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Reference

Glossary

166 key words from both courses, in plain English, each linked to the lesson that explains it.

Absolute zero
The lowest possible temperature, 0 K or −273.15 °C, where molecular motion is as small as it can be.Learn it in: Temperature & Heat →
Acceleration
How fast velocity changes. Speeding up, slowing down and turning are all acceleration.a=ΔvΔta = \frac{\Delta v}{\Delta t}Learn it in: Acceleration →
Air resistance (drag)
The push of air against a moving object. It grows with speed and is why a feather falls slower than a hammer in air.Learn it in: Free Fall →
Alternating current (AC)
Current that keeps reversing direction, made by a spinning generator. Mains electricity is AC at 50 or 60 Hz.Learn it in: Electromagnetic Induction →
Amplitude
The size of an oscillation or wave: how far it swings from the middle.Learn it in: Springs & Oscillations →
Angular momentum
The 'quantity of spin': moment of inertia times angular velocity. With no outside torque it never changes.L=IωL = I\omegaLearn it in: Spinning & Angular Momentum →
Angular speed
How fast something turns, in radians per second. A point at radius r moves at v = rω.v=r ωv = r\,\omegaLearn it in: Circles & Radians →
Antiparticle
A particle with the same mass as another but opposite charge. A particle and its antiparticle annihilate into energy when they meet.Learn it in: Particles & the Standard Model →
Bernoulli's principle
In a moving fluid, where the flow is faster the pressure is lower. It's energy conservation for fluids.Learn it in: Flow & Bernoulli →
Buoyancy
The upward force a fluid exerts on an object in it, equal to the weight of the fluid the object pushes aside (Archimedes' principle).Fb=ρVgF_b = \rho V gLearn it in: Floating & Sinking →
Centrifugal force
The outward 'force' you seem to feel in a turning car. It isn't a real force: it's your inertia, trying to carry you straight on.Learn it in: Circular Motion →
Centripetal force
The net force toward the centre that keeps something moving in a circle. Tension, gravity or friction can provide it.F=mv2rF = \frac{m v^2}{r}Learn it in: Circular Motion →
Components
The sideways (x) and up-down (y) parts of a vector. Adding vectors is easy once you split them into components.vx=vcos⁡θ, vy=vsin⁡θv_x = v\cos\theta,\ v_y = v\sin\thetaLearn it in: Vectors: Arrows With Meaning →
Conservation of energy
In a closed system the total energy stays the same; it only changes form, for example from potential to kinetic to heat.Learn it in: Energy & Its Conservation →
Cosine
In a right triangle, adjacent ÷ hypotenuse for a given angle. On the unit circle, a point's across position.cos⁡θ=adjhyp\cos\theta = \frac{\text{adj}}{\text{hyp}}Learn it in: Sine, Cosine & Tangent →
Coulomb's law
The electric force between two charges: proportional to both charges, and to one over the distance squared.F=kq1q2r2F = k\frac{q_1 q_2}{r^2}Learn it in: Electric Charge & Force →
Cross product
Multiplying two vectors to get a new vector at right angles to both, as big as the parallelogram they span. Torque is a cross product.∣A⃗×B⃗∣=∣A∣∣B∣sin⁡θ|\vec A\times\vec B| = |A||B|\sin\thetaLearn it in: Multiplying Vectors: Dot & Cross →
Current
The rate at which charge flows past a point, in amperes (coulombs per second).I=QtI = \frac{Q}{t}Learn it in: Current, Voltage & Resistance →
Decibel (dB)
A logarithmic unit of sound level. +10 dB means 10 times the intensity; +3 dB about twice.β=10log⁡10II0\beta = 10\log_{10}\frac{I}{I_0}Learn it in: Logarithms →
Density
Mass per unit volume. Water is 1000 kg/m³. Objects less dense than a liquid float in it.ρ=mV\rho = \frac{m}{V}Learn it in: Pressure & Depth →
Derivative
The instantaneous rate of change of a function: the slope of its tangent line. Velocity is the derivative of position.f′(x)=lim⁡h→0f(x+h)−f(x)hf'(x) = \lim_{h\to 0}\frac{f(x+h)-f(x)}{h}Learn it in: Rates of Change: Derivatives →
