# OCR A-Level Physics/Glossary of Terms

Definitions of keywords and terms that you will need to know.

## A
### Absolute zero

    Zero on the thermodynamic temperature scale, or 0 K (kelvin), where a substance has minimum internal energy, and is the coldest possible temperature. It is equal to -273.15 degrees Celsius.

### Absorption spectrum

    A spectrum of dark lines across the pattern of spectral colours produced when light passes through a gas and the gas absorbs certain frequencies depending on the elements in the gas.

### Acceleration

    The (instantaneous) rate of change of velocity in respects to time.

### Acceleration of free fall (g)

    The acceleration of a body falling under gravity (9.81ms$^{-2}$ on earth).

### Ammeter

    A device used to measure the electric current in a circuit. It is connected in series with the components.

### Amount of substance

    A SI quantity, measured in moles (mol).

### Ampere

    The SI unit for electric current.

### Amplitude

    The maximum displacement of a wave from its rest/mean position (measured in metres).

### Antiphase

Any odd integer of $\pi$ radians apart (for points in waves)

### Attenuation

The gradual loss of flux intensity through a medium.
Attenuation coefficient
Measure of how much an incident energy beam (ultrasound or x-ray) is attenuated through a medium. I = I$_0$e$^{-μx}$
Attenuation Mechanisms
- Simple scattering = low energy x-ray photon encounters an electron in an atom causing it to be scattered without a change in energy.
- Photoelectric effect = X-ray photon absorbed by an electron causing it to be emitted from the atom as a photoelectron.
- Compton scattering = X-ray is deflected from interaction of electron causing wavelength of photon to increase as well as emittance of electron from atom at high speed.
- Pair production = High energy x-ray photon interacts with nucleus of atom causing production of e- and e+.ω
Astronomical Unit (AU)
Average distance from the Earth to the Sun. ≈ 1.5 x 10$^{11}$ m
Atomic mass unit, u
! atomic mass unit is equal to the mass of 1/12th the mass of one carbon-12 atom.
Archimedes' Principle
For an object submerged in a fluid, the up-thrust is equal to the weight of that the object displaces.

### Average Speed (v)

Rate of change in distance (m) calculated over a completed journey, with respect to time. Measured in ms$^{-1}$.

### Antinode

    A point of maximum amplitude along a stationary wave caused by constructive interference.
Angular velocity, ω
Rate of change of angle, measured in rads$^{-1}$
Aphelion
The furthest point in an elliptical orbit from the sun.
Activity
Rate at which nuclei decay or disintegrate. It is also the number of alpha, beta or gamma photons emitted per unit time. Measured in Becquerels. Bq
Acoustic impedance (Z) Z = ρc
Fraction of ultrasound intensity reflected at the boundary depends on the acoustic impedance (Z) of both media. Product of density (P) of substance and speed of ultrasound (c) in it.

## B
### Braking Distance

    Distance travelled from the brakes being applied to the car stopping completely.
Baryon
Hadron composed of 3 quarks.
Brownian motion
Random motion of a larger particle in a gaseous system due to collisions with smaller particles.
Bremsstrahlung (or Braking) Radiation
Form of X-ray radiation produced when high energy electrons interact with a nucleus.
Black body radiation
At an given temperature above absolute zero, an object emits emr at different wavelengths and intensities.
Boyle's Law
A gas law stating a gas' pressure is inversely proportional to the volume, considering temperature is constant.
Boltzmann constant, k
A constant describing the ratio between the ideal gas constant and Avogadro's constant (R/N$_a$) ~ 1.38 x 10$^{-23}$ m$^2$kgs$^{-2}$K$^{-1}$
Binding Energy ΔE=Δmc$^2$
Minimum energy required to completely separate a nucleus into its constituent protons and neutrons.
Binding energy per nucleon
Measure of stability. The greater the value, the more tightly bound the nucleons, and the more energy needed to break them.

## C
### Charge (Q)

How much positive or negative the charge carriers are per unit volume. Measured in coulombs.
Capacitor
An electrical component which can separate (store) charge on two metallic plates, separated by an insulator (dielectric).
Capacitance (C)
The amount of charge per volt that can be stored. Measured in Farads.

### Chemical Energy (E$_c$)

Energy contained within the chemical bonds between atoms and molecules.
Characteristic Radiation
Form of X-ray radiation produced when high energy electrons interact with electrons in orbitals around a nucleus, causing a transition of another electron to fill its gap.

