IB Physics Definitions
- absolute zero: the temperature at which all random motion of molecules stops
- absorption spectrum: the set of wavelengths that can be absorbed
- acceleration: the rate of change of velocity; it is a vector
- acceleration of free fall: the acceleration, g, due to the pull of the earth on a body; g = 9.8 m s-2 near the surface of the earth
- activity: the rate of decay (number of decays per second)
- adiabatic: a thermodynamic process in which no heat enters or leaves the system
- albedo: the ratio of reflected to incident power or intensity; it has no unit
- alpha decay: a decay of a nucleus producing an alpha particle
- alternating current (ac): current which varies between positive and negative values
- alternating voltage: voltage which varies between positive and negative values
- ammeter: an instrument that measures the electric current through it
- amplitude: the maximum displacement from equilibrium of a wave or oscillation
- angular acceleration: the rate of change of angular velocity; measured in rad s^-2
- angular displacement: the change in angular position; measured in rad
- angular impulse: the change in angular momentum: angular momentum
- angular position: the angle defining the position of a particle relative to some arbitrary reference line; measured in rad
- angular velocity (ω): the rate of change of the angular position; measured in rad s^-1
- antinode a point on a standing wave where the displacement is a maximum (at some instant)
- apparent brightness: the power received from a star per unit area (of the detector)
- atmosphere: a non-SI unit of pressure
- atomic mass unit 1/12th of the mass of the neutral atom of carbon-12
- average speed: total distance travelled divided by the time taken
- average velocity: total displacement divided by total time
- Avogadro constant: the number of particles in one mole: 6.02214076 × 10^23
- background radiation: radiation from natural sources such as cosmic rays and radioactive material in rocks
- beta minus decay: a decay of a nucleus producing an electron and an anti-neutrino
- beta plus decay: a decay of a nucleus producing a positron and a neutrino
- binding energy: the minimum energy required to separate/combine the nucleons of a nucleus, E_b = µc^2
- black body: a theoretical body that absorbs all the radiation incident on it and radiates the maximum possible intensity for a given temperature
- blue-shift: the shift in wavelength towards smaller wavelengths for an approaching source
- Boltzmann constant: the ratio of the gas constant to Avogadro’s number
- buoyant force: the force acting on an object in a fluid due to a difference in pressure at the top and bottom of the body; force is opposite to weight
- Carnot cycle: a thermodynamic cycle consisting of two isothermal and two adiabatic curves
- centripetal acceleration: the rate of change of velocity of a body in circular motion; given as a=∆v/∆t = v^2/r = r^2ω = vω and its directed towards the centre of the circle
- centripetal force: the force directed towards the centre of a circular path; given as F=mv^2/r = mω^2r = mvω
- chain reaction: a reaction in which the products are used to keep the reaction going
- Chandrasekhar limit: the maximum possible mass of a white dwarf, about 1.4 solar masses
- closed system: a system that can transfer energy, but not matter (mass), into or from its surroundings
- coefficient of dynamic friction: the ratio of the force of friction to the normal contact force on a on a body that is sliding along a surface
- coefficient of static friction: the ratio of the maximum force of friction between two bodies to the normal contact force when an object is at rest
- coherent sources: sources with the same frequency and a constant phase difference between them
- compression: a point where the density of the medium is highest
- Compton scattering: the scattering of light off an electron
- condensation: when a vapour changes into a liquid (thermal energy is transferred away from the vapour)
- conduction: method of thermal energy transfer based on collisions of electrons with atoms
- conductors: materials with many free electrons per unit volume, through which thermal energy and electric current can pass easily
- conservation of angular momentum: when the net torque on a system is zero, the angular momentum is conserved; that is, it stays constant
- conservation of momentum: when the net force on a system is zero, the total momentum of the system remains constant
- conserved: a quantity that stays the same before and after an interaction
- constructive interference: superposition of waves leading to maximum amplitude
- control rods: rods that can easily absorb neutrons that are raised and lowered into the core of the reactor in order to control the rate of the reactions
- convection: method of thermal energy transfer due to the rising of lower density hot fluids
- convection currents: motion of a fluid as result of differences in fluid density
- Coulomb’s law: the force between two point charges is inversely proportional to the square of the separation and proportional to the product of the charges
- critical mass: the smallest mass of the nuclear fuel that can sustain a chain reaction
- damping: the loss of energy of an oscillating system due to the presence of resistance forces
- daughter nucleus the nucleus produced by radioactive decay
- decay constant: probability of decay per unit time
- decay series: the series of decays until a nucleus reaches a stable nuclide
- destructive interference: superposition of waves leading to zero (or minimum) amplitude
