Physics (university) glossary
103 key terms from Physics (university), A to Z. Each is explained in one plain sentence and links to its free lesson.
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A
- AccelerationMotion and vectors
- How fast your velocity changes.
- Angular momentumRotation and gravity
- Rotational momentum, L = I omega.
- Angular velocityRotation and gravity
- How fast something turns, measured in radians per second.
B
- Bohr energy levelsRelativity and quantum physics
- Electrons in hydrogen have fixed energies, E = -13.6 eV / n squared.
C
- CapacitanceElectric fields and potential
- The charge stored per volt, C = Q/V, in farads.
- Centripetal accelerationRotation and gravity
- The inward acceleration that keeps something on a circle, a = v squared / r.
- Centripetal forceRotation and gravity
- The net inward force needed for circular motion, F = m v squared / r.
- Circular motion in a fieldCircuits and magnetism
- A charge moving at right angles to a uniform field follows a circle of radius r = m v / (q B).
- CollisionsWork, energy and momentum
- Meetings where momentum is conserved.
- Conductors in equilibriumElectric fields and potential
- In a conductor at rest the electric field inside is zero and extra charge sits on the surface.
- Conservation of energyWork, energy and momentum
- Energy is never created or destroyed.
- Constant-acceleration equationsMotion and vectors
- Three formulas that link position, velocity, acceleration and time when acceleration does not change.
- Coulomb's lawElectric fields and potential
- The force between two point charges is F = k q1 q2 / r squared, with k = 8.99e9 N m squared per C squared.
D
- de Broglie wavelengthRelativity and quantum physics
- Matter has a wavelength, lambda = h/p.
- DisplacementMotion and vectors
- The straight-line change in position from start to finish, with a direction.
- Doppler effectOscillations and waves
- The pitch you hear changes when source and listener move toward or away from each other.
- Double-slit interferenceInduction, light and optics
- Light through two narrow slits makes bright and dark fringes.
E
- Electric chargeElectric fields and potential
- A property of matter that comes in two kinds, positive and negative.
- Electric currentCircuits and magnetism
- The rate that charge flows past a point, I = Q/t.
- Electric fieldElectric fields and potential
- The force per unit of positive test charge at a point, E = F/q.
- Electric potentialElectric fields and potential
- Potential energy per unit charge, V = U/q.
- Electric powerCircuits and magnetism
- Rate of energy use in a circuit, P = IV = I squared R = V squared / R.
- Electromagnetic spectrumInduction, light and optics
- The family of EM waves ordered by frequency: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.
- Electromagnetic waveInduction, light and optics
- A wave of oscillating electric and magnetic fields that travels through empty space at c = 3.00e8 m/s.
- EntropyThermodynamics
- A measure of disorder, or how many ways energy can be spread out.
- Escape velocityRotation and gravity
- The launch speed needed to leave a planet and never fall back, v = sqrt(2 G M / R), ignoring air.
F
- Faraday's lawInduction, light and optics
- A changing magnetic flux creates a voltage (emf) in a loop: emf = N x change in flux / change in time.
- Field linesElectric fields and potential
- Lines that show the direction of the field.
- Field of a long straight wireCircuits and magnetism
- A current makes circular field lines around the wire.
- First law of thermodynamicsThermodynamics
- Change in internal energy equals heat added minus work done by the system: change in U = Q - W.
- Force on a current-carrying wireCircuits and magnetism
- A wire carrying current I in a field B feels force F = B I L sin theta.
- Free fallMotion and vectors
- Motion with only gravity acting.
- Free-body diagramNewton's laws and forces
- A sketch of one object with arrows for every force acting on it.
- FrictionNewton's laws and forces
- A force along a surface that resists sliding.
G
- Gauss's lawElectric fields and potential
- The total electric flux through a closed surface equals the enclosed charge divided by epsilon zero.
- Gravitational potential energyWork, energy and momentum
- Stored energy due to height, PE = mgh.
H
- Heat enginesThermodynamics
- A machine that turns heat into work.
- Heat transferThermodynamics
- Heat moves by conduction (through touching matter), convection (moving fluid) and radiation (electromagnetic waves).
I
- Ideal gas lawThermodynamics
- For a gas, PV = nRT, with pressure in pascals, volume in cubic metres and temperature in kelvin.
- ImpulseWork, energy and momentum
- The change in momentum caused by a force acting over time: F times time equals change in p.
- Inclined planeNewton's laws and forces
- A ramp problem.
- InductanceInduction, light and optics
- A coil's resistance to changing current.
K
- Kepler's third lawRotation and gravity
- The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.
- Kinetic energyWork, energy and momentum
- The energy of motion, KE = 1/2 m v squared.
- Kinetic theoryThermodynamics
- Gas pressure comes from molecules hitting the walls.
- Kirchhoff's junction ruleCircuits and magnetism
- At a junction, the current going in equals the current going out, because charge is conserved.
- Kirchhoff's loop ruleCircuits and magnetism
- Around any closed loop, the voltage rises equal the voltage drops, because energy is conserved.
L
- Latent heatThermodynamics
- Energy absorbed or released when a substance changes state without changing temperature.
- Length contractionRelativity and quantum physics
- Objects moving fast are shorter along the direction of motion, by the factor 1/gamma.
- Lenz's lawInduction, light and optics
- The induced current flows in the direction that opposes the change in flux that made it.
M
- Magnetic fieldCircuits and magnetism
- A region where moving charges and magnets feel force.
