Paper 2 Definitions

Internal energy

The internal energy of an object is the sum of the random distribution of the kinetic and potential energies of its molecules.

First law of thermodynamics

The change of internal energy of an object = the total energy transfer due to work done and heating

Thermal equilibrium

If there is no overall energy transfer between two objects, those objects are in thermal equilibrium.

Absolute zero

0 K. The lowest possible temperature.

Absolute scale of temperature

The temperature scale in Kelvin. Based on absolute zero and the triple point of water.

Specific heat capacity

The energy required to increase the temperature of 1 kg of a substance by 1 K, without change of state.

Specific latent heat of fusion

The energy required to change the state of 1kg of a substance from solid to liquid, without change in temperature.

Specific latent heat of vaporisation

The energy required to change the state of 1kg of a substance from liquid to vapour, without change in temperature.

Gas pressure

The force per unit area that a gas exerts normally on a surface.

Boyle's Law

For a fixed mass of gas at constant temperature, pV = constant (where p = gas pressure and V = gas volume).

Charles' Law

For a fixed mass of gas at constant pressure, V/T = constant (where V =gas volume and T = temperature measured in Kelvin).

Pressure Law

For a fixed mass of gas at constant volume, p/T = constant (where p = gas pressure and T = temperature measured in Kelvin).

Ideal gas Law

pV/T =constant.An ideal gas obeys Boyle's Law. This law becomes pV = nRT, the ideal gas equation.

Avogadro's number/ constant, NA

The number of atoms in exactly 12 g of the carbon-12 isotope.

The mole

One mole of a substance is the quantity of that substance that contains NA particles.

The molar mass

The mass of 1 mol of a substance.

Gravitational field

The force field around a mass is a gravitational field. A force field is a region where a body will experience a non contact force.

Gravitational field line

The direction, and relative magnitude, of force on a small mass placed in the gravitational field of a large mass.

Gravitational field strength

The gravitational field strength at a point is the force per unit mass on a small mass at that point. (The gravitational field strength is equal to the negative of the potential gradient.)

Newton's Law of Gravitation

The force of attraction between two masses is directly proportional to the product of the two masses and inversely proportional to the square of their separation.

Gravitational potential energy

The energy of an object due to its position in a gravitational field (position for zero g.p.e. is at infinity).

Gravitational potential

The work done per unit mass in moving a small object from infinity to that point (i.e. the gravitational potential energy per unit mass)

Equipotential

A 2-dimensional surface of constant potential. No work needs to be done to move along an equipotential surface.

Potential gradient

The potential gradient at a point in a gravitational field is the change of potential per metre at that point.

Uniform field

Gravitational field strength is the same everywhere. Field lines are parallel and equally spaced.

Radial field

Due to a point mass or uniform spherical mass. Gravitational field strength follows inverse square law.

Escape velocity

The minimum velocity an object must be given to escape from a large mass (e.g. planet) when projected vertically from the surface.

Geostationary satellite

A satellite which orbits the Earth directly above the equator and has a time period of exactly 24 hours. (Note that a geosynchronous orbit is a 24 hour orbit inclined to the equator.)

Electric field

The force field around a charged object is an electrical field. A force field is a region where a body will experience a non contact force.

Electric field line

The direction, and relative magnitude, of force on a small positive charge placed in the electric field of a large charge.

Electric field strength

The electric field strength at a point is the force per unit charge on a small positive charge at that point. (The electric field strength is equal to the negative of the potential gradient.)

Coulomb's Law

The force of attraction/repulsion between two charges is directly proportional to the product of the two charges and inversely proportional to the square of their separation.

Electrical potential energy

The energy of an object due to its position in an electric field (position for zero e.p.e. is at infinity).

Electric potential

The work done per unit positive charge in moving a positive test charge from infinity to that point (i.e. the electric potential energy per unit charge)

Equipotential

A 2-dimensional surface of constant potential. No work needs to be done to move along an equipotential surface.

Potential gradient

The potential gradient at a point in an electric field is the change of potential per metre at that point.

Uniform field

Electric field strength is the same everywhere. Field lines are parallel and equally spaced.

Radial field

Due to a point charge. Electric field strength follows inverse square law.

Capacitance

The charge stored per unit potential difference of a capacitor.

Time constant

For discharge of a capacitor C through a fixed resistor R, the time constant is RC. This is the time taken for the voltage/current/charge to reduce to 1/e of its initial value.

Dielectric

An electrically insulating material that can be inserted between the parallel plates of a capacitor, that increases the ability of the capacitor to store charge.

Relative permittivity

For a fixed p.d. across a parallel plate capacitor, the ratio of the charge stored with a dielectric to the charge stored without the dielectric.

Dielectric constant

The dielectric constant of a medium is: permittivity of the medium / permittivity of free space

Magnetic field

The force field around a magnetic object is a magnetic field. A force field is a region where a body will experience a non contact force.

Magnetic field line

The direction, and relative magnitude, of force on an isolated North pole placed in a magnetic field.