Diffraction grating
A surface with many evenly spaced slits that sends each colour off at its own angle, splitting light into a spectrum.dsin⁡θ=mλd\sin\theta = m\lambdaLearn it in: Interference: Light as a Wave →
Directly proportional
Two quantities where multiplying one multiplies the other by the same factor. The graph is a straight line through the origin.y=kxy = kxLearn it in: Direct, Inverse & Square Proportion →
Displacement
The change in position: how far, and in which direction, something ended up from where it started.Learn it in: Position & Velocity →
Dot product
Multiplying two vectors to get a number: how much they line up. Work is a dot product.A⃗⋅B⃗=∣A∣∣B∣cos⁡θ\vec A\cdot\vec B = |A||B|\cos\thetaLearn it in: Multiplying Vectors: Dot & Cross →
Elastic collision
A collision where the objects bounce apart and no kinetic energy is lost.Learn it in: Momentum & Collisions →
Electric charge
A property of particles that makes them push or pull on each other electrically. It comes in two kinds, positive and negative; like charges repel.Learn it in: Electric Charge & Force →
Electric power
The energy per second a component uses, in watts.P=VIP = V ILearn it in: Current, Voltage & Resistance →
Electromagnet
A coil of wire that becomes a magnet when current flows through it, and stops being one when the current stops.Learn it in: Magnets & Magnetic Forces →
Electromagnetic induction
Making a voltage in a circuit by changing the magnetic flux through it. The basis of generators and transformers.ε=−NΔΦΔt\varepsilon = -N\frac{\Delta\Phi}{\Delta t}Learn it in: Electromagnetic Induction →
Electron
A tiny particle carrying one unit of negative charge. Moving electrons carry the current in metal wires.Learn it in: Electric Charge & Force →
Electronvolt (eV)
The energy an electron gains crossing 1 volt: 1.60 × 10⁻¹⁹ J. Visible photons carry 2 to 3 eV.Learn it in: Photons & Energy Levels →
Elimination
Solving a system by adding or subtracting the equations so one unknown cancels out.Learn it in: Simultaneous Equations →
Energy
The capacity to make things happen: to move, heat or change something. It can change form but is never created or destroyed.Learn it in: Energy & Its Conservation →
Energy level
One of the fixed energies an electron in an atom can have. Jumping between levels emits or absorbs a photon of exactly the difference.Learn it in: Photons & Energy Levels →
Entropy
A measure of how spread out energy is (how many microscopic arrangements match what we see). In an isolated system it never decreases.Learn it in: Heat Flow & Equilibrium →
Equation
A statement that two things are equal. Like a balance: whatever you do to one side, you must do to the other to keep it level.Learn it in: Equations Are Balances →
Exponent
The small raised number in a power. It tells you how many times to multiply the base by itself: 10³ = 10 × 10 × 10.Learn it in: Units & Powers of Ten →
Exponential growth
Growth by the same factor in each equal step of time, so the amount doubles every fixed doubling time.N=N0⋅2t/TN = N_0 \cdot 2^{t/T}Learn it in: Exponential Growth & Decay →
Fission and fusion
Splitting a heavy nucleus (fission) or joining light ones (fusion). Both release energy because the products are more tightly bound.Learn it in: Radioactivity & Nuclear Energy →
Flow rate
The volume of fluid passing a point each second: cross-section area times speed. In a steady flow it's the same everywhere.Q=AvQ = A vLearn it in: Flow & Bernoulli →
Focal length
The distance from a lens to the point where parallel rays meet (or seem to come from). Negative for a diverging lens.Learn it in: Lenses & Images →
Force
A push or a pull. Forces change motion: they make things speed up, slow down or turn.Learn it in: Inertia — Newton's First Law →
Fraction
