Physics definitions

Absolute scale

Temperature scale in kelvins (K) defined in terms of absolute zero, 0K, and the triple point of water, 273.15K, which is the temperature where ice, water and water vapour are in thermal equilibrium

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Absolute zero

The lowest possible temperature, the temperature at which an object has minimum internal energy

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Activity A

the number of nuclei of the isotope that disinegrate per second. Unit = becquerel (Bq) = 1 disintegration per second

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Alpha decay

Change in an unstable nucleus when it emits an alpha particle - consists of two protons and two neutrons

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Alpha radiation

easily absorbed by paper, has a range in air of a few cm and is the most ionising decay

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Amplitude

Maximum displacement from equilibrium of an oscillating object

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Angular displacement

Angle that an ovject in circular motion turns through

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Angular speed

The rate of change of angular displacement of an object in circular motion

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Angular frequency

For an object oscillating at frequeny f in SHM, its angular frequency = 2(pi)f

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Atomic mass unit, u

Referred to as the unified atomic mass constant = 1.661 x 10(-27)

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Avogadro constant N

Value = 6.02 x 10(23) mol-1

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Back emf

Emf induced in the spinning coil of an electric motor or in any coil in which the current is changing. A back emf acts against applied pd.

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Background radiation

Radiation due to naturally occurring radioactive substances in the environment - also caused by cosmic radiation

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beta- (B-) decay

change in a nucleus where a neutron changes into a proton and a B- (beta minus) particle and ANTINEUTRINO are emitted

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B- radiation

electrons emitted by unstable neutron-rich nuclei - it is easily absorbed, has a range in air of no more than a few cm, and is less ionising than alpha radiation and more ionising than gamma radiation

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beta+ (B+) decay

change in a nucleus where a neutron changes into a proton and a B+ (beta plus) particle and a NEUTRINO are emitted

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Beta+ (B+) radiation

Positrons (B+) are emitted by unstable proton-rich nuclei. Positrons emitted in solids or liquids travel no further than about 2mm before being annihilated

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Binding energy of a nucleus

The work that must be done to separate a nucleus into its constituent neutrons and protons. Binding energy = mass defect x c(sqd)

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Binding energy per nucleon

Average work done per nucleon to separate a nucleus into its constituent parts

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Boiling point

The temperature at which a pure liquid at atmospheric pressure boils

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Boyle's law

For a fixed mass of gas at constant temperature, its pressure x volume is constant. A gas obeying Boyle's law = IDEAL GAS

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Boltzmann constant k

Molar gas constant / Avogadro number

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Capacitance

Charge stored per unit pd of a capacitor, unit = farad (F) = 1C/V

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Capacitor discharge

Through a fixed resistor of resistance R, time constant = RC; exponential decrease equation for current or charge or pd

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Centripetal acceleration

a = v(sqd)/r = w(sqd)r towards the CENTRE of the circle

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Centripetal force

The resultant force (on an object that moves along a circular path) towards the centre of the circle

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Chain reactions

A series of reactions in which each reaction causes a further reaction. A stead chain reaction occurs when one fission neutron on avecage from each fission event produces a further fission event

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Charles' law

For an IDEAL gas at constant pressure, volume is proportional to its absolute temperature

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Conservation of momentum

Total momentum of objects remains constant provided no external force acts.

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Control rods

Rods made of a neutron-absorbing substance that are moved in or out of the core of a nuclear reactor to control the rate of fission events

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Coolant

Fluid that is used to prevent a machine or device from becoming dangerously hot. Transfers thermal energy to heat exchanger

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Count rate

Number of counts per unit time detected by a Gieger Muller tube

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Critical mass

Minimum mass of the fissile isotope in a nuclear reactor necessary to produce a chain reaction.

