# AS Definitions

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```					                                 AS Definitions

   Scalar. Physical Quantity with magnitude only - no direction.
   Vector. Physical quantity with magnitude and direction.
   Coplanar. In the same plane.
   Systematic Error or Uncertainty. Error in which measurements are consistently
too high or too low due to the apparatus or the way it is used.
   Random Error or Uncertainty. Statistical error with some readings being too high
or too low caused by a lack of care or lack of sensitivity of measuring instrument.
   Displacement. Distance in a particular direction
   Speed. Distance travelled /Time taken
   Velocity. Rate of change of displacement
   Acceleration Rate of change of velocity
   Force. The force on a body is the rate of change of momentum.
   The Newton. There is a net force of 1 newton acting if a one kilogram object has
an acceleration of one metre per second per second.
   Upthrust. The apparent loss of weight of a body immersed in a fluid and equal to
the weight of the fluid displaced.
   Viscosity. The internal friction of a fluid caused by molecular attraction.
   Friction. The resistance to motion between two surfaces that touch.
   Mass. A property of a body which resists change in motion.
   Weight. The effect of gravitational field on a mass.
   Inertia/Inertial Mass. Resistance to change in motion of a body.
   Centre of Gravity. The single point through which the weight may be assumed to
act.
   Moment of a force or Torque. The product of the force and the perpendicular
distance from the pivot to the line of action of the force.
   Couple. Two forces which act on a body and are equal in magnitude and opposite in
direction separated by a perpendicular distance.
   Torque of a couple. The product of ONE of the forces and the perpendicular
distance between them.
   Equilibrium. A system is in equilibrium if there is no resultant force and no resultant
torque on it.
   Principle of Moments. If a body is in equilibrium the sum of the anticlockwise
moments on it equals the sum of the clockwise moments about any pivot.
   Work. The product of the force acting on a body and the displacement in the
direction of that force.
   Joule Work done when a force of 1 Newton is moved a distance of 1 metre.
   Power. Power is defined as the rate of work done or energy transferred.
   Watt. The power when one joule per second of work is done/energy transferred.
   Kinetic Energy. The Energy that something possesses because of its motion.
   Gravitational Potential Energy. The Energy that something possesses because of
its position in a gravitational field.
   Gravitational Field Strength. The Force per unit mass felt by matter in a
gravitational field.
   Amplitude. Displacement from equilibrium position
   Period. Time taken for one complete oscillation.
   Frequency. Number of oscillations per second.
   Hertz. 1 Hz is one cycle per second.
   Phase Difference. The angular separation between two parts of a wave. It can be
expressed as the fraction of a cycle x 360o.
   In Phase. When two waves are in phase they have no phase difference.
   Anti Phase. Two waves that are 180 o out of phase.
   Refraction. The bending of waves that occurs at a boundary when they enter a
medium where their velocity changes.
   Refractive Index. Ratio of velocity of light in a vacuum to velocity of light in
medium.
   Critical Angle. Angle of incidence for which angle of refraction is 90° when light is
travelling into a less dense medium.
   Total Internal Reflection. If the angle of Incidence exceeds the critical angle there
is Total Internal Reflection.
   Progressive Wave. A wave where there is net transfer of Energy.
   Standing Wave. A wave in which there is NO net transfer of Energy. A standing
wave is formed when two identical waves travelling in opposite directions meet
   Wave Intensity. The intensity of the wave is proportional to the square of the
Amplitude. It is the rate of energy per unit area.
   Transverse Waves. A wave in which displacement moves at a right angle to the
direction of travel of the wave.
   Longitudinal Waves. A wave in which displacement is parallel to the direction of
travel of the wave.
   Wavelength. Distance between any part of the wave and the nearest part In Phase
with it.
   Polarization. If all the vibrations of a transverse wave are in a single plane the
wave is said to be plane polarized. (In an e-m wave it is the vibrations of the electric
field.)
