Topic 13- Electromagnetic Induction

Electromagnetic Induction (generator effect)

A voltage is induced in a conductor when there is relative motion between the conductor and another magnetic field

caused by coil/conductor cutting through field lines

how to produce an electric current by the generator effect on a small scale in a lab

either:

• move a conductor relative to the magnetic field of magnets

• or move a magnet through the centre of a coil

measured with a centre-zero meter (a voltmeter or ammeter that has an arow that deflects)

when a magnet is moved into a coil, the induced current will cause the ______ end to become the North pole (of the coil)

nearest

when a magnet is moved out of a coil, the nearest end to the magnet becomes the induced _____ pole.

South

the generator effect use in the large-scale generation of electricity (a.c. generator)

water heated and evaporates to form steam

steam put under pressure and forced into a turbine, connected to a huge coil of wire in a strong mag field

steam causes turbine to spin, spinning coil

current is generated in the coil by the spinning motion of the coil through the field

this is the main form of electricity found in the mains supply of a building

effect of increasing speed at which coil wire or magnet is moved

increase rate at which mag field lines are cut

increases induced p.d.

effect of increasing number of turns in the coil

more coils will cut mag field lines

so total p.d. will increase

effect of increasing size of coils (area)

more wire cutting field lines

so increased p.d.

effect of increasing strength of mag field

increased p.d.

effect of reversing orientation of the magnet poles

reversed direction of current produced

magnetic field produced in the coil by EM induction always…

opposes the original change

so will always try to stop the magnet moving in or out by repelling it

e.g. magnet is pushed N end first into a coil, the end nearest the magnet becomes the N pole of the coil, to repel the magnet

e.g. magnet is pulled out of coil south end first, the end of the coil nearest the magnet will become the south pole, to try to attract the magnet in.

how does an alternator (a.c. generator) work?

• coil spun in a uniform field

• coil connected to a circuit by metal brushes that maintain continuous contact with 2 slip rings connected to each end of coil wire

• as the coil cuts through the magnetic field lines, it induces a potential difference, causing current to flow.

• current changes direction every half turn, as the mag field of the coil also spins, cutting across the field lines in alternating directions

• this is known as alternating current

• produces a sine graph of potential difference

how does a dynamo (d.c. generator) work

• coil spun in a uniform electric field

• connected to a circuit through a split ring commutator, contact made by brushes

• as it rotates it cuts through mad field lines, inducing a p.d. in the coil, causing current to flow in circuit

• split ring commutator changes connections between coil and brushes every half turn, so current continues leaving the dynamo in the same direction

• p.d. does not reverse, so current remains positive

• known as direct current, produced in pulses (as the coil goes vertical it doesn’t induce any p.d.)

how does a microphone work with EM induction?

• when sound waves reach the microphone, the pressure variations cause the sensitive diaphragm to vibrate

• in turn, this moves the coil back and forth through the mag field of the magnet, cutting field lines

• induces an alternating p.d. in the coil → alternating current

• frequency of the current is proportional to the sound signal frequency, sent to a loudspeaker

how does a loudspeaker work using magnetic forces (motor effect)

• a.c. current passes through coil, creating a constantly changing magnetic field around the coil

• this field interacts with the field from the permanent magnet, exerting a force on the coil, moving it back and forth (oscillation)

• connected to speaker cone, which also oscillates, creating changes in pressure that are sound waves

what are transformers used for

can be used to increase or decrease the p.d. of an alternating current between two coils

how do transformers work?

a.c. supplied to primary coil

produces a changing magnetic field around itself due to changing current

iron core is easily magnetised so field passes through it

therefore there is now a moving mag field around the secondary coil, so induces an alternating p.d. of same frequency as the a.c. supplied to primary coil

if secondary connected in a complete circuit then a.c. produced

ratio equation for transformers

step up transformer

more coils on secondary

voltage increased, as changing field will cut through more wire in the secondary coil, inducing a larger p.d.

step down transformer

less coils on the secondary

voltage decreased, changing field will cut through less wire in the secondary coil, so smaller p.d. induces

what happens to electricity at high currents over long distances?

the wires heat up! (so current must be reduced)

but, to maintain the same power (because power can’t be created), the voltage can be INCREASED

because Power = Voltage x Current

national grid electrical energy

• energy transferred at high voltages from power stations

  • current low (to reduce heat loss in power lines so improve efficiency)

  • step-up transformer makes voltage high (because power is constant)

• transferred at lower voltages in each locality for domestic uses

  • current increased

  • because step-down transformer decreases voltage (because high V can be very dangerous)

what kind of transformer is used in adapters?

step-down

they lower the main voltage to lower voltages used by many electronic devices

power equation for a 100% efficient transformer

Vp x Ip = Vs x Is

AKA Pp = Ps

advantage of high voltage transmission cables

transformers cannot increased the total power output, so a step-up transformer can increase the voltage, meaning that current must decrease

lower current = less resistance and therefore energy lost

therefore more efficient

power (energy per second) lost in a wire

P = I²R

total energy lost in a certain time (t) by a wire

E = P x t