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Damped oscillations

Oscillations that reduce in amplitude due to the presence of resistive forces such as friction and drag

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DeBroglie wavelength

Particle of matter that has a wave-like nature e.g. electrons fired at a thin crystal are diffracted. = h/p = h/mv

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Decay constant

Probability of an individual nucleus decaying per second

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Decay curve

An exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time

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Diffraction

Spreading of waves whent hey pass through a gap or round an obstacle

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Dissipative forces

Forces that transfer energy which is wasted

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Dose equivalent

Comparitive measure of the effect of each type of ionising radiation, defined as energy that would need to be absorbed per unit mass of matter

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Eddy currents

Unwanted induced currents in the metal parts of ac machines

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Elastic collision

total Ek before = total Ek after collision

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Electric field strength E

Force per unit charge on a small + charge at a point in a field

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Electric potential V

Work done per unit charge on a small + charge to move it from infinity to a point in the field

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Electromagnetic induction

Generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines

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Electron capture

A proton-rich nucleus captures an inner-shell electron to cause a proton to change into a nuetron, and a NEUTRINO is emitted and a photon is also emitted

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Equilibrium

State of an object when at rest or in uniform motion

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Equipotential

A line or surface in a field along which the electric or gravitational potential is constant

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Excited state

An atom which is not in its ground state

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Explosion

When two objects fly apart, the two objects carry away equal and opposite momentum

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Exponential charge

Happens when the change of quantity is proportional to the quantity itself.

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Faraday's law of electromagnetic induction

Induced emf in a circuit = rate of change of magnetic flux linkage through the circuit

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Fission

The splitting of a Uranium nucleus or a Pu nucleus into two approximately equal fragments. Induced fission is fission caused by an incoming neutron colliding with the nucleus.

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Fission neutrons

Neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission

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Fleming's left-hand rule

A rule that relates direction of the force, magnetic field and curent on a current-carrying conductor in a magnetic field

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Fleming's right-hand rule

Rule that relates the directions of the induced current, magnetic field and velocity of the conductor when the conductor cuts across magnetic field lines and an emf is induced in it.

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Free electrons

Electrons in a conductor that move about freely inside the metal because they are not attached to a particular atom.

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Free oscillations

oscillations where there is no damping and no periodic force acting on the system so the amplitude of the oscillations is constant.

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Forced oscillations

Oscillations of a system that is subjected to an external periodic force

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Frequency

(of an oscillating object) = number of cycles of oscillation per second

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Nuclear fusion

Fusing together of light nuclei to form a heavier nucleus

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Thermal fusion

Fusin together of metals by melting them together.

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Gamma radiation

Electromagnetic radiation emitted by an unstable nucleus when it becomes more stable.

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Geostationary satellite

A satellite taht stays above the same point on the Earth's equator as it orbits the Earth because its orbit is in the same plane as the equator, its period is exactly 24h and it orbits in the same direction as the Earth's rotation.

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Gold leaf electroscope

A device used to detect electric charge.

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Gravitational constant G

Constant of proportionality in Newton's law of gravitation.

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Gravitational field

The region surrounding an object in which it exerts a gravitational force on any other object.

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Gravitational field strength, g

Force per unit mass on a small mass placed in the field.

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Gravitational force

An ATTRACTIVE force that acts equally on any two objects due to their mass.

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Gravitational potential V

Work done per unit mass to move a small object from infinity to a point in a field.

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Gravitational potential energy

Work done to move a small object from infinity to that point.

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Grid system

Network of transformers and cables that is used to distribute electrical power from power stations to users.

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Ground state

The lowest energy state of an atom.

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Half-life, T1/2

Time taken for mass/activity/nucleon number of a radioactive isotope to decrease to half the initial mass/activity/nucleon number.

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Hall probe

A device used to measure magnetic flux density.

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Heat Q

Energy transfer due to a difference of temperature.

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Heat capacity

The energy needed to change the temperature of an object by 1K.

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Heat exchanger

A steel vessel containing pipes through which hot coolant in a sealed circuit is pumped, causing water passing through the steel vessel in spearate pipes to turn to steam, used to drive turbines.

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Ideal Gas

A gas under conditions such that it obeys Boyle's law.

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Ideal gas equation

pV = nRT.

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Impulse

Force x time.

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Insulating materials

A material that cannot conduct electricity/heat.

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Integration

Mathematical process of finding the area under a curve from its mathematical equation.

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Intensity of radiation

Radiation energy per second per unit area at normal incidence to the surface.

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Internal energy

Sum of the random distribution of the kinetic and potential energies of its molecules.