A division written as one number over another. The bottom (denominator) says how many equal parts; the top (numerator) how many you take.34=3÷4\frac{3}{4} = 3 \div 4Learn it in: Fractions, Ratios & Percentages →
Free fall
Motion under gravity alone, with no air resistance. Everything in free fall speeds up by the same 9.8 m/s every second near Earth.Learn it in: Free Fall →
Free-body diagram
A sketch of one object with an arrow for every force acting on it. The first step in almost every force problem.Learn it in: F = ma — Newton's Second Law →
Frequency
How many cycles happen each second. Measured in hertz (Hz).Learn it in: Springs & Oscillations →
Friction
A force between surfaces in contact that opposes sliding. It is roughly the friction coefficient times the normal force.f=μNf = \mu NLearn it in: F = ma — Newton's Second Law →
Fundamental theorem of calculus
Integrating and differentiating undo each other: to find an area, find a function whose derivative is the curve.∫abf dx=F(b)−F(a)\int_a^b f\,dx = F(b) - F(a)Learn it in: Adding Up Slices: Integrals →
g (gravitational acceleration)
The acceleration of falling objects near a planet's surface. On Earth, about 9.8 m/s².Learn it in: Free Fall →
Gravity
The attraction between any two masses. It gets weaker with the square of the distance.F=Gm1m2r2F = \frac{G m_1 m_2}{r^2}Learn it in: Gravity & Orbits →
Half-life
The time for an exponentially decaying quantity to halve. After two half-lives a quarter is left.N=N0⋅(12)t/TN = N_0 \cdot (\tfrac12)^{t/T}Learn it in: Exponential Growth & Decay →
Half-life (radioactive)
The time for half the radioactive nuclei in a sample to decay. Carbon-14: 5730 years.N=N0(12)t/T1/2N = N_0 \left(\tfrac12\right)^{t/T_{1/2}}Learn it in: Radioactivity & Nuclear Energy →
Heat
Energy that flows from a hotter object to a colder one because of their temperature difference.Q=mcΔTQ = m c \Delta TLearn it in: Temperature & Heat →
Hertz (Hz)
One cycle per second. Named after Heinrich Hertz, who first produced radio waves in a lab in 1887.Learn it in: Waves →
Hypotenuse
The longest side of a right triangle, opposite the right angle.Learn it in: Right Triangles & Pythagoras →
Ideal gas law
The link between a gas's pressure, volume, amount and temperature.PV=nRTPV = nRTLearn it in: Gases & Pressure →
Inelastic collision
A collision where some kinetic energy turns into heat, sound or deformation. If the objects stick together, it is perfectly inelastic.Learn it in: Momentum & Collisions →
Inertia
An object's resistance to any change in its motion. Mass is the measure of inertia.Learn it in: Inertia — Newton's First Law →
Integral
The exact area under a curve, the limit of adding up thinner and thinner slices. Distance is the integral of velocity.∫abf(x) dx\int_a^b f(x)\,dxLearn it in: Adding Up Slices: Integrals →
Intercept
Where a line crosses the y-axis: the value of y when x = 0. The 'starting value'.Learn it in: Straight Lines & Slope →
Interference
What happens when waves overlap: in step they add up (constructive), half a wave out of step they cancel (destructive).Learn it in: Interference: Light as a Wave →
Inverse-square law
A rule where something gets weaker with the square of distance: twice as far means one quarter as strong.Learn it in: Gravity & Orbits →
Inversely proportional
Two quantities where multiplying one divides the other by the same factor. Their product stays constant.y=kxy = \frac{k}{x}Learn it in: Direct, Inverse & Square Proportion →
Isotope
Atoms of the same element (same number of protons) with different numbers of neutrons, like carbon-12 and carbon-14.Learn it in: Radioactivity & Nuclear Energy →
Joule (J)
The SI unit of energy. About the energy needed to lift a 100 g apple by 1 metre.Learn it in: Energy & Its Conservation →
Kelvin (K)
The temperature scale that starts at absolute zero. Same step size as Celsius: K = °C + 273.15.Learn it in: Temperature & Heat →
Kinetic energy