### Coefficient of Friction ($\mu$)

Ratio between the frictional force and normal contact force acting on an object.

### Conservation of Energy Law

Total energy in a closed system of interacting objects remains constant - can be transferred into alternate forms.

### Conservation of Momentum Law

For a closed system of interacting objects, the total momentum in a specified direction remains constant considering no external resultant force acts on it.

### Conventional current

The concept of current as the transfer of positive charge. I.e. from ve+ to ve- in a circuit.
Contrast medium μ α Z$^3$
Used to improve visibility of soft-tissues in x-rays. e.g. iodine and barium compounds: due to large atomic number.
Cosmological Principle
When viewed on a large scale, the universe is homogenous and isotropic, and the laws of physics are universal.

### Coulomb's Law

    The force between two charges is directly proportional to the product of the charges and inversely proportional to the distance between them squared

### Conductor

A material that allows the transfer of energy through it (e.g. electrical).
Control rods
Absorb thermal neutrons to control the rate of reaction and maintain it at a steady rate.
Coolant
High specific heat capacity to take away heat energy from reactor.

### Compression

Moving region in which the medium is more dense or of higher pressure than the surrounding.

### Critical Angle ($\theta$)

Angle of incidence at which from that point on, no more refraction will occur - only total internal reflection. Measured in degrees.

### Critical Temperature

The temperature in which the electrical resistance of a material falls to zero.

### Current (I)

Rate of flow of charge. Measured in amperes, A.

### Couple

    Two equal, opposite and parallel forces which create rotational force.
Charles' Law
A gas law stating that the volume of a gas is directly proportional to the temperature of the gas, considering pressure remains constant.
Centripetal acceleration
Acceleration causing an object to move in a circular path, i.e. always acting towards the centre. Measured in ms$^{-2}$
Centripetal force, F$_c$
The force responsible for an object moving in a circular path. Measured in Newtons.
c
Speed of light in a vacuum constant. ~ 3.00 x 10$^8$ ms$^{-1}$

## D
### Displacement

    The distance in a given direction.
    Displacement (Oscillations)
    Distance from equilibrium position. Measured in meters.

### Diffraction

Property of a wave in which when it passes through a gap, it spreads out.

### Density

    Density is the mass of a body per unit volume

### Drag

The resistive force on an object when it travels through a fluid.

### Drift velocity (v)

The average velocity of electrons travelling within a material. Measured in ms$^{-1}$

### Ductility

The ability of a material to plastically deform under tensile forces without fracturing.
Dopple effect
The apparent shift in frequency of a wave caused by relative motion between the source and the observer.

### Decay Constant

    The probability of decay of a nucleus per unit time
Decay types:
- Alpha = Heavier elements release alpha particles to reduce their proton to neutron ratio, becoming more stable.
- Beta- = Due to neutron rich isotopes by which the weak nuclear force is responsible for a neutron decaying into a proton: which releases a B- electron.
- Beta+ = Due to proton rich isotopes by which the weak nuclear force is responsible for a proton decaying into a neutron: which releases a B+ positron.
- Gamma = Due to a surplus of energy within the nucleus usually following an alpha or beta decay.
Damping
A reduction in amplitude of an oscillation as a result of energy being drained from the system to overcome friction or other resistive forces.
Driving frequency
The frequency of the forced oscillation system.

## E
### Elastic energy (E$_e$)

Energy stored within an object due to an elastic change in its shape (i.e. stretch or compression). Measured in joules.

### Elastic deformation

A change in objects shape in which it will return to its original shape once the force is removed.

### Electrical energy

Energy of electrical charges due to their position in an electric field. Measured in joules.

### Electric field strength (E)

    The force per unit charge that a positive point charge would experience at a specified point in an electrical field. Measured in Volts per metre or Newtons per Coulomb

### Electric potential (V)

    The energy per unit charge that a positive point charge would require to move from infinity to a specified point in an electrical field.. Measured in Volts

### Elementary Charge (e)

Amount of charge carried by a single proton or electron (1.6x10$^{-19}$ C), which may be positive or negative.

### Energy

    The stored ability to do work

### Electromagnetic Radiation

A form of progressive wave with perpendicularly oscillating magnetic and electric fields.

### Electromotive Force (e.m.f)

Energy available to be transferred from other forms (e.g. chemical) into electrical, per unit charge.

### Equilibrium position

The resting position for particles in a medium.
Equipotential Field Lines
Field lines that represent lines of equal electrical potentials. Perpendicular to electric field lines.

