Topic 13 - Electromagnetic Induction

• A magnet has a magnetic field around it. • Once it moves throughout the wire, the field induces a P.d. in the wire which creates a magnetic field opposite to original change.

• A coil of wire wrapped around an iron core is rotated inside a magnetic field • Wires from the coil are connected to slip rings on the axle of the coil • The slip rings make electrical contact through carbon brushes • This produces an alternating current

Same concept as an alternator except: • The wires are connected to a split ring commutator • The commutator is connected to the carbon brushes • Every half-turn the connections are swapped • Thus, this produces a direct current

• (The opposite to loudspeakers) • Sound waves cause vibrations in air pressure • The pressure causes a diaphragm to move back and forth • The diaphragm moves a coil of wire back and forth • This causes the coil which is wrapped around a permanent magnet to gain an induced voltage and current • The amplitude of the wave affects the volume • The frequency affects the pitch • Converting sound energy into electrical energy

• (The opposite to microphones) • An a.c. voltage passes througha coil • This moves the coil along a permanent magnet • The coil hits the diaphram causing it to vibrate • This creates soundwaves • The higher the current, the larger the frequency and amplitude • And so the volume and pitch is higher • Converting electrical energy into sound energy

The system that connects power stations to homes to provide electricity nationally

Transformers are (not robots in disguise) devices that alter the voltage and current of electricity. • SU increase voltage; decrease current • SD decrease voltage; increase current

• Electricity is generated at the power station (11kV) • Stepped up (400kV) for transmission lines • Stepped down (33kV) Where some is used in large factories • Some is stepped down further (11kV) for small factories • Rest is stepped down instead (230V) to be used in houses shops etc.

• A higher voltage means a lower current. • A higher current would mean a higher power loss (P = I²xR). • So the lowest possible current is attained so that the least amount of energy is lost by heating. • Low resistance wires are also used (thicker).

A primary coil (connected to an a.c supply) and a secondary coil with differing amounts of turns, wrapped around an iron core.

• Changing a.c current supplied • This produces a changing magnetic field in primary coil • The iron core is induced and carries this changing magnetic field • This creates a changing magnetic field in the secondary coil • This induces a changing a.c voltage in the wires connected to the secondary coil.

V(p) x I(p) = V(s) x I(s) [assuming that the transformer is 100% efficient]

V(p)/V(s) = T(p)/T(s) V = Voltage across coil T = Turns in coil

• P = I x V • E = P x t , so • E = I x t x V