   Wavefront. Line or surface in the path of a wave motion on which the disturbances
at every point have the same phase.
   Principle of Superposition. The displacement of the resultant of two waves is the
sum of the displacements of the individual waves.
   Interference. Interference of waves takes place when two or more waves of the
same type meet in space in such a way that there is a constant phase relationship
between them and they are of similar amplitude and not oppositely polarized.
   Coherence. Two waves are coherent if there is a constant phase difference between
them and therefore they must have the same frequency.
   Diffraction. Spreading out of waves into a region of geometric shadow experienced
by waves as they pass through an aperture or around an obstacle. Diffraction is
maximized if the gap/obstacle size is similar to the wavelength.
   Nodes. Positions of no displacement in standing waves.
   Anti-nodes. Positions of maximum displacement in standing waves.
   The Tesla. The magnetic Flux Density of a field in which a force of one Newton acts
on a one metre length of a conductor in the field which is carrying a current of one
Ampere and is perpendicular to the field.
   Magnetic Flux Density. Is the magnetic force per unit current per unit length of
conductor in the field which is lying at right angles to the field.
   Density. The Density of a substance is the ratio of its mass to its volume.
   Crystalline Solid. In a crystalline solid the atoms or molecules are arranged in a
regular 3-dimensional array.
   Non-crystalline. In a non-crystalline solid the atoms or molecules are not arranged
in a regular 3-dimensional array.
   Metallic Solid. Metals comprise an array of positive ions in a sea of electrons.
   Polymer. A polymer has large molecules in the form of long chains, each chain
consists of a large number of small molecules and is called a monomer.
   Amorphous Solid. An amorphous solid has no order – the molecules are randomly
arranged.
   Pressure. The pressure acting on a surface is defined as the force acting per unit
cross-sectional area.
   Stress. Force per unit cross-sectiional area.
   Strain. Change in length divided by original length.
   Elastic Behaviour. A material is said to be elastic if it returns to its original size and
shape when the load which is deforming it has been removed.
   Plastic Behaviour. A material is said to be plastic if it does not return to its original
size and shape when the load which has been deforming it is removed.
   Ductile. A ductile material is one which can be permanently stretched.
   Stiffness. The resistance which a material offers to having its size or shape
changed.
   Elastic Limit. The maximum load which a body can experience and still retain its
original size and shape once the load has been removed.
   Breaking Stress. Stress at which material breaks.
   Ultimate tensile strength. Maximum stress which can be applied to a material
without causing it to break.
   Brittleness. A brittle material cannot be permanently stretched – it breaks soon
after its elastic limit has been reached.
   Young Modulus. Provided that the stress is not too high and that the limit of
proportionality has not been exceeded the ratio stress/strain is a constant for a given
material and is known as the Young modulus.
   Charge. The property of a body which causes it to feel a force in an electric field.
   Charging by friction. Separation of charges by rubbing.
   Charging by Induction. Separation of charge by position in an Electric Field.
   Coulomb. The charge which flows past any one point in any one second when one
ampere of current flows.
   Electric Current. Flow of charged particles.
   The Ampere is defined in terms of the magnetic force between two parallel current
carrying conductors.
   Potential Difference. The p.d. between two points in an electric field is the energy
converted from electrical energy to other forms of energy when unit charge passes
between the two points or the electrical energy lost by the charge per unit charge.
   The Volt. The p.d. between two points when one Joule of work is done in moving
one Coulomb between the two points.
   Electrical Resistance. …defined in a conductor by the equation R=V/I i.e. it is the
ratio of potential difference across a conductor to the current flowing through the
conductor.
   The Ohm. The resistance of a conductor through which a current of 1 amp is flowing
when a p.d. across it is 1 Volt.
   Ohm's Law. The current through an Ohmic Conductor is directly proportional to the
p.d. across it provided there is no change in the physical conditions of the conductor
i.e the temperature.
   Resistivity. Defined by the equation =Rl ...in words the resistance x unit cross-
sectional area per unit length.