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Ionising radiation

Radiation that produces ions in the substances it passes through - destroys cell membranes and damages vital molecules (e.g. DNA) directly or indirectly by creating 'free radical' ions which react with vital molecules.

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Isotopes

Atoms which have the same number of protons in each nucleus but different number of neutrons.

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Inverse square laws

Newton's law of gravitation; Coulomb's law of force; Intensity of gamma radiation inverseley proportional to the square of the distance.

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Kepler's thrid law

For any planet, the cube of its mean radius of orbit r is directly proportional to the square of its time period T.

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Kinetic energy

The energy of a moving object due to its motion.

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Kinetic energy of the molecules of an ideal gas

Mean Ek = 3/2kT, Total Ek of n moles = 3/2nRT.

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Kinetic theory of a gas - Assumptions;

1. a gas consits of identical point molecules which do not attract one another. 2.The molecules are in continual random motion colliding elastically with each other and the container

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Latent heat of fusion

The energy needed to change the state of a solid to a liquid without change of temperature

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Latent heat of vaporisation

The energy needed to change the state of a liquid to a vapour without change of temperature

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Lenz's law

When a current is induced by electromagnetic induction, the direction of the induced current is always such as to oppose the change that casues the current.

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Line of force/field line

A line followed by a small mass in a gravitational field (or a small +vley charged object in an electric field or a free north pole in a magnetic field) acted on by no other forces than the force due to the field.

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Logarithmic scale

A scale such that equal intervals correspond to a change by a constant factor or multiple (e.g. x10).

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Magnetic flux

= BA for a uniform magnetic field of flux density B, perpendicular to an area A.

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Magnetic flux density B

THe magnetic force per unit length per unit current on a current-carrying conductor, at right anles to the field lines. Unit = tesla, T. B = sometimes magnetic field strength

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Magnetic flux linkage N#

Through a coild of N turns, = NBA. Unit = Weber, WB = 1T per metre sqd or 1 Vs

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Mass defect

The difference between the mass of the separated nucleons and the nucleus.

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Mean kinetic energy

=3/2kT.

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Melting point

The temperature at which a pure substance melts.

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Mestastable state

An excited state of the nuclei of an isotope that lasts long enough after a or B emission for the isotope to be separated from the parent isotope.

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Moderator

Substance in a thermal nuclear reactor that slows the fission neutrons down so they can go on to produce further fission.

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Mole

1 Mole of a substance consisting of identical particles = quantity of substance that contains N.a. particles of the substance.

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Molarity

The number of moles in a certain quantity of the substance. Unit = mol.

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Momentum

Mass x velocity, unit = kgm per second.

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Motor effect

Force on a current-carrying conductor due to a magnetic field.

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Natural frequency

The frequency of free oscillations of an oscillating system.

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Newton's law of Gravitation

The gravitational force F between two point masses M and m at distance r apart is given by F = GMm/rsqd.

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Newton's laws of motion: 1

Object continues at rest or in uniform motion until it is acted on by a resultant force.

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Newton's laws of motion: 2

The rate of change of momentum of an object is proportional to the resultant force on it.

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Newton's laws of motion: 3

When two objects interact, they exert equal and opposite forces on one another.

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Nucleus

The relatively small part of an atom where all the atom's positive charge and most of its mass is concentrated.

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Pair production

When a gamma photon changes into a particle and an antiparticle.

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Periodic force

A force that varies regularly in magnitude with a definite time period.

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Permittivity of free space, Eo

The charge per unit area in coulombs per square metre on oppositely charged parallel plates in a vacuum when the electric field strength between the plates is 1V/m.

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Phase difference

In radians, is the time between successive instants whent eh two objects are at maximum displacement in the same direction.

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Photon

Each photon is a wave packet of electromagnetic radiation. The energy of a photon, E = hf.

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Positron

a particle of antimatter taht is the antiparticle of an electron.

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Potential gradient

Change of potential per unit change of distance along the field line at that point. Potential gradient = -field strength at any point.

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Power

Rate of transfer of energy. P = E/t.

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Pressure

The force per unit area that a gas or a liquid or a solid at rest exerts at right angles to a surface. Unit = pascals, Pa = 1N/msqd.

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Pressure law

For a fixed mass of an ideal gas at constant volume, its pressure is directly proportional to its absolute temperature.

