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Electricity Def
Physics definitions
| Term | Definition |
|---|---|
| Insulator | any substance through which electric charge cannot flow |
| Conductor | any substance through which electric charge can flow |
| Electric field | an electric field is any reegion of space where a static electric charge experiences a force other than the force of gravity. It's always caused by other static charges in the vecinity |
| Line of Force | a line drawn in an electric field showing the direction of the force on a positive charge placed in the field |
| Electric field strength at a point in an electric field | is the force per unit charge at that point ie force per coulomb |
| Electric current | is a flow of electric charge |
| Potential difference (between two points) | in an electric field is the work done in bringing a charge of +1C from one point to the other |
| Potential differennce (in a circuit) | between two points is the amount of energy converted from electrical to other forms between the two points when 1C charge passes any point of that circuit |
| Capicitance | of a conductor is the ratio of the charge on the conductor to its potential C=Q/V |
| The Farad | when placing a charge of 1C on a conductor raises its potential by 1 volt. 1farad=1C/V |
| Size of an electric current | in a conductor is the amount of charge passing any point of that conductor per second |
| Electromagnetic force | a voltage whe applied to a circuit |
| Resistance | of a conductor is the ratio of the p.d across it to the current flowing through it R=V/I |
| Ohm | a conductor has a resistance of one ohm if the current through it is 1 Ampere when th p.d across it is 1 Volt |
| Resistivity | if a conductor of length l and cross-section area A has a resistance R the constant p is given by p=RA/l |
| Ion | an atom or molecule that has lost or gained one or more electrons is called an ion |
| Kilowatt hour | the amount of energy used by 1,000W appliance in one hour |
| Semi-conductor | a substance whose resistivity is between between that of a good conductor and a good insulator. The resistivity of a semiconductor decreases as its temperature increases |
| Holes | when an electron breaks free from a covalent bond, it leaves behind a gap in the atom from which it came- this gap is a positively charged hole |
| Intrinsic conduction | conduction in a pure semi-conductor due to electrons moving from negative to positive and an equal number of holes moving in the opposite direction |
| Extrinsic conduction | increased conduction in a semi-conductor due to the addition of impurities |
| The thermistor | is a semi-conductor whose resistance decreases rapidly with increasing temperature |
| LDR | conductivity is increased when light shines on it |
| Doping | the adding of small controlled amounts of certain impurities to a pure semi-conductor to increase its conductivity |
| N-type semiconductor | the impurity added produces more free electrons available for conduction ie phosphorus |
| P-type semiconductor | the impurity added produces extra holes which are available for conduction ie boron |
| P-N junction | is a piece of semiconductor with part of it doped p-type and the rest doped n-type |
| Depletion layer | is the region at both sides of a p-n junction that contains no free majority charge carriers. It thus behaves as an insulator |
| Junction voltage | the p.d that exists across a p-n junction caused by holes and electrons moving across the junction when it was formed is called the junction voltage |
| Magnetic field | is any region of space where magnetic forces can be felt. The direction of the magnetic field at a point is the direction of the force on a north pole if it were placed at that point |
| Magnetic field line | a line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point |
| the magnetic flux density (B) | at a point in a magnetic field, is a vector whose magnitude is equal to the force that would be experienced by a conductor of length 1m |
| carrying a current or 1A at right angles to the field at the point and whose direction is the direction of the force on a north pole placed at that point | |
| The tesla | the magnetic flux density at a point is 1 tesla if a conductor of length 1m carrying a current of 1A experiences a force of 1N when placed perpendicular to the field |
| The Ampere | is that constant current which if maintained in two straight parallel conductors of infinite length of negligible cross section and placed 1m apart in a vacuum, would produce a force on each conductor of 2x10^-7 newtons per metre of length |
| The Coulomb | is the amount of charge that passes any point in a circuit when a current of 1A flows for 1second |
| Electromagnetic induction | whenever the magnetic field passing through a coil changes an emf appears in the coil |
| The Weber | if the magnetic flux density over an area of 1m^2 is 1 tesla then the flux through the area is 1 weber |
| Electric generator | is a device that converts mechanical energy to electrical energy |
| Mutual Induction | if a changing magnetic field in one coil causes an induced emf to appear in a nearby coil there is said to be mutual induction between the two coils |
| Self Induction | whenever the current passing through a coil changes the magnetic field surrounding |
| Transformer | is a device used to change the value of an alternating voltage |
| Thermionic emission | is the giving off of electrons from that surface of a hot metal |
| Cathode rays | streams of high speed electrons moving from the cathode |
| The electronvolt | is the emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency |
| Threshole frequency | metal the frequency below which photoemmision will not occur for a given. Light of frequency above the threshold frequency will cause photoemission |
| Work function | of a metal is the minimum energy needed to remove the loosest electron from the surface of that metal |
| Photon | is a packet of electromagnetic energy. The energy E of a photon is given by E=hf, where f is its frequency and h is Planck's constant |
| X-rays | are high frequency electromagnetic radiation produced when high speed electrons in a cathode ray tube strike a metal target that has a high melting point |
| Emission spectrum | the resulting pattern when light from a luminous source undergoes dispersion |
| Energy level | is a fixed energy value an electron in an atom can have |
| Atomic number | of an element is the number of protons in the nucleus of an atom of that element |
| Mass number | the total number of protons and neutrons in the nucleus of an atom |
| Isotopes | atoms of an element that have the same number of protons but different numbers of neutrons |
| Radioactivity | is the disintegration or decay of the nuclei of certain atoms with the emission of one or more types of radiation |
| Alpha α radiation | is fast moving hekium nuclei ejected from the nuclei of radioactive atoms |
| Beta β radiation | is high-speed electrons ejected from the nuclei of radiation atoms |
| Gamma γ radiation | is high frequency electromagnetic radiation (with frequencies above those of normal X-rays) emitted from the nucleus of a radioactive atom |
| Activity | of a radioactive substance is the number of nuclei of that substance decaying per second |
| Half-life | of a radioactive isotope/element is the time taken for: half of the undecayed atoms to undergo decay, :its activity to decrease by half |
| The Mole | of any substance is the amount of that substance that contains as many particles as there are atoms in exactly 12g of the c-12 isotope (avagadro's number) |
| Nuclear Fussion | is the joining of two small nuclei to form a larger nucleus |
| Nuclear Fission | is the splitting up of a large nucleus into two smaller nuclei of roughly the same size |
| Pair production | is the creating of two particles from energy. In the process a particle and its anti-particle are produced. Momentum and electric charge are conserved |
| An elementary particle | is a particle that has no other particles inside it or has no subparts to it |
| Leptons | is a particle that does not feel the strong force |
| Hadron | a particle that feels the strong force |
| Quarks | are elementary particles out of which baryons and mesons are made. They have a charge of +-1/3 or +-2/3 e |
| A meson | is made up of any one quark and any one anti-quark |
| A baryon | is made up of any three quarks |
| An anti-baryon | is made of any three anti-quarks |
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