   Internal Resistance. The chemicals in a cell or the structure of a power pack offer
resistance when a current is drawn. This resistance is called the internal resistance.
   Electromotive Force EMF. …of a cell is the energy converted into electrical energy
from other forms when unit charge passes through it or the amount of electrical
energy gained by the charge per unit charge.
   Terminal Potential Difference. The p.d. across the terminals of a power pack or
cell. It is equal to the e.m.f. when no current is drawn.
   Quantisation of Charge. Electric charge can never exist in fractions of “e”.
   Photoelectric Effect. Emission of electrons from the surface of a metal irradiated
by e-m radiation., usually ultraviolet.
   The Photon. A quantum of e-m radiation (of energy E=hf, where h is Planck’s
constant).
   Work Function Energy. The Work function energy of a surface is the minimum
amount of energy that has to be given to an electron to release it from the surface.
   Threshold Frequency. Minimum value of frequency of incident radiation necessary
to cause Photoelectric emission.
   The Electronvolt. A unit of energy equal to the kinetic energy gained by an electron
in being accelerated through a potential difference of 1 volt.
   De Broglie Wavelength. Any particle of momentum mv has an associated
wavelength, the de Broglie wavelength, given by =h/mv.
   The kilowatt-hour. A unit of electrical energy equal to the energy transferred
when an electrical device of power one kilowatt operates for one hour.
A2 PHYSICS DEFINITIONS

   Gravitational Potential Energy. The Energy that something possesses because of
its position in a gravitational field.
   Field of force. A region in space in which an object experiences a force.
   Gravitational Field Strength. The Force per unit mass felt by matter in a
gravitational field.
   Newton’s Law of Gravitation. F = GM1M2/r2 where M1 and M2 are point masses
and r is the separation between them. G = the universal gravitational constant =
6.67 x 10-11 Nkg-2m2.
   Gravitational Field strength of a point mass. ...is proportional to the mass and
inversely proportional to the SQUARE of the distance from the point mass.
   Angular Displacement. Angle turned through IN A SPECIFIED DIRECTION.
   Angular Velocity. Rate of change of angular displacement.
   Centripetal Acceleration. The acceleration experienced by a body moving in a
circle which always acts towards the centre of the circle. a = v 2/r
   Centripetal Force. The force which causes centripetal acceleration. It is the
resultant of all the forces acting on the body.
   Geostationary Orbits. Satellite with an orbital period of 24 hours above the
equator and moving in the same direction as the earth is rotating. Thus it stays
above the same point on the Earth’s surface.
   Free Oscillations. An oscillating system which is not forced to move.
   Simple Harmonic motion. A system oscillates with SHM if a restoring force acts on
it proportional to its displacement from its equilibrium position and always towards its
equilibrium position. Acceleration is a = - (2f)2x.
   Forced Oscillations. A system is undergoing forced oscillation if it is being forced to
oscillate by some other system at the frequency of that system
   Damping. Oscillation of reducing amplitude owing to energy from the system being
absorbed by the surroundings.
   Resonance. A system which is being forced at its natural frequency is said to be
resonating – its amplitude is maximum.
   Electric field strength, E, is electric force per unit charge experienced by a positive
test charge in the field. (E at a point is proportional to the potential gradient at the
point i.e. the p.d. per unit distance.)
   Electric field strength of a parallel plate field is the potential difference
between the plates divided by their separation. E = V/d.
   Coulomb’s Law. The electric force between two pint charges is proportional to the
product of the charges and inversely proportional to the square of their separation. F
= 1/40 x Q1Q2/r2.
   Electric field strength of a point charge = 1/40 x Q/r2.
   Capacitance. Ratio of charge stored to potential difference across a capacitor.
   Farad. The capacitance of a capacitor which stores one Coulomb of charge when
there is one Volt across it.