### Extension (x)

    The change in length of an object when a force is applied to it
Eccentricity
How 'squashed' an ellipse is. I.e. the closer the foci are, the less eccentric an ellipse is.
Escape Velocity
The minimum velocity of an object required to escape the gravitational field of a larger body.

## F
### Faraday's Law

    The emf induced in a conductor is directly proportional to the rate at change of magnetic flux linkage.

### Force

    A force causes a mass to change motion.

### Frictional Force

Force arising when two objects rub against each other.

### Frequency

    The number of oscillations per second. Measured in hertz, Hz.
Free oscillation
When a pendulum is pulled to one side and allowed to oscillate freely,
Forced oscillation
Periodic driver is applied to an oscillator.
Fundamental particle
Particle with no internal structure.
Fuel rods
Bombardment with neutrons causes fission and the release of energy.

## G
### Gravitational Field Strength (g)

    The force that a unit mass would experience at a specified point. ms$^{-2}$ or Nkg$^{-1}$

### Gravitational Potential

    The work done per unit mass to move and object from infinity to a specified point in a gravitational field. Measured in Jkg$^{-1}$

### Gravitational Potential Energy

    the energy an object has due to its relative position in a gravitational field. Found by mass x gravity (or gravitational field strength) x height
    or force per unit mass at a set point in a gravitational field. Measured in joules.
Gravitational Constant, G
Constant ensuring direct proportionality in newtons laws of gravitation. ~ 6.67x10$^{-11}$m$^3$kg$^{-1}$s$^{-2}$
Geostationary orbit
An orbit with equal period to that of the Earth (i.e. equal angular velocity). The orbit must be above the equator and rotate in the same direction as the Earth.
Geosynchronous
An orbit with equal period to that of the Earth. Doesn't have to be above the equator.

## H
### Hardness

Ability of an object to resist surface scratching and pressure.
Half life
Average time it takes for the number of radioactive nuclei in a sample to half.
Hadron
Particle composed of quarks in which interacts via the strong force.

### Heat

    A form of energy transfer, also known as 'Thermal Energy'.

### Hooke's Law

    An approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.
Hubble's Law
The recessional velocity of a galaxy is almost directly proportional to the distance, d, from Earth. (V≈H$_0$ d)
Homogenous
The universe is composed of the same thing.
Heat flux, q, (Heat flow density)
Rate at which thermal energy is transferred per unit area, per unit time.
Heat transfer coefficient, h
How well heat is conducted over a series of resistant mediums.

## I
### Inertia

Property of matter by which it continues in its existing state, rest, or uniform motion, unless acted on by an external resultant force.

### Inelastic Collision

A collision in which some kinetic energy is transferred to other forms (e.g. heat).

### In-phase

Points in waves in which have no phase difference.

### Instantaneous acceleration

    acceleration at a specific time; slope of tangent to velocity- time graph.

### Instantaneous position

    position of an object at specific time.

### Instantaneous velocity

    Velocity over an infinitesimal interval of time or displacement.

### Intensity (I)

Describes how much energy per unit time is received per unit area from a wave. Measured in Wm$^{-2}$

### Internal resistance (r)

Resistance at the source of e.m.f (e.g. cell) due to its construction. Energy is lost as heat etc which reduces the terminal p.d.

### Interference

The superposition of waves when they interact, leaving a resultant amplitude.

### Impetus

The force or energy of which a body moves with.

### Impulse ($\Delta$p)

Described as the change in momentum. Calculated by the product of force and change in time. Measured in Ns.

### Internal energy

    Sum of random potential and kinetic energies of atoms within a system. Measured in joules.
Isochronous
An oscillation with a constant period.
Isotropic
The composition of the universe is equal throughout (i.e. equal density)
Isotope
Nuclei of the same element that has equal number of protons but different number of neutrons.

## J
### Joule

    The SI unit of work done, or energy. One joule is the work done when a force of one newton moves an object one metre.

## K
### Kirchhoff's 1st Law

The sum of current entering a junction in a circuit, equals the total leaving it - conservation of charge law.

### Kirchhoff's 2nd Law

The sum of e.m.f's equals the sum of p.d's across components in a closed loop of a circuit - conservation of energy law.

### Kinetic Energy

    The energy an object possesses due to its motion, given by KE = 0.5 x mass x velocity²
Kepler's Laws:
1st: Planets orbit the sun in elliptical orbits with the sun at one foci.
2nd: A line segment joining a planet and the sun sweeps out equal areas in equal times.
3rd: The square of the orbital period, T, of a planet is directly proportional to the cube of its average distance from the sun.
K lines
Peak intensities of specific energy photons produced from characteristic radiation.

