Magnetism and Electromagnetism

Topic 7- Magnetism and Electromagnetism

what is a magnet

any material or object that produces a magnetic field

examples of magnets

bar magnet, Earth

what do all magnets have

• two poles; north and south

• a magnetic field around them

what do you use to represent the magnetic field around a magnet

field lines

what direction do field lines point (N to S or S to N)

north to south

what does it mean if field lines are closer together

stronger magnet

stages of drawing field lines around a magnet

1. draw one straight line in one end of the magnet and out of the other

2. draw two curved lines, one on either side of the first straight line into the magnet on the south and out of it on the north

3. draw some curved lines that loop all the way from north to south

4. double check all arrows pointing the right way

what do field lines show us

• where the magnetic field is

• where the magnetic field is acting

• the strength of the magnetic field

how can you find the poles of a magnet and its magnetic field if the magnet isn’t labelled

• use a compass

• needle of compass always lines up with the field lines of whichever magnetic field it is

• compass always points towards the south pole of a magnet

• repeatedly doing this and marking the direction gives a field line pattern

why does the needle of a compass always line up with the field lines of a magnet

the needle of a compass is a tiny bar magnet

what do like poles do

repel

what do opposite poles do

attract

why do like poles repel

the magnetic fields of each are pushing each other away, which is why they repel and cannot stay together

why do opposite charges attract

their field lines are all going north to south in the same direction and so pull each other together

what is a magnetic material

any object that can be influenced by magnetic fields, and has the potential to become a magnet

examples of magnetic materials

• nickel

• iron

• cobalt

• alloys of these e.g. steel

are all magnetic materials magnets

no

what is a permanent magnet

a magnet that produces their own magnetic field all of the time

what is an induced/temporary magnet

given a magnetic field by a permanent magnet and only have it temporarily

how is a magnetic material induced

• magnetic material is put in the field of a permanent magnet

• induces the material to have a north and south pole

why can magnetic materials be induced

• they have domains inside, which line up when in the field of a permanent magnet

• when the domains line up to act in the same direction, it makes them a magnet

what are domains

atoms that act like small magnets

what will the force between the permanent and induced magnet be

always attracted

what happens when an induced magnet leaves the magnetic field of a permanent magnet

it will begin to lose its magnetism and become normal again

what magnetic materials lose their magnetism very quickly

e.g. nickel and iron

what is it called if a magnetic material loses their magnetism very quickly

magnetically soft

example of a magnetically hard material

steel

what happens if you break a magnet in half

forms two small magnets as the domains remain unchanged

what are the two ways that a magnet loses its magnetism

• induced one loses if it moves out of the magnetic field of a permanent magnet

• if the domains are distorted

what are the other ways an induced magnet can be formed

• stroking it with a permanent magnet

• placing it within a strong electromagnetic field

• placing it in a magnetic field then heating it

how can you make an induced magnet a permanent magnet

heat it

what is electromagnetism

where electric currents produce their own magnetic field

what happens when a current is passed through a wire

a magnetic field around the wire is produced

what is the shape of the magnetic field around a wire presented as

concentric circles around the wire

why are field lines closer together closer to the wire

this is where the magnetic field is strongest

what happens if the current is reversed

the magnetic field is also reversed

what does the right hand grip rule show

• thumb shows the direction of the current

• fingers show the direction of the magnetic field

what happens to the field lines if it is a circular coil

field lines become ellipses rather than circles

what happens when the two magnetic fields from each side of the coil come together and interact

they form one magnetic field, acting straight through the coil

what is a solenoid

a wire which is curved into a coil

what is the magnetic field inside a solenoid like

• strong

• uniform

• straight lines all in the same direction

what is the magnetic field like outside the solenoid

the same as it would in a bar magnet; goes around in circles

why is a solenoid an electromagnet

the electricity and current flowing through the solenoid is what gives it its magnetic field

when do electromagnets have their magnetic fields

only when there is a current flowing through the wire

how can we reverse the direction of the electromagnets magnetic field

by reversing the direction of the current

ways to increase the strength of an electromagnet

• increase the current flowing through the solenoid

• increase the number of coils, keeping the length the same

• decreasing the length but keeping number of turns the same

• add an iron core to the inside of the solenoid

why does an iron core being added increase the strength of an electromagnet

iron is a soft magnetic material and becomes induced when the current is turned on

examples of electromagnetic devices

• relay switch

• electric bell

• loudspeaker

• electromagnetic circuit breaker

• door lock

• door chime

what is the purpose of a relay switch

used so that a large current can be switched off and on by a much smaller current

where are relay switches used

• car-ignition switches

• electric systems which require a unit that delivers a significant amount of current or power to be switched off or on by a low current voltage switch

positives of relay switches

makes the device much safer and cheaper (look at notes for diagram)

how does an electric bell work

• current in the coil creates an electromagnet, which attracts the hammer to strike the bell

• disconnects the circuit and the armature moved away but springs back into position afterwards and the cycle repeats

how does a loudspeaker work

• alternating current in the coil

• magnetic field is produced also alternates

• interaction causes the coil to move back and forth

• cone of the speaker also moves with the coil

• air in front of the cone also moves and a longitudinal wave produced

how does an electromagnetic circuit breaker work

• spring loaded push switch held in closed position by spring-loaded soft iron bolt

• electromagnet arranged so it can pull the bolt away from the switch

• when live wire carries usual operating current, the electromagnet not strong enough to separate the contacts