   Magnetic Flux. …through a region is a measure of the number of magnetic field
lines passing through the region. It is defined as the magnetic flux density normal to
the area x the area.  = BA
   The Weber. 1 weber = 1 tesla-metre squared.
   Magnetic Flux Linkage. The product of the number of turns and the flux through a
coil. It is the magnetic flux threading the coil. Its unit is the weber-turn.
   Faraday’s Law. The magnitude of the emf induced in a coil is equal to the rate of
change of flux-linkage through the coli - in a conductor it is equal to the rate of
cutting of magnetic flux.
   Lenz’s Law. The direction of the emf is such as the current it induces opposes the
change causing it.
   Internal Energy, U. The sum of the random potential and kinetic energies of the
molecules of a system is called the internal energy and is determined by the state of
the system.
   Temperature. The temperature of a body is a measure of how hot it is i.e. the
mean kinetic energy of its molecules NOT how much heat it contains. For an ideal
gas it is proportional to the internal energy of the gas
   Absolute or Thermodynamic Temperature Scale. A temperature scale which
does not depend on the properties of any substance. Its fixed points are absolute
zero (0K) and the triple point of water (273.16K). Its unit is the Kelvin.
   Empirical Temperature Scale. A temperature scale which does depend on the
properties of a particular substance. Its unit is the centigrade.
   Celsius temperature. /oC = T/K – 273.15.
   Specific Heat Capacity. The Specific Heat Capacity of a substance is the heat
required to produce unit temperature rise in unit mass of the substance.
   The Equation of State or Ideal Gas Equation. Relates the pressure, volume,
absolute temperature and number of moles of an Ideal gas i.e. pV = nRT, where R
= molar gas constant 8.31 Jkg-1mol-1.
   An Ideal Gas. Defined as one which obeys the Ideal gas Equation exactly. It has
only internal kinetic energy since it is assumed there are no interatomic forces in an
ideal gas. (See any textbook for assumptions made about ideal gases).
   The Mole. The amount of a substance which contains the same number of
molecules or elementary units as there are atoms in 12g of Carbon-12. i.e.
Avogadro’s number.
   The Avogadro Constant - Numerically equal to the number of atoms in one mole
NA = 6.02 x 1023.
   The Relative Molecular Mass. Ratio of average mass of a molecule of a substance
to one twelfth the mass of a Carbon-12 atom.
   The Molar Mass. The mass per mole of a substance.
   Thermal Equilibrium. If two bodies are in thermal contact and there is no net flow
of heat energy between them they are said to be in Thermal Equilibrium and so at
the same temperature.
   STP. Standard temperature and pressure (273K and 1.01 x 105 Pa).
   The average translational Kinetic Energy of the molecules of a gas is
proportional to its absolute temperature. ( U = 3/2kT where k is Boltzmann’s
constant and equals R/ NA, and T is Absolute Temperature.)
   Nuclear Radiation. Emission of particles and/or waves from an unstable nucleus to
enable it to achieve a more stable or lower energy state.
   The Nuclear Atom. The positively charged core of an atom containing most of the
mass of the atom. It is surrounded by orbiting electrons.
   Nucleon. An inhabitant of the nucleus i.e. protons and neutrons.
   Isotopes. Nuclides with the same proton number but different nucleon numbers.
   Binding Energy. Energy required to separate nucleons in a nucleus to infinite
separation.
   Binding Energy per Nucleon. Average energy which needs to be supplied to
nucleus to remove a nucleon.
   Nucleon or Mass Number. Number of neutrons and protons in the nucleus of an
atom.
   Proton or Atomic Number. Number of protons in the nucleus of an atom.
   Activity. The rate of decay of a radioactive nuclide.
   Decay Constant. Ratio of activity to the number of undecayed nuclei in a nuclide.
=A/N It represents the probability of a nucleus decaying in the next second.
   Half Life. Time taken for half the undecayed nucleii in a sample to decay.
   Parent Nucleus. Emitting nucleus.
   Daughter Nucleus or decay product. New nucleus formed when decay occurs

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