Magnetic field strength / flux density, B

Force per unit length per unit current on a current-carrying conductor perpendicular to the magnetic field lines.

The Tesla

The strength of the magnetic field that produces a force of 1 N in a wire of length 1 m with 1 Ampere flowing perpendicular to the field.

Motor effect

When a current carrying wire placed at a non-zero angle to the field lines of an external magnetic field experiences a force due to the field.

Fleming's Left Hand rule

Used to determine the direction of the force on the wire relative to the direction of the field and the direction of the current.

Electromagnetic induction / generator effect

When a wire cuts across the lines of a magnetic field, an e.m.f. Is induced. If the wire is connected in a circuit, a current is induced.

Fleming's Right Hand rule

Used to determine the direction of the induced current relative to the direction of the field and the direction of the motion of the wire.

Lenz's Law

The direction of the induced emf when there is a change of flux linkage is such that it will try to oppose the change of flux that is producing it.

Magnetic flux,

The product of the magnetic flux density, B, and the area swept out. A. Can be thought of as the net number of field lines passing through - and perpendicular to - the area.

Magnetic flux linkage

Magnetic flux linkage through a coil of N turns is the product of N and the magnetic flux through each turn. Can be thought of as an extension to the idea of magnetic flux for a coil.

Faraday's Law of Induction

The induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.

Back e.m.f.

An e.m.f. induced in the spinning coil of an electric motor because the flux linkage through the coil changes.

R.M.S. current/ voltage

The root-mean-square value of an alternating current is the value of direct current that would give the same heating effect as the alternating current in the same resistor. The R.M.S. of an alternating current or p.d.

Step up transformer

Has more turns on the secondary coil than on the primary coil, so increases the voltage.

Step down transformer

Has fewer turns on the secondary coil than on the primary coil, so decreases the voltage.

Eddy currents

Loops of electric current induced within conductors by a changing magnetic field in the conductor or by the relative motion of a conductor in a magnetic field.

Nucleon

A proton or neutron in the nucleus of an atom

Nucleon number/ Mass number

The number of protons and neutrons in the nucleus of an atom

Proton number/ Atomic number

The number of protons in the nucleus of an atom

Isotope

An atom of the same element (i.e. same number of protons) but with a different number of neutrons

Alpha particle

Two protons and two neutrons. Alpha decay results in proton number decreasing by 2, and nucleon number decreasing by 4.

Beta (minus) particle

An electron. In beta (minus) decay, a fast moving electron and an antineutrino are emitted from the nucleus. Beta minus decay results in proton number increasing by 1 and nucleon number remaining the same.

Beta (plus) particle

A position (anti electron). In beta (plus) decay, a fast moving positron and a neutrino are emitted from the nucleus. Beta plus decay results in proton number decreasing by 1 and nucleon number remaining the same.

Gamma radiation

High frequency e-m radiation emitted by an unstable nucleus.

Ionising radiation

Radiation that produces ions in the substances it passes through. It destroys cell membranes and damages vital molecules such as DNA directly or indirectly by creating 'free radical' ions which react with vital molecules.

Count rate

Number of counts (e.g. recorded by a Geiger tube) per unit of time.

Intensity (of radiation)

Radiation energy per unit time passing normally through unit area.

Background radiation

A measure of the level of ionising radiation present in the environment at a particular location.

Decay curve

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

Half-life

The half-life of a radioactive isotope is the time taken for the mass of the isotope to decrease to half the initial mass.

Activity

The activity of a radioactive isotope is the number of nuclei of the isotope that disintegrate per second. Measured in Becquerel (Bq) where 1 Bq = 1 disintegration per second.

Decay constant

The probability of an individual nucleus decaying per second.

Nuclear ground state

The lowest energy state of a nucleus.

Nuclear excited state

A nucleus which is an energy state higher than its ground state.

Nuclear metastable state

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

Binding energy

The work that must be done to separate a nucleus into its constituent neutrons and protons.

Binding energy per nucleon

The average work done per nucleon to separate a nucleus into its constituent neutrons and protons.

Mass defect

The mass defect of a nucleus is the difference between the mass of the separated nucleons and the nucleus.

Nuclear fission

The splitting of a large nucleus (e.g. of uranium-235) into two approximately equal fragments. Induced fission is fission caused by an incoming neutron colliding with the nucleus.

Nuclear fusion

The fusing together of light nuclei to form a heavier nucleus.

Fission neutrons

Neutrons released when a nucleus undergoes fission.

Reactor core

The fuel rods and the control rods together with the moderator substance are in a steel vessel through which the coolant (which is also the moderator in a pressurised water reactor) is pumped.

Control rods

Made of a neutron-absorbing substance such as cadmium or boron, the controls rods are moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor.

Moderator

The moderator substance in a thermal nuclear reactor slows the fission neutrons down so they can go on to cause further fission. (A thermal nuclear reactor is simply a nuclear reactor that has a moderator in the core.)

Critical mass

The minimum mass of a fissile isotope in a nuclear reactor necessary to produce a chain reaction.