## L
### Longitudinal wave

Form of progressive wave that oscillates parallel to the direction of energy transfer.

### Lenz's Law

    An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.
Lepton
Particle not composed of quarks and considered a fundamental particle by which interacts via the weak force.
Light year (ly)
Distance travelled by light in a vacuum in a year. ≈9.46 x 10$^{15}$ m

## M

Maxima
Point of constructive interference due to path difference being whole integer of wavelengths (from coherent waves).
Minima
Point of destructive interference due to path difference being an odd number of half wavelengths (from coherent waves).
Malleability
Ability to plastically deform under compressive stress.
Moment (M)
Product of the force applied and perpendicular distance between force and pivot point or axis of rotation. Measured in Nm.
Momentum (p)
Vector quantity describing the 'inertia of movement'. Product of velocity and mass of an object. Measured in kgms$^{-1}$
Moderator
Reduces speed of thermal neutrons by collisions with moderator atoms. e.g. water.
Magnetic Flux
Magnetic flux is the product of magnetic flux density and the projection of area onto a surface at right angles to the magnetic field. measured in Weber's (ωb)
Magnetic flux density
The force acting per unit charge per unit length on a wire placed at right angles to the magnetic field. Measured in Tesla's (T)
Magnetic flux linkage
The product of magnetic flux and number of turns in a current-carrying coil. Measured in Weber Turn's (NΦ)
Magnetic Field
A field surrounding a permanent magnet or a current carrying conductor in which magnetic objects experience a force.
Mass defect Δm
Difference between the mass of the completely separated nucleons and the mass of the nucleus.
Meson
Hadron composed of a quark and an antiquark.

## N
### Node

A point of minimum amplitude of a stationary wave due to deconstructive interference.

### Normal contact force

The force that arises when an object rests on another. Measured in N.

### Newtons 1st Law of motion

States that a body will remain at rest or at constant velocity unless an external resultant force acts on it.

### Newtons 2nd Law of motion

The resultant force acting on an object is directly proportional to the rate in change of momentum with respect to time, and is in the same direction.

### Newtons 3rd Law of motion

For interacting objects, they will exert equal and opposite forces on each other.

### Nuclear energy (E$_n$)

The energy stored within the nuclei of atoms. Measured in joules.

### Number density (n)

The number of free charge carriers within a material per unit volume. Unit: m$^{-3}$

### Newton

    Unit in which force is measured. Symbol "N". One Newton is the force required to give a mass of 1kg an acceleration of 1ms$^{-2}$
Neutrinos
Fundamental particle with 3 'flavours', almost massless with 0 net charge.
Natural frequency
The frequency of a freely oscillating system.

## O

Oscillation
A regular variation in displacement of particles in a medium.
Out of Phase
Points in waves with phase difference.
Ohm's Law
Voltage is directly proportional to current as long as all other physical properties remain constant (V =IR) for an ohmic conductor.

## P
### Phase difference ($\Phi$)

Describes the difference in positions of points on a wave, or multiple waves. Measured in radians or degrees.
Parsec (Pc)
The distance which a radius of 1AU subtends an angle of one arcsecond. ≈ 3.1 x 10$^{16}$ m
Parallax
The apparent shift in position of a nearby star against the background of distant stars. A method used to determine the distance, d, between the Earth and a nearby star.

### Photoelectric Effect

The emission of photoelectrons from a material into a vacuum due to the absorption from EM radiation. (above threshold frequency).

### Photovoltaic Effect

Generation of a current/p.d. form the absorption of EM radiation.

### Photoconductive Effect

A material becomes more electrically conductive from the absorption of EM radiation.

### Period (T)

    The time taken for one complete oscillation. Denoted by 'T'. T=1/f

### Perfectly Elastic collision

A collision in which no kinetic is lost (i.e. sum of kinetic energies before collision equals sum of kinetic energies after).
A universal constant. ≈ 8.85 x 10$^{-12}$ Fm$^{-1}$

### Power

    The rate at which work is done. Measured in Watts.

### Polarization

Action of restricting the oscillations of a transverse wave - only oscillate in one plane.
Polar Orbit
An orbit which passes above or close to both poles on the Earth.

