**Coulomb**– the unit of electric charge**Insulator**– a substance in which electricity does not flow**Conductor**– a substance in which electricity flows**All static charge lies on the**– outside of a conductor**Static charge on a conductor accumulates**– where the conductor is more pointed**Coulomb’s law**– force equals charge 1 times charge 2 divided by 4 pi epsilon distance squared**Relative permittivity of a medium**– permittivity divided by the permittivity of free space**Electric field**– a region of space where a static charge experiences a force due to its charge**Electric field line**– a line drawn in an electric field showing the direction of the force on a positive charge**Electric field strength**– force / charge = charge divided by 4 pi epsilon distance squared**Unit of electric field strength**– Newton / Coulomb or Volt / metre**Potential difference**– work done in moving +1C of charge in an electric field**Unit of potential difference**– Joule / Coulomb = Volt**Electric current**– flow of electric charge**Potential at a point**– potential difference between a point and the earth**All points on a conductor carrying static charge**– are at the same potential**Capacitance**– charge divided by voltage**Unit of capacitance**– Farad**Farad**– Coulomb divided by Volt**Capacitance of parallel plates**– permittivity times area divided by distance**Energy stored in a charged capacitor**– 0.5 times charge times voltage squared**The size of an electric current**– the amount of charge passing a point in one second**Conventional current**– flows plus to minus**Sum of currents flowing into a junction**– equals the sum of currents flowing out of the junction**Potential difference**– energy converted or work done when a charge of one coulomb is moved in an electric field**Electromotive force**– sum of all voltages around a complete circuit**Voltages in series**– V1 + V2 + V3**Voltages in parallel**– V1 = V2 = V3**Primary cell**– a cell which cannot be recharged**Secondary cell or accumulator**– a cell which can be recharged**Simple cell**– two different metal plates placed in an electrolyte**Resistance**– voltage divided by current**Unit of resistance**– ohm**Ohm’s law**– voltage divided by current is constant at constant temperature**Resistors in series**– R total = R1 + R2 + R2**Resistors in parallel**– inverse of total resistance equals sum of inverses of resistances**Resistance of a metal**– increases linearly with temperature**Resistance of a semiconductor**– decreases non linearly with temperature**Thermistor**– semiconductor resistance decreases rapidly as temperature increases**Resistivity**– resistance times area divided by length**Unit of resistivity**– ohm metre**Balanced wheatstone bridge**– R1 / R2 = R3 / R4**Potential divider**– gives any voltage from zero to the voltage supplied**Heat energy from electric current**– current squared times resistance times time**Electric power**– current squared times resistance**Chemical effect of electric current**– electric current can cause chemical change**Chemical changes**– chemical changes can give electric energy**Copper anode**– copper metal dissolves to give copper 2+ ions and 2 electrons**Copper cathode**– copper 2+ ions gain 2 electrons to form copper metal**Ion**– atom or molecule which has lost or gained one or more electrons**Charge carriers in an electrolyte**– positive and negative ions**Charge carriers in metal**– electrons**Charge carriers in semiconductors**– electrons and holes**Applications of chemical effect**– electrolysis electroplating extraction of ores purifying metals electrolytic capacitors**Live wire**– voltage varies from + 325 volts to – 325 volts**Neutral wire**– voltage about zero when device off**Earth wire**– safety wire connecting metal parts of device down to ground in case of accident**Ring circuit**– power fed along both sides of the ring to the sockets**Switch always connected in the**– live wire**Fuse**– a piece of wire which melts when a certain current passes through it**Fuse always connected in the**– live wire**MCD miniature circuit breakers**– contain a bimetallic strip and an electromagnet**MCD bimetallic strip trips**– small current**MCD electromagnet trips**– large current**RCD residual current device trips**– difference between current in live and neutral exceeds a given value usually 30 millAmps**RCD residual current device trips**– very quickly and so protects against electrocution**Bonding**– all metal water pipes in a house must be connected to earth**Kilowatt hour**– the amount of energy used by a 1000 Watt device in one hour**MCD miniature circuit breakers work**– contacts pulled apart when a specific current is exceeded**Kilowatt hour in joules**– 1000 Watts times 3600 seconds equals 3.6 Megajoules

**Did this raise a question for you?**Get involved in the discussion.