- Magnetic fluxInduction, light and optics
- How much magnetic field passes through a surface, flux = B A cos theta.
- Magnetic force on a chargeCircuits and magnetism
- A moving charge in a magnetic field feels a force F = q v B sin theta, at right angles to both.
- Mass on a springOscillations and waves
- A spring pulls with force F = -kx.
- Mass-energy equivalenceRelativity and quantum physics
- Mass is a form of energy, E = m c squared.
- Moment of inertiaRotation and gravity
- How hard an object is to spin.
- MomentumWork, energy and momentum
- Mass times velocity, p = mv.
N
- Net forceNewton's laws and forces
- The vector sum of every force on an object.
- Newton's first lawNewton's laws and forces
- An object keeps its velocity (including staying still) unless a net force acts on it.
- Newton's second lawNewton's laws and forces
- Net force equals mass times acceleration (F = ma).
- Newton's third lawNewton's laws and forces
- When object A pushes on object B, B pushes back on A with an equal and opposite force.
- Normal forceNewton's laws and forces
- The push a surface gives back, at right angles to it, when something presses on it.
O
- Ohm's lawCircuits and magnetism
- For many conductors, voltage equals current times resistance, V = IR.
- Orbital speedRotation and gravity
- The speed that keeps a satellite in circular orbit, v = sqrt(G M / r).
P
- Period and frequencyOscillations and waves
- Period is the time for one full cycle.
- Photoelectric effectRelativity and quantum physics
- Light above a threshold frequency knocks electrons out of a metal.
- PhotonRelativity and quantum physics
- A packet of light with energy E = h f = h c / lambda, where h = 6.63e-34 J s.
- Postulates of relativityRelativity and quantum physics
- Einstein's two ideas: the laws of physics are the same for everyone moving at constant velocity, and light in a vacuum has the same speed c for all observers.
- Potential differenceElectric fields and potential
- Voltage between two points.
- PowerWork, energy and momentum
- How fast energy is transferred, P = work / time.
- Projectile motionMotion and vectors
- Motion of a thrown object under gravity alone.
R
- Radioactive half-lifeRelativity and quantum physics
- The time for half of a radioactive sample to decay.
- RC circuitCircuits and magnetism
- A resistor and a capacitor together.
- Relative velocityMotion and vectors
- The velocity of one object as seen from another moving object.
- ResonanceOscillations and waves
- A big response when a driving force matches an object's natural frequency.
- Rotational second lawRotation and gravity
- Net torque equals moment of inertia times angular acceleration, tau = I alpha.
S
- Series and parallel resistorsCircuits and magnetism
- In series, resistances add.
- Simple harmonic motionOscillations and waves
- Back-and-forth motion where the restoring force is proportional to the displacement and points back to the middle.
- Simple pendulumOscillations and waves
- A mass on a string swinging through small angles.
- Snell's lawInduction, light and optics
- When light crosses between materials its direction bends: n1 sin theta1 = n2 sin theta2.
- Sound intensity levelOscillations and waves
- Loudness measured in decibels (dB), a log scale where +10 dB means ten times the intensity.
- Specific heatThermodynamics
- The energy needed to raise 1 kg of a material by 1 K.
- Standing wavesOscillations and waves
- A fixed pattern of nodes and antinodes made by two waves moving in opposite directions.
- SuperpositionOscillations and waves
- When waves meet, their displacements add.
T
- TemperatureThermodynamics
- A measure of the average kinetic energy of the particles in something.
- TensionNewton's laws and forces
- The pulling force along a rope or string.
- Thermal expansionThermodynamics
- Most solids lengthen when heated by an amount change in L = alpha x L x change in T.
- Thin lens equationInduction, light and optics
- Lenses form images with 1/f = 1/object distance + 1/image distance.
- Time dilationRelativity and quantum physics
- A moving clock runs slow as seen by a stationary observer, by the factor gamma = 1/sqrt(1 - v squared / c squared).
- TorqueRotation and gravity
- The turning effect of a force, torque = r F sin theta.
- Total internal reflectionInduction, light and optics
- Light going from a dense to a less dense material reflects completely beyond the critical angle, sin theta c = n2/n1.
- TransformerInduction, light and optics
- Two coils on one iron core change AC voltage.
- Transverse and longitudinal wavesOscillations and waves
- In a transverse wave the medium moves at right angles to the wave direction.
U
- Uncertainty principleRelativity and quantum physics
- You cannot know both a particle's position and momentum exactly: change in x times change in p is at least h-bar / 2.
- Uniform field between platesElectric fields and potential
- Between two parallel charged plates the field is steady and equals voltage divided by plate gap, E = V/d.
- Universal gravitationRotation and gravity
- Every two masses attract with a force F = G M m / r squared, where G is 6.67e-11 in SI units.
V
- Vector componentsMotion and vectors
- A vector has size and direction.
- VelocityMotion and vectors
- How fast your position changes, with a direction.
W
- Wave functionRelativity and quantum physics
- A mathematical description of a quantum particle.
- Wave speedOscillations and waves
- For any wave, speed equals frequency times wavelength: v = f lambda.
- WeightNewton's laws and forces
- The pull of gravity on an object, found with W = mg.
- WorkWork, energy and momentum
- The energy a force transfers when it moves something.
- Work-energy theoremWork, energy and momentum
- The net work done on an object equals its change in kinetic energy.