### Pressure

    The load applied to an object per unit surface area. Measured in Pa.
Principle of superposition
When two or more waves meet, the resultant displacement for a single point is equal to the sum of individual displacements for the waves at that point.
Principle of Moments
For a system in dynamic equilibrium, the sum of clockwise moments about any pivot point must equal the sum of anticlockwise moments about the same pivot point.

### Progressive wave

An oscillation that travels through a medium or vacuum - transferring energy from one place to another.

### Plastic Deformation

A change in an objects shape in which it will not return to normal once the force is removed.

### Potential divider circuit

A simple circuit in which includes resistors to supply variable p.d's.

### Potentiometer

Sliding dial which alters resistance and hence p.d.

### Potential difference

    The work done in moving a unit positive charge from one point to the other. The unit is volt.
Perihelion
Closest point in elliptical orbit from the sun.
Piezoelectric effect
Electric charge that accumulates in piezoelectric materials when subjected to applied mechanical stress e.g. sound waves.

## Q
### Q or q

    Often used as the symbol for charge in equations
Quark
Considered a fundamental particle which forms hadrons, mesons etc and are bound by the strong nuclear force.

## R
### Restoring force

The force in which tries to return a system to its equilibrium position.

### Resistivity

    The property of a material that measures it resistance to electric current. It is defined as the resistance a wire of the material would have if it had a cross sectional area of one metre square and a length of one meter.

### Resistance (R)

Describes the resistance of flow of charge carriers within a material/circuit. Measured in ohms, $\Omega$.

### Refractive Index (n)

The ratio of the speed of light in a vacuum to the speed of light in a material. given by n = c/v. n>1.

### Refraction

When a wave travels into a new medium at a non-zero angle of different optical density (refractive index), it changes speed and hence direction. Frequency is constant.

### Reflection

When a wave hits another medium that is smooth enough for it to bounce off at an equal angle to the normal. Frequency, wave speed and wavelength are constant.

### Rare-fraction

A moving region in which the medium is less dense or of less pressure than surrounding medium.

### Radiant energy

Energy associated with all EM radiation, stored in oscillating electric and magnetic fields.

### Radian

    A radian is the angle subtended at the centre of the circle when the arc length is equal in length to the radius.
R.M.S
The root mean squared speed of gaseous atoms.
Resonance
A phenomenon which occurs when a driving frequency on an oscillator matches the natural frequency of the system. When this occurs, amplitude will increase due to energy being supplied by the driver.
Radial field pattern
Planets are modelled as a point mass with a centre of gravitation with field lines around a spherical object.

## S
### Scalar

    A quantity with magnitude but no direction.

### Speed

    A scalar quantity, speed is the rate of change of distance with respect to time. Measured in ms$^{-1}$
NB s can also mean displacement.

### Stopping Distance

    Stopping distance = Thinking distance + Braking distance
       thinking distance (distance travelled while reacting) = time taken to react X velocity
       braking distance (distance travelled while braking)
Superposition
The process of waves interfering and their amplitudes being added to calculate a resultant.
Superconductor
Material in which can conduct electrical charge with zero resistance.
Stiffness
Ability of an object to resist plastic deformation when force is applied.
Stefan's Law (Stefan-Boltzmann Law)
States the total power radiated per unit surface area of a black body is directly proportional to the fourth power of absolute temperature. (L = 4π r$^2$ σ T$^4$ )
Stefan's constant
Fundamental constant relating to stefan's law ( ≈ 5.67 x 10 $^{-8}$ Wm$^{-2}$ K$^{-4}$ )
Sound energy
Energy of mechanical waves due to movement of atoms.
Specific Heat Capacity

### Specific Heat Capacity

    The amount of heat energy required to raise the temperature of a substance per unit of mass by 1 degree, without going through a change in state.

### Specific Latent Heat

    The amount of heat energy required to change the state of 1 kg of the mass of a substance, without a change in temperature.

   ### Fusion

          The amount of heat energy required for a phase change of 1 kg of the mass of a substance from solid to liquid or from liquid to solid, without a change in temperature.

   ### Vaporization

          The amount of heat energy required for a phase change of 1 kg of the mass of a substance from liquid to gas or from gas to liquid, without a change in temperature.

## T
### Temperature

    A SI quantity, measured in kelvin (K)
Temperature scale
A scale with two fixed temperatures at each extreme. Other values are then allocated at equal intervals in between.

### Tensile force

The forces being applied onto a material (usually a wire) on two opposite sides in order to stretch it. Both forces' values are the same as the tensile force value.