- diffraction: the spreading of a wave through an aperture or past an obstacle
- diffraction grating: a very large number of closely separated slits of negligibly small width
- direct current (dc): rate of flow of charge through the cross sectional area of a conductor
- discrete energy: energy taking a set of specific values rather than a continuous set of values
- displacement: change in position; the difference between the position of the oscillating body and the position at equilibrium
- distance: length of path followed
- Doppler effect: the change in observed frequency when there is relative motion between the source and the observer
- drag force: the force acting against the motion of an object that is moving through a fluid (gas or liquid)
- driven oscillations: oscillations when an external periodic force acts on the system
- driving frequency: the frequency of the external force acting on the system
- duality of matter: matter has both particle and wave-like characteristics
- dynamic friction: a force opposing motion when a body moves
- efficiency, η: the ratio of useful work or power to input work or power; η=(useful energy out)/(useful energy in) or (useful power out)/(useful power in)
- Einstein’s postulates: 1. All the laws of physics are the same in all inertial frames; 2. the speed of light in vacuum is the same for all inertial observers:
- elastic collision: a collision in which the total kinetic energy before the collision equals the total kinetic energy after the collision; that is, E_k is conserved
- elastic potential energy ( E H ): the energy stored in a spring that has been stretched or compressed; E_H = 1/2 kx^2
- electric charge: a conserved property of matter
- electric field: a region of space where an electric charge experiences an electric force
- electric field lines: curves, tangents to which give the direction of the electric field strength
- electric field strength: the electric force per unit charge on a small positive point charge
- electric potential: the work done by an external agent per unit charge in bringing a small point positive charge from infinity to a point in an electric field
- electric potential energy: the work that needs to be done by an external agent in order to bring a set of charges from where they were separated by an infinite distance to their current position
- electric power: the energy per unit time dissipated in a conductor
- electrical resistance: the ratio of voltage across conductor to current through it
- electromagnetic (EM) wave: a transverse wave moving at the speed of light in vacuum and consisting of oscillations of an electric and a magnetic field at right angles to each other
- electromagnetic interaction: force given by Coulomb’s law; acts on any particle that has electric charge; has infinite range
- electromotive force (emf): the work done per unit charge in moving charge across a battery’s terminals
- electroweak interaction: represents the unification of the electromagnetic and weak nuclear interactions
- emission spectrum: the set of wavelengths of light emitted by an atom
- emissivity: the ratio of the intensity radiated by a body to the intensity radiated by a black body of the same temperature
- energy balance equation: an equation expressing the equality of incoming and outgoing intensities of radiation
- energy level diagram: a representation of the discrete energies of an atom
- enhanced greenhouse effect: the augmentation of the greenhouse effect due to human activities
- entropy: a measure of a system’s disorder
- equations of kinematics: suvat, check formula booklet A1
- equilibrium: the state when the net force on a system is zero
- equilibrium position: the position where an oscillator has no net force acting on it
- equipotential surfaces: surfaces where all points are at the same potential
- escape speed: the minimum speed v_esc, of an object in a gravitational field so that the object reaches infinity
- evaporation: vaporisation taking place at the surface of a liquid at any temperature
- event: something that happens at a particular point in space and time
- excited state: any state above the ground state
- Faraday’s law (of electromagnetic induction): the induced emf is the rate of change of magnetic flux linkage
- first harmonic: the longest wavelength (and the lowest frequency) at which a standing wave forms
- fluid resistance force: a speed dependent force opposing the motion of a body through a fluid
- flux linkage: magnetic flux multiplied by the number of turns in a loop
- force: the action of one body on a second body; unbalanced forces cause changes in velocity or shape
- free-body force diagram: a diagram showing a body in isolation with all forces acting on it drawn as arrows
- freezing: when a liquid changes into a solid (thermal energy is transferred away from the liquid)
- frequency f: the number of complete revolutions or full waves made in one second: f=1/T
- friction laws: empirical ‘laws’ about frictional forces
- Galilean transformation equations: x'= x-vt; t'= t; u'= u-v
- gamma decay: a decay of a nucleus producing a short wavelength photon
- gas constant: the constant, R that appears in the ideal gas law
- gravitational field: a region of space where a mass experiences a gravitational force
- gravitational field strength: gravitational force per unit mass exerted on a small point mass
- gravitational interaction: force that acts on any particle with mass; has infinite range