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Principle of conservation of momentum

When two or more bodies interact, the total momentum is unchanged, provided no external foce acts on the bodies.

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Principle of conservation of energy

In any change, the total amount of energy after the change is always equal to the total amount of energy before the change.

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Radial field

A field in which the field lines are straight and converge/diverge as if from a single point.

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Reactor core

The fuel rods, control rods, and absorber rods of a nuclear reactor which together with the moderator substance are ina steel vessel through which the coolant is pumped.

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Renewable energy

Energy from a source that is continually renewed - e.g. hydroelectricity, tidal power, wave power, geothermal power, solar power, wind power.

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Resonance

The amplitude of vibration of an oscillating system subjected to a periodic force has the same frequency as the resonant frequency of the system. At resonance, the system vibrates such that its velocity is in phase with the periodic force.

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Resonant frequency

The frequency of an oscillating system in resonance.

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Root mean square speed

The square root of the mean value of the square of the molecular speeds of the molecules of a gas.

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Rutherford's a-particle scattering experiment

Demonstrated that every atom contains a positively charged nucleus which is much smaller than the atom and where all the positive charge and most of the mass is located.

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Satellite

A small object in orbit round a larger object.

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Simple electric motor

An electric motor with an armature consisting of a single coil of insulated wire.

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Simple harmonic motion

Motion of an object if its acceleration is proportional to the displacement of the object from equilibrium and is always directed towards the equilibrium position.

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Sinusoidal curve

Any cuve with the same shape as a sine wave (e.g. a cosine curve)

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Specific heat capacity, c

Energy needed to raise the temperature of 1kg of the substance by 1K without change of state.

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Specific latent heat of fusion

Energy needed to change the state of unit mass of a solid to a liquid without change of temperature.

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Specific latent heat of vaporisation

Energy needed to change the state of unit mass of a liquid to a vapour without change of temperature.

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Strong nuclear force

Force that holds the nucleons together. Range of 2-3fm and is attractive down to distances of about 0.5fm. Below this distance, it is repulsive.

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Sublimation

The change of state when a solid changes to a vapour directly.

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Temperature

The degree of hotness of an object.

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Thermal energy

The internal energy of an object due to temperature.

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Thermal equilibrium

When no overall heat transfer occurs between two objects at the same temperature.

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Thermal nuclear reactor

Nuclear reactor which has a moderator in the core.

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Time constant

The time taken for a quantity that decreases exponentially to decrease to 0.37 (=1/e) of its initial value. For the discharge of a capacitor through a fixed resistor, the time constant = resitance x capacitance.

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Time period, or Period

Time taken for one complete cycle of oscillations.

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Transformer

Converts the amplitude of an alternating pd to a different value. Consists of two insulated coils, the primary coil and the secondary coil, wound round a soft iron laminated core.

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Step-down transformer

A transformer in which the rms pd across teh secondary coil is less than the rms pd applied to the primary coil.

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Step-up transformer

A transformer in which the rms pd across the secondary coild is greater than the rms pd applied to the primary coil.

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Transformer rule

The ratio of the secondary voltage to the primary voltage is equal to the ratio of no. of secondary turns to the no. of primary turns.

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Transformer efficiency

Output power = secondary voltage x secondary current. Input power = primary voltage x primary current.

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Transformer inefficiency

Due to: resistance heating of the current, in each coil; heating effect of eddy currents (unwanted induced currents) in the core; repeated magnetistaion and demagnetisation of the core.

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Uniform circular motion

Motion of an object moving at constant speed along a circular path.

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Uniform field

A region where the field strenght is the same in magnitude and direction at every point in the field.

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Velocity

Change of displacement per unit time.

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Work done

Energy transferred by means of a force. W = Fd.

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Wave-particle duality

Matter particles have a wave-like nature, e.g. electrons diffracted by the crystal, and particle-like behaviour, e.g. electrons deflected by a magnetic field. Photons also have a wave-particle duality.

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X-rays

Electromagnetic radiation of wavelength less than about 1 nm. Emitted from X-ray tube as a result of fast-flowing electrons. X-rays are ionising and penetrate matter - can be stopped by thick lead plates.

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