s6 magnetism and electromagnetism

8/13/2019 S6 Magnetism and Electromagnetism

1/42

Section 6: Magnetism andelectromagnetism


2/42

Units

Ampere (A) currentVolt (V) voltageWatt (W) - power


3/42


4/42

Magnetically hard materials Are used as permanent magnets.Retain its magnetism once it has been magnetised.E.g. steel

Magnetically soft materials Are used as temporary magnets.

Lose their magnetism easily.E.g. iron


5/42

Magnetic field

Magnetic field is a volume of space around everymagnet where magnetism can be detected.


6/42

Magnetic field lines (flux lines)

Does not actually exist, but visualises the mainfeatures of a magnetic field1. Show the shape of the magnetic field2. Show the direction of the magnetic field the field

lines travel from north to south 3. Show the strength of the magnetic field the field

lines are closest together where the magnetic fieldis strongest


7/42

Magnetic induction

Magnetism can be induced in a magnetic materialwhen they are placed in a magnetic field (may bedone deliberately)It can also happen if a magnetic material is left in

one position, since the Earths magnetic field willgradually induce magnetism in them.


8/42

Field of cylindrical bar magnet


9/42


10/42

Field of parallel magnets


11/42

Field of attracting magnets


12/42

Field of repelling magnets


13/42

Field of anti-parallel magnets


14/42

Uniform magnetic field pattern

When North and South poles are placed near eachother, there is an almost uniform field between thetwo poles


15/42

Electromagnetism

An electric current in a conductor, e.g. a wire,produces a magnetic field around it.A soft metal, e.g. iron core, is made into amagnet by the passage of electric current

through a coil surrounding it.


16/42

Wire

The field around the wire is quite weak and circularin shape.The direction of the magnetic field depends upon thedirection of the current, and can be found using the

right-hand grip rule.The strength of the magnetic field around a current-carrying wire can be increased by:

Increasing the current in the wire

Wrapping the wire into a coil or solenoidWrapping the coil of wire around an iron core


17/42

Magnetic field pattern for straight wire


18/42

Magnetic field pattern for flat circular coil


19/42

SolenoidThe shape of the magnetic field around a solenoid is thesame as that around a bar magnet.The position of the poles can be determined using theright-hand grip rule.The poles of the solenoid can be reversed by reversing

the direction of the current. (direction of magnetic field isreversed)The strength of the field around a solenoid can beincreased by:

Increasing the current flowing through the solenoidIncreasing the number of turns on the solenoidWrapping the solenoid around a magnetically soft core e.g. iron

this combination of soft iron core and solenoid is oftenreferred to as an electromagnet


20/42

Magnetic field pattern for solenoid


21/42

There is a force on a charged particle when it movesin a magnetic field, as long as its motion is notparallel to the field.


22/42

Movement from electricity overlappingmagnetic fields

If we pass a current through a piece of wireperpendicular to the magnetic field of a magnet, thewire will move.This motion is caused by a force created by

overlapping magnetic fields around the wire and themagnet. (motor effect)


23/42

The motor effect A cylindrical magnetic field is created around a wirewhen current flows along it.In certain places, the fields are in the same direction.They reinforce each other, producing a strongmagnetic field.In other places, the fields are in opposite directions,producing a weaker field.The wire experiences a force, pushing it from thestronger part of the field to the weaker part.The force exerted on the wire/current-carryingconductor in a magnetic field can be increased by:

Increasing the strength of the fieldIncreasing the current


24/42

Catapult field


25/42

Flemings Left Hand Rule

The direction of motion of the wire can be predictedusing Flemings Left Hand Rule. (when a wire carriesa current perpendicular to a magnetic field)


26/42

Moving-coil loudspeaker

It uses the motor effect to change electrical energyinto sound energy.Signals from a source, e.g. amplifier, are fed into thecoil of the speaker as currents that are continually

changing in size and direction.The overlapping fields of the coil and the magnettherefore create rapidly varying forces on the wire ofthe coil, which cause the speaker cone to vibrate.

These vibrations create the sound waves we hear.


