, but nonzero even far below the barrier.
Physics · L5 · Nuclear Astrophysics — Stellar Nucleosynthesis
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Physics · L5 · Nuclear Astrophysics — Stellar NucleosynthesisFlashcards7 cards
How does quantum optics treat a single mode of the electromagnetic field, in terms of familiar quantum mechanics?1 / 7
Each mode of the quantised EM field is mathematically equivalent to a quantum harmonic oscillator, with photon number states |n⟩ playing the role of oscillator energy eigenstates, and raising/lowering (creation/annihilation) operators ↠and â that add or remove one photon of energy ℏω.
Physics · L5 · Quantum Optics — Coherent States & Cavity QED
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Physics · L5 · Quantum Optics — Coherent States & Cavity QEDQuiz3 questions
You open a drawer. Is that a push or a pull?
APushBPullCBoth at onceDNeither
Physics · L1 · Push, Pull, and Magnets
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Physics · L1 · Push, Pull, and MagnetsQuiz3 questions
What causes a shadow to form?
ALight bends around objectsBAn object blocks lightCSound bounces off wallsDThe Sun moves
Physics · L1 · Light and Sound
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Physics · L1 · Light and SoundQuiz4 questions
What does a thermometer measure?
AHow heavy something isBHow hot or cold something isCHow loud something isDHow big something is
Physics · L1 · Heat & Temperature
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Physics · L1 · Heat & TemperatureQuiz4 questions
What does a battery do in a circuit?
AIt blocks electricity from flowingBIt pushes electricity around the loopCIt only works when the bulb is brokenDIt makes the wires longer
Physics · L1 · Batteries & Circuits
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Physics · L1 · Batteries & CircuitsQuiz4 questions
What is gravity?
AA force that pulls things down toward the EarthBA kind of windCA force that pushes things upDSomething only astronauts feel
Physics · L1 · Gravity & Falling
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Physics · L1 · Gravity & FallingQuiz4 questions
What do we call it when light bounces off a surface?
AShadowingBReflectionCMeltingDAbsorbing
Physics · L1 · Mirrors & Reflections
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Physics · L1 · Mirrors & ReflectionsQuiz4 questions
A heavy steel ship floats, but a small heavy stone sinks. What is the main reason?
AThe ship weighs less than the stoneBThe ship's hollow, spread-out shape makes it float, even though it is heavyCSteel is not really a metalDThe stone is made of a magic material
Physics · L1 · Floating & Density
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Physics · L1 · Floating & DensityQuiz4 questions
What makes a drum sound louder?
AHitting it harder, so it vibrates moreBPainting it a different colourCMaking it colderDTurning it upside down
Physics · L1 · Sound & Music
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Physics · L1 · Sound & MusicQuiz4 questions
Why is a rock a solid?
ABecause it keeps its own shape wherever you put itBBecause it takes the shape of its containerCBecause it spreads out to fill a roomDBecause you cannot see it
Physics · L1 · Solids, Liquids & Gases
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Physics · L1 · Solids, Liquids & GasesQuiz4 questions
When is a shadow usually at its shortest?
AEarly in the morningBAround middayCLate in the eveningDAt midnight
Physics · L1 · Shadows Through the Day
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Physics · L1 · Shadows Through the DayQuiz4 questions
On a bar magnet, where is the pulling force strongest?
AExactly in the middleBAt the two ends (the poles)CNowhere — it pulls the same everywhereDOnly on the flat sides
Physics · L1 · Magnets: A Closer Look
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Physics · L1 · Magnets: A Closer LookQuiz4 questions
What does a simple machine need to work?
ABatteriesBAn engine or motorCEffort from a personDAn electrical plug
Physics · L1 · Simple Machines
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Physics · L1 · Simple MachinesQuiz4 questions
In tug of war, both teams pull exactly as hard as each other. What happens to the rope?
AIt moves toward the stronger teamBIt stays stillCIt moves toward both teams at onceDIt snaps in half
Physics · L1 · Balanced & Unbalanced Forces
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Physics · L1 · Balanced & Unbalanced ForcesQuiz4 questions
A swing is pulled back and held still in the air. What kind of energy does it have?