- gravitational potential: the work done per unit mass by an external agent in bringing a small point mass from infinity to a point
- gravitational potential energy (E_P): the work done by a force in moving a body to a position above its initial position; for an earth-mass system
- greenhouse effect: the phenomenon in which re-radiated energy from the greenhouse gases returns to earth, warming the earth
- greenhouse gases: gases in the atmosphere that are capable of absorbing infrared radiation
- ground state: the state of lowest energy
- half-life: time after which the activity is reduced by a factor of 2
- harmonics: standing wave with a frequency that is an integral multiple of the first harmonic frequency
- heat engine: a device that transfers thermal energy to mechanical or electrical energy, which can then be used to do mechanical work
- heat exchanger: The part of the reactor where the thermal energy generated in the moderator is extracted
- Hooke’s law: the tension in a spring is proportional to the extension or compression
- ideal ammeter: an ammeter with zero resistance
- ideal gas: a theoretical gas in which the particles do not exert forces on each other except during contact
- ideal gas law: (aka equation of state) equation relating pressure, volume and temperature of an ideal gas: PV=nRT
- ideal voltmeter: a voltmeter with infinite resistance (takes no current when connected to a resistor)
- impulse: the product of force and the time interval for which the force acts; it equals to the change in momentum, J=∆p
- induced fission: the splitting of a large nucleus into two smaller nuclei after it has absorbed a neutron
- inelastic collision: a collision in which the total kinetic energy after a collision is less than the total kinetic energy before the collision; that is, some E_K is lost during the collision
- inertia: the tendency of a massive body to remain in its current state of motion
- inertial reference frame: a reference frame which is not accelerating
- instability region: a region on the HR diagram with stars of variable luminosity
- instantaneous speed: the magnitude of the instantaneous velocity
- instantaneous velocity: the rate of change of position; it is a vector
- insulators: materials with few free electrons per unit volume, through which thermal energy and electric current cannot readily pass
- intensity: the power of radiation: received or emitted per unit area; its unit is W m^-2
- interactions: a process in which two or more systems exchange energy or momentum
- interference: when two or more waves meet to form a resultant wave of greater, lower or the same amplitude
- internal energy: the sum of the random kinetic energy of particles and the intermolecular potential energy in a system
- internal resistance: a resistance in series to the cell due to the chemicals in the cell
- isobaric: a thermodynamic process in which pressure stays constant
- isolated: a system whose total energy stays constant
- isolated system: a system where no energy or matter (mass) can be transferred into or from its surroundings
- isothermal: a thermodynamic process in which temperature stays constant
- isothermal curve: a curve on a pressure-volume diagram where all points have the same temperature
- isotopes: nuclei of the same element containing different numbers of neutrons
- isovolumetric: a thermodynamic process in which volume stays constant
- Kepler’s first law: planets move on ellipses with the sun at one of the foci of the ellipse
- Kepler’s second law: the line joining a planet and the sun sweeps equal areas in equal times
- Kepler’s third law: the period of revolution of a planet aroudn the sun is proportional to the 3/2 power of the semi-major axis of the ecllipse
- kinetic energy: the energy possessed by a body that is moving: E_K = 1/2 mv^2
- kinetic energy of rotational motion: E_K = 1/2 Iω^2
- law of conservation of energy: energy cannot be created or destroyed - it can only be transferred from one form to another
- law of conservation of total mechanical energy: in the absence of resistive forces and other external forces the total mechanical energy of a system stays the same
- law of reflection: the angle of incidence i is equal to the angle of reflection r
- length contraction: the length of an object that moves past an observer is shorter than the object in a frame where it is at rest; L'=L/γ
- Lenz’s law: the induced emf will be such so as to oppose the change in the magnetic flux; it is equivalent to energy conservation
- light dependent resistor (LDR): resistor where resistance decreases as light intensity increases
- linear momentum: (vector)P, is the product of the mass of a body and its velocity; p = mv
- longitudinal: a wave where the displacement is parallel to the direction of energy transfer
- luminosity: the power radiated by a star
- magnetic field: a region of space where a bar magnet would experience a magnetic force
- magnetic field lines: imaginary lines the tangents to which give the direction of the magnetic field
- magnetic flux: the product of the magnetic field flux density times the area of a loop times the cosine of the angle between the magnetic field direction and the normal to the loop
- magnetic flux density: the force per unit charge on a charge moving with unit velocity at right angles to the field
- main sequence: the stable phase in the life of a star; the stars produce energy in their core, from nuclear fusion of hydrogen into helium
- mass defect: the difference between the mass of the nucleons and the mass of a nucleus