### Tensile stress

    The tensile force per unit cross-sectional area. Measured in Pa.

### Tensile Strain

Ration between extension and original length of an object.

### Tensile strength

Ability of a material to absorb tensile forces without fracturing.

### Terminal Velocity

    maximum velocity a body can travel. When resistive forces = driving force, acceleration = 0, so it cannot travel any faster.

### Thermistor

    An electrical component that changes its resistance depending on its temperature.

### Thinking distance

    The distance travelled from seeing the need to stop to applying the brakes.

### Threshold frequency

    The lowest frequency of electromagnetic radiation that will result in the emission of photoelectrons from a specified metal surface.

### Thermo-ionic emission

Emission of electrons from the absorption of heat.
Thermal Equilibrium
When a system reaches thermal equilibrium, there is no net transfer of energy.

### Thrust

    A type of force due to an engine (usually forward force).

### Time interval (t)

    A SI quantity, measured in seconds (s).

### Time Constant

    Time taken for current/charge/voltage to fall to 1/e of its initial value.

### Torque / moment

    Moment = force x perpendicular distance from the pivot to the line of action of the force
    Torque = one of the forces x the distance between them

### Total internal reflection

Occurs when angle of incidence > critical angle.

### Transverse Wave

    A progressive wave that transfers energy as a result of oscillations/vibrations that are perpendicular to direction of energy transfer.

### Triangle of forces

    If three forces are acting at a point that can be represented by the sides of a triangle, the forces are in equilibrium.

### Turning forces

    More than one forces that if unbalanced will cause a rotation.

## U
### Ultimate tensile strength

    The maximum tensile force that can be applied to an object before it breaks.

### Ultimate tensile stress

    The maximum stress that can be applied to an object before it breaks.

### Ultraviolet

    A form of electromagnetic wave (wavelengths 10$^{-9}$-3.7x10$^{-7}$m). It may cause sun tanning. Usually classified into three categories: UV-A, UV-B and UV-C.

### Up-thrust

    A force experienced due to the pressure difference of the fluid at the top and bottom of the immersed portion of the body.
Uniform field pattern
At surfaces of planets, considering modest ranges of height compared to the radius, the gravitational field is approximately uniform and field lines are modelled as parallel and equally apart.

## V
### Vector

    A quantity with magnitude and direction.

### Velocity

    The (instantaneous) rate of change of displacement with respect to time. Velocity is a vector. Measured in ms$^{-1}$

### Velocity-time graph

    A motion graph which shows velocity against time for a given body.

### Vibration

Mechanical oscillation.

### Volt (V)

    The unit of potential difference (p.d.) or electromotive force (e.m.f.)
potential difference=energy/charge

### Voltmeter

    A device used to measure the potential difference across a component. It is connected in parallel across a component.

### Volume(V)

    A physical quantity representing how much 3D space an object occupies, measured in cubic metres(m$^3$)

## W
### Watt(W)

    The unit of power.
    power = energy / time

### Wave

    Series of vibrations that transfer energy from one place to another.

### Wavelength(λ)

    The smallest distance between one point of a wave and the identical point of the next wave, measured in metres (m).

### Wave speed (v)

Product of frequency and wavelength of a wave. Measured in ms$^{-1}$

### Wave-particle duality

    The theory which states that all objects can exhibit both wave and particle properties.

### Weight

    The gravitational force acting on a body, measured in newtons (N).
weight=mass x gravitational force
Wein's Displacement Law
States the product of maximum intensity wavelength and temperature of a body is constant. (ʎ$_{max}$T = constant (constant ≈ 2.9x10$^{-3}$)

### Work Done

    The energy transferred when an object is moved through a distance by a force. Can be calculated by multiplying the force involved by the distance moved in the direction of the force.
Alternatively, [work done = transfer of energy].
i.e., work is done when energy is transferred from one form to another. [OCR do not accept this definition if asked "Define work done by a force"]

### Work function energy (Φ)

    The minimum energy that is required for a material to release an electron, measured in joules(J).

## X
### X rays

    A form of electromagnetic wave (wavelengths:10$^{-12}$-10$^{-7}$m). It is used in X-ray photography.

## Y
### Yield Strength

Amount of force needed to change an object into its plastic state.

### Young's double slit experiment

    An experiment to demonstrate the wave nature of light via superposition and interference.

### Young Modulus

    Ration of tensile stress to tensile strain. Stress per unit Strain, units: Pascals or N/m$^2$