• if something goes wrong with the appliance and a large current flows, the electromagnet will pull hard enough to separate the contact and break the circuit

how does a door lock work

1. electromagnet mounted on a door frame while armature plate mounted on the door

2. when door closed, the armature plate in contact with the electromagnet

3. electromagnet energised by electrical current which creates magnetic field

4. this magnetic field attracts armature plate, which locks the door

how does a door chime work

1. iron striker attached to electromagnet and strikes bell

2. as striker moves towards the bell, the contact is broken; electricity stops flowing through the coil which loses magnetism

3. spring returns striker to original position which makes new contact so electricity flows again

4. back to number one and cycle repeats; bell continues to ring as long as switch held closed

what is the motor effect

a current carrying wire in the presence of a magnetic field will experience a force

what happens when you place a current carrying wire in between two magnets

magnetic field of the magnets and magnetic field of the wire will interact, producing a force on the wire, pushing it out of the field

at what angle does the wire have to be at in the magnets to receive the full force

90 degrees to the magnetic field

what angle does the wire have to face for there to be no force experienced

the same direction as the magnetic field

what can Flemings left hand rule be used for

• direction of magnetic field

• direction of current

• direction of force

how do you do the Flemings left hand rule

1. point your thumb upwards

2. point your first finger out at a right angle to your thumb

3. point your second finger out at a right angle to your first finger

what does the direction of the thumb represent

the direction of the force

what does the direction of the first finger represent

the direction of the magnetic field (points north to south)

what does the direction of the second finger represent

the direction of the current

explain the horseshoe magnet motor effect

• between the north and south poles of the horseshoe magnet are placed three metal rails, through which current can flow

• two rails parallel to each other on the sides are fixed but the other is free to roll

• as the power source is turned on and the rails generate their own magnetic field, this field interacts with the field of the horseshoe magnet and experiences a force

equation for force strength

force = magnetic flux density x current x length (F=BIL)

what is force measured in

newtons (N)

what is magnetic flux density measured in

teslas (T)

what is current measured in

amperes (A)

what is the length of the wire measured in

metres (m)

what is an electric motor

an electromagnetic device which spins round, which can be used in almost all equipment that needs a motor

parts of an electric motor

• a coil passes in and out of two magnets next to each other

• wire and power supply

• a split-ring commutator

how does an electric motor work

• current flows out of the positive terminal and into negative

• as current flows through different directions on each side of the coil, it means that the force they experience will be in opposite directions

• as the forces act in opposite directions, it causes the coil to spin

what happens after the coil has spun 180 degrees

the current then flows through the opposite direction with the positive and negative terminals swapping

what does the swap in positive and negative terminals mean for the forces and direction of spin

• means the forces acting on each side have swapped too

• the coil will now spin in the opposite direction and go back to where it begun; repeating 180 degree spins there and back

what does a split-ring commutator do

swaps the positive and negative connections every half term

why is a split-ring commutator needed; what problem does it solve

• solves the problem of the motor only spinning 180 degrees

• means that the direction of current changes every half term so the positive and negative terminals stay on the same side

• means it always spins the same direction

examples of where electric motors are used

• lorry

• fan

• hard drives

how can the speed of rotations of an electric motor be increased

• increasing the current passing through the wire

• add more turns to the wire

• increase the magnetic flux density by using a more powerful magnet

how can the direction of rotation of the electric motor be changed

• reversing the direction of the current supply

• reversing the poles

how can the force in an electric motor be increased

• increase the current in the coil

• increasing the strength of the magnetic field

• adding more turns to the coil

what is the generator effect

where a wire is given an electromagnetic force aka electromagnetic induction

how does the generator effect happen

• you move a coil of wire in between two magnets

• the movement induces a potential difference in the wire

what happens when the moving wire in the generator effect stops moving e.g. at the top and bottom

the potential difference disappears so is not induced

what happens to the direction of PD each time we change direction

it swaps

what happens if a full circuit is moved up and down in between two magnets

the induced potential difference generates a current as electrons able to flow around the circuit

what happens if the wire remains still but the magnets move up and down and why

the same thing happens and it is induced because the magnetic field is still changing therefore the potential difference is still occurring

what happens if the circuit moves horizontally between two magnets instead of vertically

nothing as the magnetic field acting on the circuit is staying the same

how can you change the size of the potential difference

• change the strength of the magnetic field; stronger magnetic field means larger potential difference

• move the wire/magnets quicker; faster they move the faster the magnetic field changes therefore the bigger the potential difference

• shape the wire into a coil; more turns the bigger the induced PD will be

what happens when a single magnet is moved in and out of a coil

• movement still induces a PD in the coil, generating a current

what happens each time we change the direction of the magnet into the coil

we change the direction of the current

how else can we swap the direction of the current

changing the poles of the magnet

who’s law outlines the process of electromagnetic induction

Faraday’s law

how can you reverse the induced current

• turn the magnet around (reverse the poles)

• insert the magnet into the other end of the coil

• reverse the direction of motion

what are two devices that rely on electromagnetic induction and the generator effect

• dynamo

• alternator

how does a dynamo work

• magnet turns around, causing the wires in the surrounding coil to cut through its magnetic field lines

• induces a voltage across the end of the wire

• this is then used to power devices

example of a device that a dynamo could power

lights on a bike

what is a key part of an alternator that a dynamo doesn’t have

slip rings and brushes

how does an alternator work

• coil rotates, its wires cut through the magnetic field lines, inducing a voltage across the ends of the wire

• two slip rings make contact with brushes

• because each side of the coil will go up and down through the field lines, the induced voltage alternates (charge flows one way then the other)