- mechanical wave: a disturbance that transfers energy and momentum through oscillations of the particles of a medium
- melting: when a solid changes to a liquid (thermal energy is transferred to the solid)
- moderator: the fast neutrons produced in fission must be slowed down and this is achieved by collisions of the neutrons with the moderator atoms
- molar mass: the mass in grams of one mole of a substance
- mole: a quantity of a substance containing as many particles as atoms in 12 g of carbon-12
- moment of inertia: a measure of the distribution of mass of an extended body about an axis of rotation; I = ∑mr^2
- monochromatic light: light of a single wavelength
- motional emf: an emf produced due to the motion of a conductor in a magnetic field
- natural frequency: the frequency of free oscillations of a body
- Newton’s first law of motion: when the net force on a body is zero, the body will move with constant velocity (which may be zero); in other words, it will move on a straight line with constant speed (which may be zero)
- Newton’s law of gravitation: there is a force of attraction between any two point masses that is proportional to the product of the masses and inversely proportional to the square of their separation; the force is directed along the line joining the two masses
- Newton’s second law of motion: the net force on a body of constant mass is proportional to that body’s acceleration and is in the same direction as the acceleration
- Newton’s third law of motion: if body X exerts a force on body Y, then body Y will exert an equal and opposite force on body X
- node: a point on a standing wave where the displacement is always zero
- normal contact force: the force that arises when two bodies are in contact directed normally to the surface creating the force
- nuclear fusion: the joining of light nuclei to form heavier nuclei, releasing energy
- nucleon: a proton or a neutron
- nucleon number: the total number of protons and neutrons in a nucleus
- nucleosynthesis: the formation of the elements
- nuclide: a nucleus with a specific atomic and nucleon number
- Ohm’s law: at constant temperature the current through a metallic conductor is proportional to the voltage across the conductor
- Oppenheimer–Volkoff limit: the maximum possible mass of a neutron star, about 3 solar masses
- parallax: the fact that when you look at an object from two different positions, the object appears to shift relative to the very distant background
- parallel connection: resistors connected so that they have the same potential difference across them
- parent nucleus: the decaying nucleus
- path difference: the difference in distance of a point from each of the two sources
- peak wavelength: the wavelength corresponding to the peak of the black body spectrum curve
- period, T: the time it takes for an orbiting object to make one complete revolution; the time for one full oscillation or one full wave
- phase: the state of a substance depending on the separation of its molecules; we consider the solid, liquid and vapour phases in this course
- phase angle: the angle that appears in the formula for displacement, determined by the initial position and velocity
- phase difference: ∆ϕ = ∆t/T x 2π, where T is the period and ∆t the time difference between two neighbouring peaks
- photo-surface: technical term for the metallic surface that releases electrons when light is incident on it
- photoelectric effect: the emission of electrons from a metallic surface when light is incident on the surface
- photoelectrons: the electrons emitted in the photoelectric effect
- photon: the particle of light (a quantum of energy), a massless particle moving at the speed of light
- planetary nebula: the explosion of a red giant hurling mass and energy into space
- point mass: an idealised body with no physical size
- position: the coordinate on the number line
- position vector: the vector from the origin of a coordinate system to the position of a particle
- potential difference: the work done per unit charge in moving a point charge from one point to another
- power: the rate at which work is being done (or energy being transferred or dissipated); P=W/t
- pressure: the normal force on an area per unit area
- principle of superposition: when two or more waves of the same type arrive at a given point in space at the same time, the displacement of the medium at that point is the algebraic sum of the individual displacements
- proper length: the length of an object measured by an observer in a frame of reference in which the object is stationary with respect to the observer
- proper time interval: the time interval between two events that occur in a reference frame in which both events occur at the same position
- proton number: the number of protons in a nucleus
- proton–proton chain: the production of helium through the fusion of hydrogen
- quantised energy: discrete energy given by a formula involving an integer
- quantised quantity: a quantity whose magnitude is an integral multiple of a basic unit
- quantum: a small quantity of something, for example, energy
- radial: the direction towards or away from the centre of a spherical body
- radiation: method of thermal energy transfer though the emission of electromagnetic waves from a hot surface
- radioactivity: the phenomenon where particles and energy are emitted by an unstable nucleus
- random: it cannot be predicted when or which nucleus will decay
- rarefaction: a point where the density of the medium is lowest
- ray: a line perpendicular to a wavefront that gives the direction of energy transfer of the wave