27/42

Electric motor As current passes around the loop of wire, one side of itwill experience a force pushing it upwards.The other side will feel a force pushing it downwards, sothe loop rotates.Because of the split ring commutator, when the loop isvertical, the connections to the supply through thebrushes swap over, so that the current flowing througheach side of the loop changes direction.The wire at the bottom is now pushed upwards, and thewire at the top is pushed downwards this makes theloop carry on turning.The arrangement of the brushes and split ringchanges direction of the current flowing through theloop every half turn, making the rotation continuous.


28/42

The rate at which the motor turns can be increasedby:

Increasing the number of turns of wire (to make a coil)Increasing the strength of the magnetic fieldIncreasing the current flowing through the loop of wire

Practical motors differ from that described above:The permanent magnets are replaced with curvedelectromagnets capable of producing very strongmagnetic fields.

The single loop is replaced with several coils of wirewrapped on the same axis. This makes the motor morepowerful and allows it to run more smoothly.The coils are wrapped on a laminated soft iron core. Thismakes the motor more efficient and more powerful.


29/42


30/42

Electromagnetic induction A voltage is induced in a conductor/wire when wemove it across a perpendicular magnetic field.If the wire is part of a complete circuit, a currentflows.

The size of the induced voltage (and current) can beincreased by:

Moving the wire more quicklyUsing a stronger magnet

Increasing the number of turns of the coil so that morepieces of wire move through the magnetic field

The direction of the induced current can be reversedby reversing the direction of movement.


31/42

A voltage and current can also be generated bypushing a magnet in a coil.The size of the induced voltage can be increased by:

Moving the magnet more quickly

Using a stronger magnetUsing a coil with more turnsUsing a coil with a larger cross-sectional area


32/42


33/42


34/42

Dynamo a simple generator As the cyclist pedals, the wheel rotates and a smallmagnet within the dynamo spins around.

As the magnet turns, its magnetic field cuts throughthe surrounding coil inducing a current in it. This

current can be used to work the cyclists lights. The size of the induced voltage can be increased by:

Moving the magnet more quicklyUsing a stronger magnet

Increasing the number of turns of the coil


35/42

Generators As the coil rotates, its wires cut through magneticfield lines and a current is induced in them.The wire on each side moves up through the fieldand then down for each turn of the coil.

As a result, the current induced in the coil flows firstin one direction then in the opposite direction.This is called alternating current .

A generator that produces alternating current iscalled an alternator .The frequency of an alternating current is thenumber of complete cycles it makes each second.(Hz)


36/42


37/42

Transformers An alternating current passing through the primarycoil causes a changing magnetic field in the soft ironcore.

As it cuts through the wires of the secondary coil, an

alternating voltage is induced across that coil.The size and direction of the induced voltagechanges as the voltage applied to the primary coilchanges.

An alternating voltage applied across the primarycoil therefore produces an alternating voltageacross the secondary coil. This combination oftwo magnetically linked coils is called a

transformer.


38/42

The closer together the two coils, the stronger themagnetic field.The laminated iron core are strips of iron joinedtogether

There is an extended magnetic field from the ironcore.


39/42

Transformers and the UK National GridThe UK National Grid is a network of wires andcables that carries electrical energy from powerstations to consumers, e.g. factories and homes.Electricity has to be transferred at a high voltage so

that the current is very low, in order to preventenergy loss in the cables. (P = IV)If current is high in the cables, the wires carrying thecurrent will heat up and a lot of electrical energy will

be lost as heat energy.


40/42

Transformers are therefore used to step up the voltage atthe power station end and to step down the voltage at theusers end. Electricity can only be transmitted as alternating current,but not direct current, because transformers do not workwith DC.

A transformer changes the size of an alternating voltageby having different numbers of turns on the input andoutput sides. (primary and secondary coils)If there are less turns in the primary coil than thesecondary coil, the voltage is stepped up .If there are more turns in the primary coil than thesecondary coil, the voltage is stepped down .


41/42


42/42

Input power = output power

OR

P in = P outOR

IpVp = I sVs

(for 100% efficiency)

s6 magnetism and electromagnetism

Documents