topic 7 - magnetism and electromagnetism

state what the poles of a magnet are

places where the magnetic forces are strongest

state what happens when two magnets are brought close together

they exert a force on each other

state what happens when two like poles are brought close together

they repel each other

state what happens when opposite poles are brought close together

they attract each other

state what kind of force attraction between magnetic poles is

non-contact force

state what kind of force repulsion between magnetic poles is

non-contact force

state what induced magnetism causes

a force of attraction

state what happens when an induced magnet is removed from a magnetic field

it loses most/all of its magnetism quickly

state what a permanent magnet is

an object where its magnetism cannot be induced or removed

state what an induced magnet is

an object where its magnetism can be induced or removed

explain what a magnetic field is

• a region around a magnet

• where a force acts on another magnet/magnetic material

state what the force is between a magnet and magnetic material

attraction

state what the strength of a magnetic field depends on

the distance from the magnet

state where a magnetic field is strongest

at the poles of the magnet

state what determines the direction of a magnetic field at any given point

• direction of force

• that would act on

• another north pole

• placed at that point

state the direction of magnetic field lines

north pole → south pole

state what a magnetic compass contains

small bar magnet

state what kind of field the earth has

magnetic field

state what direction a magnetic compass’ needle points in

direction of earth’s magnetic field

describe how to plot the magnetic field pattern of a magnet using a compass

1. place a magnet on top of a piece of paper and draw a dot at one corner of the magnet

2. place a plotting compass next to the dot, so that one end of needle of the compass points away from the dot

3. use a pencil to draw a new dot at the other side of the compass needle

4. move the compass so that it points away from the new dot and draw a new dot at the other side of the compass needle

5. repeat this process until there is a chain of dots going from one end of the magnet to the other

6. repeat the entire process several times to create several other magnetic field lines

explain how the behaviour of a magnetic compass is related to evidence that the core of the earth must be magnetic

• on earth, in the absence of any magnets or magnetic materials

• a magnetic compass will always point north

• this evidences that earth’s core is magnetic

• and creates its own magnetic field

state where the magnetic poles of earth’s magnetic field are

• magnetic north pole = antarctica

• magnetic south pole = arctic

state what happens when current flows through a conducting wire

• magnetic field is produced

• around the wire

state what the strength of a conducting wire’s magnetic field relies upon

• current through the wire

• distance of the field from the wire

state what shaping a wire to form a solenoid does

• increases the strength

• of the magnetic field

• created by a current

• through the wire

state the nature of a magnetic field in a solenoid

• strong

• uniform

state the shape of the magnetic field around a solenoid

similar to a bar magnet

state what increases the strength of a solenoid’s magnetic field

adding an iron core

state electromagnet definition

• solenoid

• with an iron core

describe how the magnetic effect of a current can be demonstrated

1. place a conducting wire in between two bar magnets

2. allow current to flow through the wire

3. this will create a magnetic field

4. this causes a force to push the wire at right angles

explain how a solenoid arrangement can increase the magnetic effect of current

• increase current

• increase number of turns in coil

• add an iron core

describe how to determine the direction of a magnetic field

• use right-hand thumb rule

• thumb points in direction of current flow

• rest of fingers give direction of the field lines

state what the magnetic field of a conducting wire is made up of

• concentric circles

• with no poles

state what happens to the magnetic field of a conducting wire as distance from the wire increases

concentric circles get further apart

state what happens to the magnetic field of a conducting wire when you reverse the direction of current

the magnetic field direction will reverse

state motor effect definition

• when a conductor carrying a current

• is placed in a magnetic field

• the magnet producing the field

• and the conductor

• will exert a force on each other

describe how fleming’s left hand rule shows the orientation of force, current in a conductor and magnetic field

• direction of a force on

• a conductor carrying current

• depends on the direction of current

• and the direction of the magnetic field

• all three are PERPENDICULAR to each other

• thumb - force orientation

• pointer finger - magnetic field orientation

• middle finger - current orientation

state the factors that affect the size of the force on a conductor

• current - larger current = larger force

• length - larger length = larger force

• magnetic flux density - larger density = more magnetic field lines = larger force

state symbol equation to calculate force for a conductor

F (N) = B (T) x I (A) x L (m)

state the basis of an electric motor

• coil of wire carrying a current

• in a magnetic field

• tends to rotate

state what the motor effect can be used to make

simple d.c. electric motor

describe what a simple d.c. electric motor consists of

• coil of wire

• positioned in a uniform magnetic field

• the coil of wire, when horizontal, forms a complete circuit with a cell

• coil of wire attached to a split-ring commutator

• split ring is connected in a circuit with a cell via contact with conducting carbon brushes

explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor

• current flowing through the coil produces a magnetic field around the coil

• this magnetic field interacts with the uniform external field

• so a force is exerted on the wire

• forces act in opposite direction on each side of the coil

• causing it to rotate and the coil is now in a vertical position

• on the side of the coil on top, current is flowing towards the cell

• so the force acts downwards

• on the side of the coil on the bottom, current is flowing away from the cell

• so the force acts upwards

• once the coil has rotated 90°, the split ring is no longer in contact with the brushes