- real gas: a gas obeying the gas laws approximately for limited ranges of pressures, volumes and temperatures
- red giants: very large, cool stars with a reddish colour; the core is hot enough for the helium to fuse to form carbon
- red-shift: the shift in wavelength towards larger wavelengths for a source getting further away
- reference frame: a set of co-ordinate axes and a set of clocks at every point in space
- resistivity: the resistance of a conductor of unit length and unit cross-sectional area
- resonance: the condition when the driving frequency is equal or slightly less than the natural frequency of a system, resulting in maximum amplitude
- rest frame: a reference frame in which a particular object is at rest
- resultant or net force: the one force whose effect is the same as that of a number of forces combined
- rotational equilibrium: the net torque on the body is zero
- series connection: wire connecting resistors does not have any junctions so both resistors take the same current
- simultaneity: events that take place at the same time are said to be simultaneous: solar constant, S
- the intensity of solar radiation received at the upper atmosphere of the earth
- spacetime diagram: a graph in which the y-axis represents time (although it is more usually given as: ct), and the x-axis represents position,: x; single events are then shown by a dot on the diagram
- spacetime interval: (∆s)^2 = (c∆t)^2 - (∆x)^2, has the same value in all intertial reference frames
- specific heat capacity: the energy required to change the temperature of a unit mass by one degree
- specific latent heat of fusion: the energy needed to change a unit mass from the solid to the liquid phase at constant temperature
- specific latent heat of vaporisation: the energy needed to change a unit mass from the liquid to the vapour phase at constant temperature
- spontaneous: the decay cannot be prevented from happening or forced to happen
- spontaneous fission: the splitting of a large nucleus into two smaller nuclei without neutron absorption
- standing wave: a wave formed by the superposition of two identical travelling waves in opposite directions
- state of a gas: a gas with a specific value of pressure, volume, temperature and number of moles
- states of matter: matter can exist in one of three states: solid, liquid or gaseous form
- static friction: a force opposing the tendency to motion when a body is at rest
- Stefan–Boltzmann law: the radiated intensity is proportional to the fourth power of the kelvin temperature
- stopping voltage: the voltage in the photoelectric effect that makes the current zero
- strong nuclear interaction: force that acts on protons and neutrons to keep them bound to each other inside nuclei; has short range (10^-15m)
- supernova: the explosion of a red super giant hurling mass and energy into space
- temperature: a measure of the average random kinetic energy of particles
- tension: the force arising when a body is being stretched or compressed
- terminal speed: the constant speed attained when the resistance force becomes equal to the force pushing the body
- thermistor: temperature dependent resistor where resistance decreases as temperature increases
- time dilation:
- torque: the product of the magnitude of the force and the perpendicular distance between the line of action of the force and the axis of rotation; τ = Fd or τ = Fr sinθ
- total mechanical energy: the sum of the kinetic energy, gravitational potential energy and elastic potential energy of a body
- transfer of thermal energy: the transfer of energy from one body to another as a result of a temperature difference
- translational equilibrium: the net force of the body is zero
- transverse: a wave where the displacement of the particles of the medium is at right angles to the direction of energy transfer
- travelling wave: A wave that transfers energy from one place to another through oscillations of a medium
- total internal reflection: when the angle of incidence is greater than the critical angle, the incident ray only reflects with no refracted ray
- uniform motion: motion with constant velocity
- vaporisation (or boiling): when a liquid changes into vapour (thermal energy is transferred to the liquid)
- velocity: displacement divided by time to achieve that displacement; v = ∆d/∆t, the rate of change of position
- voltage: the potential difference across a conductor
- voltmeter: an instrument that measures the potential difference across its ends
- wavefront: a surface through crests and normal to the direction of energy transfer of the wave
- wavelength: the length of one full wave
- weak nuclear interaction: force that acts on protons, neutrons, electrons and neutrinos in order to bring about beta decay; has very short range (10^-18m)
- weight: the gravitational force between the mass of a planet and a body; W = mg
- white dwarfs: small, hot stars no longer undergoing fusion; they get dimmer as they cool down
- Wien’s law: the peak wavelength is inversely proportional to the kelvin temperature
- work done: product of the force in the direction of the displacement multiplied by the distance travelled; W = Fs cosθ
- work–energy relation: the net work done on a system is equal to the change in kinetic energy of the system:
- work function: ϕ, the amount of energy required to free an electron from a photo-surface
- worldline: a line on a spacetime diagram relating a sequence of events, the angle of which is related to the speed at which something is moving; tanθ = x/ct = v/c