The energy of motion.KE=12mv2KE = \tfrac12 m v^2Learn it in: Energy & Its Conservation →
Length contraction
A moving object is measured shorter along its direction of motion.L=L0γL = \frac{L_0}{\gamma}Learn it in: Special Relativity →
Lenz's law
An induced current always flows so as to oppose the change that caused it. It's energy conservation in disguise.Learn it in: Electromagnetic Induction →
Lepton
An elementary particle that isn't made of quarks and doesn't feel the strong force: the electron, muon, tau and the neutrinos.Learn it in: Particles & the Standard Model →
Limit
The value an expression settles on as something (like a gap h) gets as close to a number as you like.Learn it in: Rates of Change: Derivatives →
Line of best fit
The single straight line that best represents scattered data, passing through or near every error bar.Learn it in: Best-Fit Lines & Straightening Curves →
Linear
Changing by the same amount for every equal step, so the graph is a straight line.y=mx+by = mx + bLearn it in: Straight Lines & Slope →
Linearising
Choosing what to plot so that a law becomes a straight line, like T² against L for a pendulum. The gradient then carries the physics.Learn it in: Best-Fit Lines & Straightening Curves →
Logarithm
The power a base must be raised to in order to give a number. log₁₀ counts powers of ten. It undoes exponentials.log⁡101000=3\log_{10} 1000 = 3Learn it in: Logarithms →
Logarithmic scale
A scale on which each equal step multiplies the value by the same factor, so huge ranges fit on one line.Learn it in: Logarithms →
Lorentz factor
How much moving clocks slow and moving lengths shrink. 1 at rest, 2.29 at 0.9c, infinite at c.γ=11−v2/c2\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}Learn it in: Special Relativity →
Magnetic field
The region around a magnet or a current where magnetic forces act, measured in teslas (T). Drawn as lines from north to south.Learn it in: Magnets & Magnetic Forces →
Magnetic flux
How much magnetic field passes through an area, in webers (Wb): roughly the number of field lines through a loop.Φ=BAcos⁡θ\Phi = BA\cos\thetaLearn it in: Electromagnetic Induction →
Magnetic force
The sideways push a magnetic field gives a moving charge, at right angles to both the velocity and the field. A charge at rest feels none.F=qvBsin⁡θF = qvB\sin\thetaLearn it in: Magnets & Magnetic Forces →
Magnitude
The size of a vector: the length of its arrow, ignoring direction.Learn it in: Vectors: Arrows With Meaning →
Mass
How much matter something contains, and how hard it is to accelerate. Measured in kilograms. Unlike weight, it is the same everywhere.Learn it in: F = ma — Newton's Second Law →
Mass–energy equivalence
Mass is a form of energy: a little mass holds a huge amount of it.E=mc2E = mc^2Learn it in: Special Relativity →
Mole
A counting unit for atoms and molecules: 6.02 × 10²³ of them.Learn it in: Gases & Pressure →
Moment of inertia
How hard something is to spin up or slow down. It grows with mass and, even more, with how far the mass is from the axis.I=mr2I = m r^2Learn it in: Spinning & Angular Momentum →
Momentum
Mass times velocity. The 'quantity of motion'. The total momentum of colliding objects stays the same.p=m vp = m\,vLearn it in: Momentum & Collisions →
Net force
The total of all forces on an object, taking directions into account. Only the net force changes motion.Learn it in: Inertia — Newton's First Law →
Newton (N)
The SI unit of force. 1 N gives a 1 kg mass an acceleration of 1 m/s². About the weight of a small apple.Learn it in: F = ma — Newton's Second Law →
Newton's first law
An object keeps moving at constant velocity (or stays at rest) unless a net force acts on it.Learn it in: Inertia — Newton's First Law →
Newton's second law
The net force on an object equals its mass times its acceleration.F=m aF = m\,aLearn it in: F = ma — Newton's Second Law →
Newton's third law
