9.3 Motors & Generators

Pregunta	Respuesta
force on a current-carrying conductor	F = BIlsintheta
torque	the turning moment of a force T = Fd T = nBIAcostheta
the motor effect	a current-carrying conductor in a magnetic field experiences a force
Faraday's discovery	when a conductor moves such that it cuts across a magnetic field, a current will flow in the conductor
magnetic flux	the number of magnetic field lines passing through an imaginary area in a given time
magnetic flux density	amount of magnetic flux per unit area / the strength of the magnetic field B
Lenz's law	the direction of any induced emf will always be such that it opposes the change that caused it. when a conductor moves through a magnetic field, current flows in the conductor, inducing a second magnetic field thus producing a force on the conductor
DC motor	electrical --> kinetic split-ring commutator
back emf in electric motors	as the rotor rotates, coils of wire are cutting flux, which induces a "back emf" between the ends of the wires. back emf is in the opposite direction and opposes the voltage (Lenz's law)
eddy currents	current produced by the back emf of a motor proportional to the speed of the motor minimised using a laminated soft iron core (insulating oxide layer disrupts eddy currents)
electromagnetic braking	electromagnets brought close to metal wheels, where eddy currents induced in the wheels create a magnetic field which opposes the field producing it, thus slowing the train/ride
induction cooktops	when AC passes through the coil under the cooktop, a changing magnetic field is created that induces eddy currents in metal pans. the resistance of metal in saucepan transforms electricity into heat energy, cooking the food.
DC generator	mechanical --> electrical split-ring commutator
energy loss in transmission lines	mainly due to resistance in transmission lines which converts electrical energy into heat
effect of AC generators on society	positive: E transmitted over long distances with minimal power loss, lowered cost of E, led to development of household appliances negative: increasing atmospheric pollution which is adverse to health, reliance on crude oil leading to the risk of war
effect of AC generators on the environment	positive: power generation now away from population centres, increased efficiency reduces pollution, technology leading to renewable sources of energy negative: CO2 emissions resulting from electricity generation, thermal pollution from power stations, health risks
AC generator	kinetic --> electrical slip-ring commutator
protection for transmission	shield conductors (intercept lightning strike & divert to earth), insulation chains (join wires to metal tower so tower E can't be conducted to wires), earth cable, metal tower, distance between towers
transformers	device used to change voltages within a circuit by changing the current. changing AC voltages in the primary coil create flux in the iron core, 'linking' with the secondary coil (which has a changing magnetic field through it) hence inducing voltage.
conservation of energy in transformers	Vp/Vs = Np/Ns as the current and magnetic field increase in the primary coil, the current in the secondary coil increases in a direction which produces an opposing magnetic field. when the current in the primary coil is switched off, the current in the secondary coil will increase in an attempt to maintain the field at its last stable level
impact of transformers on society	benefits: reduced power loss in transmissions, electricity can meet demands of appliances, fewer transmission lines required problems: increased demand, automation of industry (loss of jobs), risk of electrocution, dependence on electricity
AC electric motor	electrical --> kinetic no commutator since direction of current reverses 50 times/sec
AC induction motor	principle: a rotating magnetic field will exert a torque on a stationary coil current induced due to changing magnetic field in the stator; interaction of electric field + magnetic field results in torque

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