• so no current flows through the coil

• thus no forces act on the coil

explain how momentum of a conductor in a magnetic field causes the continued rotation of the coil in an electric motor

• with the coil in vertical position, even with no forces acting upon it

• momentum of the coil causes it to slightly rotate

• this causes the split ring to make contact with the carbon brushes again

• and current flows through the coil again

• current flows towards the cell on the left side of the coil

• and away from the cell on the right

• even though the coil has flipped

• the left side of the coil experiences an upwards force

• and the right side of the coil experiences a downwards force

• causing the coil to continue to rotate in the same direction

• forming a continuously spinning motor

describe why loudspeakers and headphones use motor effect

• to convert variations in current

• in electrical circuits

• to the pressure variations

• in sound waves

explain how a moving-coil loudspeaker/headphones works

• loudspeaker/headphones consists of a coil of wire wrapped around one pole of a permanent magnet

• an alternating current flows through the coil, creating a changing magnetic field around the coil

• magnetic field around the coil interacts with the field from the permanent magnet

• interacting magnetic fields will exert a force on the coil

• direction of the force is determined by fleming’s left-hand rule

• as direction of the magnetic field is constantly changing, the force exerted on the coil will constantly change direction

• which makes the coil oscillate

• the oscillating coil causes the speaker cone to oscillate

• which makes the air oscillate, creating sound waves

state what induces a potential differences across the ends of a conductor

• if the conductor moves relative to a magnetic field

• or if there is a change in the magnetic field around the conductor

state generator effect definition

• if a conductor with induced potential difference across its ends

• is part of a complete circuit

• where current is then induced in the conductor

state what an induced current generates

• magnetic field

• that opposes the original change

• which was either the movement of the conductor relative to a magnetic field

• or the change in the conductor’s magnetic field

state factors that increases the size of induced potential difference/current

• magnetic field strength increases

• number of turns of the coil increases

• speed conductor is moved at increases

state factors that affect the direction of induced potential difference/current

• direction of magnetic field

• direction of wire/coil

state how the generator effect is used in an alternator

to generate a.c.

state how the generator effect is used in a dynamo

to generate d.c.

state what a simple alternator is

• device

• which converts energy

• from motion

• into an electrical output

state what a simple alternator consists of

• rotating coil of wire

• between the poles of a permanent magnet

• slip rings and brushes

• connected to an external circuit

explain why a permanent magnet is used in a simple alternator/dynamo

to provide a uniform magnetic field

explain why a rotating coil is used in a simple alternator/dynamo

• to cut the magnetic field as it rotates

• and to allow an induced current to flow

explain why slip rings are used in a simple alternator

• to allow the alternating current to flow

• between the coil

• and the external circuit

explain why carbon brushes are used in a simple alternator/dynamo

• to provide a good electrical connection

• between the coil and the external circuit

explain how the generator effect is used to generate a.c. in an alternator

• rectangular coil rotates in a unform magnetic field due to the motor effect

• coil is connected to an external circuit via slip rings and carbon brushes

• the induced potential difference in the coil is measured by adding a galvanometer to the external circuit

• potential difference is induced in the coil as the coil’s rotation cuts the magnetic field lines

• the pointer on the galvanometer defects one way, then the opposite way, then back again

• which indicates the size and direction of the induced potential difference is constantly changing

• as a result of the alternating potential difference, an alternating current is produced as the coil rotates

• this continues as long as the coil keeps rotating in the same direction

state when and explain why a maximum potential difference is induced in a simple alternator

• position of the coil is horizontal

• motion of the coil is perpendicular to the magnetic field

• because the largest number of magnetic field lines are cut

• when the motion of the coil is perpendicular

state when no potential difference is induced in a simple alternator

• position of the coil is vertical

• motion of the coil is parallel to the magnetic field

• because no magnetic field lines are cut

• when the motion of the coil is parallel

state what a dynamo is

• device

• that converts

• an electrical input

• into motion

state what a simple dynamo consists of

• rotating coil of wire

• between the poles of a permanent magnet

• split ring commutator and brushes

• connected to an external circuit

explain why a split ring commutator is used in simple dynamos

• to allow the connection

• between the coil and the external circuit

• to change

• every half turn

explain how the generator effect is used to generate d.c. in a dynamo

• as the coil rotates, due to the motor effect, it cuts through the magnetic field lines