Forces come in pairs: if A pushes on B, B pushes back on A with an equal force in the opposite direction.Learn it in: Action & Reaction — Newton's Third Law →
Ohm's law
Current through a component is proportional to the voltage across it.V=IRV = I RLearn it in: Current, Voltage & Resistance →
Orbit
The path of an object falling around a planet or star: it moves sideways so fast that it keeps missing the ground.Learn it in: Gravity & Orbits →
Oscillation
Repeated motion back and forth around a resting (equilibrium) position, like a spring or a pendulum.Learn it in: Springs & Oscillations →
Parabola
The curved path of a projectile, made by steady sideways motion combined with steadily accelerating vertical motion.Learn it in: Projectile Motion →
Parallel circuit
Components connected on separate branches across the same voltage. Currents add; the combined resistance is lower.Learn it in: Series & Parallel Circuits →
Pascal (Pa)
The SI unit of pressure: one newton per square metre. Air pressure is about 101 000 Pa.Learn it in: Pressure & Depth →
Percentage
A fraction out of 100: part ÷ whole × 100%.Learn it in: Fractions, Ratios & Percentages →
Period
The time for one complete cycle of a repeating motion.T=1fT = \frac{1}{f}Learn it in: Springs & Oscillations →
Phase
Where a wave is in its cycle at the start. Changing it slides the wave sideways in time.Learn it in: Sine Waves →
Photoelectric effect
Light knocking electrons out of a metal. Each photon frees at most one electron, and only if its energy beats the work function.KEmax⁡=hf−φKE_{\max} = hf - \varphiLearn it in: Photons & Energy Levels →
Photon
A single packet of light energy. Its energy depends only on its frequency.E=hfE = hfLearn it in: Photons & Energy Levels →
Pivot
The fixed point or line something turns around, like a door's hinges or a see-saw's middle.Learn it in: Torque & Balance →
Position
Where something is, measured from a chosen starting point (the origin), with a sign for direction.Learn it in: Position & Velocity →
Potential energy
Stored energy due to position, like a raised object in gravity.PE=mghPE = m g hLearn it in: Energy & Its Conservation →
Power
Repeated multiplication of a number by itself. The exponent counts the copies.25=2×2×2×2×22^5 = 2 \times 2 \times 2 \times 2 \times 2Learn it in: Powers & Roots →
Precision and accuracy
Precise: repeated readings agree closely. Accurate: they are centred on the true value. A miscalibrated scale can be precise but not accurate.Learn it in: Measurement & Uncertainty →
Pressure
Force spread over an area. Measured in pascals (N/m²).P=FAP = \frac{F}{A}Learn it in: Pressure & Depth →
Projectile
An object thrown or launched that then moves only under gravity. Its path is a parabola.Learn it in: Projectile Motion →
Pythagorean theorem
In a right triangle, the squares on the two legs add up to the square on the hypotenuse.a2+b2=c2a^2 + b^2 = c^2Learn it in: Right Triangles & Pythagoras →
Quadratic
An expression whose highest power is 2. Its graph is a parabola.y=ax2+bx+cy = ax^2 + bx + cLearn it in: Parabolas & Quadratics →
Quadratic formula
Gives the roots of any quadratic. The part under the root, the discriminant, tells you whether there are 2, 1 or 0 of them.x=−b±b2−4ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}Learn it in: Parabolas & Quadratics →
Quark
An elementary particle with a charge of +⅔ or −⅓. Never found alone: three make a proton or neutron.Learn it in: Particles & the Standard Model →
Radian
The angle that cuts off an arc one radius long. A full turn is 2π radians, so 180° = π.s=rθs = r\thetaLearn it in: Circles & Radians →
Radioactive decay
An unstable nucleus changing into a more stable one, giving out alpha, beta or gamma radiation.Learn it in: Radioactivity & Nuclear Energy →
Range
How far a projectile travels horizontally before landing.R=v2sin⁡2θgR = \frac{v^2 \sin 2\theta}{g}Learn it in: Projectile Motion →