ANo energy at allBKinetic (moving) energyCPotential (stored-up) energyDSound energy
Physics · L1 · Energy in Motion
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Physics · L1 · Energy in MotionQuiz4 questions
Which of these is usually the fastest?
AA snailBA walking personCA carDA crawling baby
Physics · L1 · Fast & Slow
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Physics · L1 · Fast & SlowQuiz4 questions
Which one will spring back to its original shape after you stretch or squish it: a rubber band or a stick of clay?
AA stick of clayBA rubber bandCBoth doDNeither does
Physics · L1 · Springs & Stretchy Things
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Physics · L1 · Springs & Stretchy ThingsQuiz4 questions
On a balance scale, what happens to the side holding the heavier object?
AIt goes upBIt goes downCIt spins aroundDNothing happens
Physics · L1 · Weighing & Balancing
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Physics · L1 · Weighing & BalancingQuiz4 questions
What is white light, like sunlight, actually made of?
AJust one plain color with no others hidden insideBA mix of all the rainbow colors blended togetherCOnly red and blue lightDNo color at all
Physics · L1 · Rainbows & Mixing Colors
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Physics · L1 · Rainbows & Mixing ColorsQuiz6 questions
What is surface tension?
AA stretchy, skin-like pull at the top of waterBA force that only happens in soapCThe temperature of waterDA kind of wind over water
Physics · L1 · Bubbles & Surface Tension
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Physics · L1 · Bubbles & Surface TensionQuiz6 questions
What is friction?
AA rubbing force that slows down sliding thingsBA force that only exists in spaceCA kind of lightDA force that speeds things up
Physics · L1 · Friction: Why Things Slow Down
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Physics · L1 · Friction: Why Things Slow DownQuiz6 questions
What is a heat conductor?
AA material that lets heat move through it easilyBA material that stops all heatCA material that makes its own heatDA material that is always cold
Physics · L1 · Keeping Warm: Insulators & Conductors of Heat
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Physics · L1 · Keeping Warm: Insulators & Conductors of HeatQuiz6 questions
What is a light source?
ASomething that makes its own lightBAnything you can seeCAnything that is bright coloredDOnly the Sun
Physics · L1 · Where Light Comes From: Sources vs Reflections
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Physics · L1 · Where Light Comes From: Sources vs ReflectionsQuiz6 questions
What is a pendulum?
AAnything that hangs from a point and swings freely back and forthBAny object that is roundCA type of lightDAnything that floats on water
Physics · L1 · Swings & Pendulums
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Physics · L1 · Swings & PendulumsQuiz6 questions
What happens when you rub a balloon on your hair?
ANothing changes at allBThe balloon and your hair pick up opposite chargesCThe balloon becomes magneticDYour hair disappears
Physics · L1 · Static Electricity: Balloons & Hair
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Physics · L1 · Static Electricity: Balloons & HairQuiz6 questions
What is wind?
AAir that is movingBA kind of lightCWater vapor onlyDA force that only exists near the ocean
Physics · L1 · Wind & Moving Air
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Physics · L1 · Wind & Moving AirQuiz6 questions
What shape is the glass in a typical magnifying glass?
APerfectly flat, like a windowBThicker in the middle and thinner at the edgesCThinner in the middle and thicker at the edgesDA perfect cube
Physics · L1 · Magnifying Glasses & Simple Lenses
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Physics · L1 · Magnifying Glasses & Simple LensesQuiz3 questions
A cyclist travels 30 km in 2 hours. What is the cyclist's speed?
A60 km/hB15 km/hC32 km/hD28 km/h
Physics · L2 · Speed, Distance, and Time
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Physics · L2 · Speed, Distance, and TimeQuiz3 questions
A boulder sitting on a cliff has which type of energy?
AKinetic energyBSound energyCGravitational potential energyDChemical energy
Physics · L2 · Energy Forms and Changes
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Physics · L2 · Energy Forms and Changes