physics - electromagnetic induction (13.1 - 13.11)

13.1 how to produce electric current - small scale in lab

changing magnetic field can induce voltage/p.d. in wire - causes current to flow

p.d. also induced if wire moved in magnetic field

13.1 how to produce electric current - large-scale generation of electrical energy

generator: coil of wire rotated inside magnetic field

as coil turns, voltage induced in wire

causes current to flow

generators in power stations have rotating electromagnet surrounded by coils of wire

13.2 factors that affect size & direction of induced p.d.

number of turns in coil of wire

strength of magnetic field

how fast magnetic field changes/moves past coil

13.2 how does number of turns in coil of wire affect size & direction of induced p.d.

more turns in coil of wire = larger induced p.d.

13.2 how does strength of magnetic field affect size & direction of induced p.d.

stronger magnetic field = larger induced p.d.

13.2 how does how fast magnetic field changes/moves past coil affect size & direction of induced p.d.

magnetic field changes/moves past coil faster = larger induced p.d.

13.2 what does magnetic field produced do to original change?

p.d. causes current to flow - magnetic field of current opposes original change

13.3 how is electromagnetic induction used in alternators to generate a.c.?

1. coil of wire rotated inside magnetic field

2. induces p.d. in coil of wire

3. induces current in coil of wire

ends of coil connected to slip rings

electrical contact with external circuit made with carbon brushes (press on slip rings)

slip rings & brushes - connections don’t switch every half turn

produces a.c.

13.3 how is electromagnetic induction used in dynamos to generate d.c.?

1. coil of wire rotated inside magnetic field

2. induces p.d. in coil of wire

3. induces current in coil of wire

coil connected to split-ring commutator - connections switch every half turn

produces d.c.

13.4 how do microphones work?

convert pressure variations in sound waves → variations in current in electrical circuits

1. sound waves cause variations in air pressure

2. pressure variations make diaphragm vibrate

3. diaphragm moves coil of wire back & forth

13.4 how do loudspeakers & headphones work?

convert variations in electrical current → pressure variations in sound waves

1. varying current flows through coil in magnetic field

2. causes force on coil - moves back & forth as current varies

3. coil connected to diaphragm - movements of diaphragm produce sound waves

transformer

transformer: 2 coils insulated wire wound onto iron core - no electrical connection between 2 coils of wire

electricity supplied to primary coil

electricity in second coil at diff. voltage

13.5 how can a.c. in one circuit induce current in another circuit in transformer?

a.c. in primary coil creates continuously changing magnetic field

iron core carries magnetic field to secondary coil

changing magnetic field induces changing p.d. in secondary coil

13.6 what can transformer change size of?

transformer can change size of alternating voltage

(secondary coil has more turns than primary coil - p.d. greater in secondary coil)

13.7 turns ratio equation for transformers - calculate missing voltage or missing number of turns

13.8 in national grid why is electrical energy transferred at high voltages from power stations then at lower voltages in each locality for domestic uses?

electricity flows through wire - wire heats up

amount of energy wasted by heating in wires significant for transmission lines in national grid

p.d. of electricity passing through wire increased = current decreased

current smaller = less energy transferred by heating = efficiency improved

13.9 where are step-up transformers used in transmission of electricity in national grid?

after electricity leaves power station

before electricity travels through transmission lines

13.9 where are step-down transformers used in transmission of electricity in national grid?

after electricity travels through transmission lines

before electricity reaches factories, homes, shops, offices

13.9 why are step-up transformers used in transmission of electricity in national grid?

increases voltage

decreases current

13.9 why are step-down transformers used in transmission of electricity in national grid?

decreases voltage

increases current

13.10 power equation (for transformers with 100% efficiency)

p.d. across primary coil (V) x current in primary coil (A) = p.d. across secondary coil (V) x current in secondary coil (A)

V_p x I_p = V_s x I_s

13.11 advantages of power transmissions in high-voltage cables (using equations)

P = E/t

P = IV

P = I²R