Rate
An amount of one thing for each unit of another, found by dividing. Every 'per' (km per hour, kg per m³) is a rate.Learn it in: Fractions, Ratios & Percentages →
Ratio
A comparison of two amounts, written a : b. It's a recipe: to find each share, divide the total by the sum of the parts.Learn it in: Fractions, Ratios & Percentages →
Real image
An image where light rays actually meet. It can be caught on a screen and, through one lens, is upside down.Learn it in: Lenses & Images →
Rearranging a formula
Using the balance rule to get a different letter on its own, undoing operations in reverse order.Learn it in: Rearranging Formulas →
Refractive index
How many times slower light travels in a material than in a vacuum. Water 1.33, glass about 1.5, diamond 2.42.n=cvn = \frac{c}{v}Learn it in: Reflection & Refraction →
Resistance
How strongly something opposes current, in ohms (Ω).Learn it in: Current, Voltage & Resistance →
Riemann sum
An estimate of the area under a curve made by adding up thin rectangles.Learn it in: Adding Up Slices: Integrals →
Right-hand rule
Fingers along the first vector, curl them toward the second: your thumb points along their cross product (for example, the force on a positive charge).Learn it in: Magnets & Magnetic Forces →
Root
A value of x that makes an expression equal zero: where its graph crosses the x-axis.Learn it in: Parabolas & Quadratics →
Scalar
A quantity that has only a size, like mass, time or temperature.Learn it in: Vectors: Arrows With Meaning →
Scientific notation
Writing a number as a value between 1 and 10 times a power of ten, like 3 × 10⁸, so very big and very small numbers stay readable.Learn it in: Units & Powers of Ten →
Second law of thermodynamics
Heat never flows from a colder object to a hotter one by itself. Energy naturally spreads out.Learn it in: Heat Flow & Equilibrium →
Series circuit
Components connected one after another on a single path. Same current through each; resistances add.Learn it in: Series & Parallel Circuits →
SI units
The international system of units used in science: metre (m), kilogram (kg), second (s), and others built from them.Learn it in: Units & Powers of Ten →
Significant figures
The digits that carry real information. Keep about as many as the least precise measurement you used.Learn it in: Measurement & Uncertainty →
Sine
In a right triangle, opposite ÷ hypotenuse for a given angle. On the unit circle, a point's height.sin⁡θ=opphyp\sin\theta = \frac{\text{opp}}{\text{hyp}}Learn it in: Sine, Cosine & Tangent →
Sine wave
The smooth wave traced by the height of a point turning steadily round a circle. Springs, sound and light follow it.y=Asin⁡(2πft+φ)y = A\sin(2\pi f t + \varphi)Learn it in: Sine Waves →
Slope
The steepness of a line: rise divided by run. In physics it's a rate, such as velocity on a position–time graph.m=ΔyΔxm = \frac{\Delta y}{\Delta x}Learn it in: Straight Lines & Slope →
Snell's law
The rule for how much light bends when it crosses from one material into another.n1sin⁡θ1=n2sin⁡θ2n_1\sin\theta_1 = n_2\sin\theta_2Learn it in: Reflection & Refraction →
Special relativity
Einstein's 1905 theory of space and time for observers moving steadily, built on the speed of light being the same for all of them.Learn it in: Special Relativity →
Specific heat capacity
The energy needed to warm 1 kg of a substance by 1 °C. Water's is large: 4186 J/(kg·°C).Learn it in: Temperature & Heat →
Speed
How fast something moves: distance covered per unit of time. It has no direction.Learn it in: Position & Velocity →
Square root
The number that, multiplied by itself, gives the original. It undoes squaring and equals the ½ power.a=a1/2\sqrt{a} = a^{1/2}Learn it in: Powers & Roots →
Square–cube law
Scale an object's lengths by k and its areas grow by k², its volume (and weight) by k³.Learn it in: Powers & Roots →
Standard Model
The theory listing every known elementary particle (quarks, leptons, force carriers and the Higgs boson) and how they interact.Learn it in: Particles & the Standard Model →
System of equations
Two or more equations that must all be true at once. On a graph, their solution is where the lines cross.Learn it in: Simultaneous Equations →
Systematic error
An error that pushes every reading the same way, like a scale that doesn't start at zero. Averaging can't remove it.Learn it in: Measurement & Uncertainty →
Tangent (trigonometry)
In a right triangle, opposite ÷ adjacent for a given angle: the slope of the hypotenuse.tan⁡θ=oppadj\tan\theta = \frac{\text{opp}}{\text{adj}}Learn it in: Sine, Cosine & Tangent →
Temperature
A measure of the average kinetic energy of the molecules in something.Learn it in: Temperature & Heat →
Tesla (T)
The SI unit of magnetic field. Earth's field is about 0.00005 T; a hospital MRI scanner about 1.5 to 3 T.Learn it in: Magnets & Magnetic Forces →
Thermal equilibrium
The state two objects reach when they've been in contact long enough to be at the same temperature, so no more heat flows.Learn it in: Heat Flow & Equilibrium →
Thin lens equation
Links the focal length to where the object and its image are.1f=1d0+1di\frac{1}{f} = \frac{1}{d_0} + \frac{1}{d_i}Learn it in: Lenses & Images →
Time dilation
A moving clock runs slow compared with clocks at rest, by the Lorentz factor γ.Δt=γ Δt0\Delta t = \gamma\,\Delta t_0Learn it in: Special Relativity →
Torque
The turning effect of a force: the force times its distance from the pivot.τ=rF\tau = r FLearn it in: Torque & Balance →
Total internal reflection
When light inside a slower material hits the boundary beyond the critical angle and is all reflected back. It keeps light inside optical fibres.sin⁡θc=n2n1\sin\theta_c = \frac{n_2}{n_1}Learn it in: Reflection & Refraction →
Transformer
Two coils on one iron core that change an AC voltage up or down in proportion to their turns.VsVp=NsNp\frac{V_s}{V_p} = \frac{N_s}{N_p}Learn it in: Electromagnetic Induction →
Uncertainty
How far the true value could plausibly be from a measurement: the ± part, as in (2.01 ± 0.08) s.Learn it in: Measurement & Uncertainty →
Unit
An agreed standard amount used to measure something. A metre is a unit of length; a second is a unit of time.Learn it in: Units & Powers of Ten →
Variable
A symbol, usually a letter, that stands for a quantity whose value can change, like the speed of a car.Learn it in: Equations Are Balances →
Vector
A quantity with both a size (magnitude) and a direction, drawn as an arrow. Velocity, force and displacement are vectors.Learn it in: Vectors: Arrows With Meaning →
Velocity
Speed with a direction. It tells you how fast position changes, and which way.v=ΔxΔtv = \frac{\Delta x}{\Delta t}Learn it in: Position & Velocity →
Vertex
The turning point of a parabola: its lowest or highest point.x=−b2ax = -\frac{b}{2a}Learn it in: Parabolas & Quadratics →
Virtual image
An image that light only seems to come from, like the one in a mirror or a magnifying glass. No screen can catch it.Learn it in: Lenses & Images →
Voltage
The energy each coulomb of charge gets from a battery or gives up in a component. 1 volt = 1 joule per coulomb.Learn it in: Current, Voltage & Resistance →
Wave
A disturbance that carries energy from place to place without carrying matter along with it.Learn it in: Waves →
Wave speed
How fast a wave travels: frequency times wavelength.v=f λv = f\,\lambdaLearn it in: Waves →
Wavelength
The distance between two neighbouring crests of a wave.Learn it in: Waves →
Weight
The force of gravity on an object: its mass times g. It changes from planet to planet; mass doesn't.W=m gW = m\,gLearn it in: F = ma — Newton's Second Law →
Work function
The least energy needed to pull an electron out of a metal's surface. A few electronvolts.Learn it in: Photons & Energy Levels →