• this induces a potential difference between the end of the coil

• the split ring commutator changes the connections between the coil and the brushes every half turn

• in order to keep the current leaving the dynamo in the same direction

• this happens each time the coil is perpendicular to the magnetic field lines

• thus, the induced potential difference doesn’t reverse its direction (unlike in alternators)

• instead, the potential difference varies from zero to a maximum value twice each cycle of rotation

• and never changes polarity

• meaning that the current is always positive/negative

state what bicycle dynamo is used for

• to supply electricity

• to bike lights

• whilst in motion

explain how the generator effect is used to generate d.c. in a bicycle dynamo

• a bicycle dynamo consists of a rotating magnet placed inside a coil

• the magnet is rotated by its connection to the bicycle inside the coil

• the magnetic field lines cut through the sides of the coil

• inducing a potential difference in the coil

• since the magnetic field direction is constantly changing as it rotates

• the potential difference alternates direction

• meaning the output current alternates direction

• causing the bike light to illuminate

explain how to interpret graphs of potential difference generated in the coil against time (ALTERNATOR)

• the shape of the graph is a sine or cosine curve (depending on coil starting position)

• when the coil is vertical at 0°, its motion is parallel to the magnetic field, causing the induced p.d. size to be 0

• when the coil has rotated by 90°, its position is horizontal, its motion is perpendicular to the magnetic field, causing the induced p.d. size to be its maximum value

• when the coil has rotated by 180°, its position is vertical again, its motion is parallel to the magnetic field, causing the induced p.d. size to be 0

• when the coil has rotated by 270°, its position is horizontal again, its motion is perpendicular to the magnetic field, causing the induced p.d. size to be its maximum value and opposite in direction to the value at 90°

• when the coil has rotated by 360°, its position is at its starting point, its motion is parallel to the magnetic field, causing the induced p.d. size to be 0

explain how to interpret graphs of potential difference generated in the coil against time (DYNAMO)

• the shape of the graph is a sine curve and is always in the same direction

• when the coil is vertical at 0°, its motion is parallel to the magnetic field, causing the size of the induced p.d. to be 0

• when the coil has rotated by 90°, its position is horizontal, its motion is perpendicular to the magnetic field, causing the induced p.d. size to be its maximum value

• when the coil has rotated by 180°, its position is vertical again, its motion is parallel to the magnetic field, causing the induced p.d. size to be 0

• when the coil has rotated by 270°, its position is horizontal again, its motion is perpendicular to the magnetic field, causing the induced p.d. size to be its maximum value and in the same direction to the value at 90°

• when the coil has rotated by 360°, its position is at its starting point, its motion is parallel to the magnetic field, causing the induced p.d. size to be 0

state the factors affecting alternator/dynamo output

• frequency of coil rotation

• number of turns on the coil

• strength of magnet

• inserting a soft iron core into the coil

describe why microphones use generator effect

• to convert the pressure variations

• in sound waves

• into variations in current

• in electrical circuits

explain how a moving-coil microphone works

• when sound waves reach the microphone

• the pressure variations cause the diaphragm to vibrate

• this causes the coil to move back and forth

• through the magnetic field produced by the magnet

• as the coil moves, it cuts through the field lines

• inducing a potential difference in the coil

• the induced potential difference will be alternating

• because the coil is continually changing direction

• due to the vibrations of the diaphragm

state what a basic transformer consists of

• primary coil

• and a secondary coil

• wound on an iron core

state why iron is used in transformers

because it’s easily magnetised

state the ratio of the potential difference in a transformer to number of turns in a coil

V_p / V_s = N_p / N_s

state the ratio of voltage in a step-up transformer

V_s > V_p

state the ratio of voltage in a step-down transformer

V_p > V_s

state what would happen if transformers were 100% efficient

electrical power output = electrical power input

state ratio of voltage to current (power input : power output) in a transformer

V_p x I_p = V_s x I_s

explain how a transformer works

• an alternating current is supplied in the primary coil

• meaning it will produce a changing magnetic field around the primary coil

• the iron core is easily magnetised

• so the changing magnetic field passes through it

• thus, there is now a changing magnetic field in the secondary coil

• this changing field cuts through the secondary coil

• and induces an alternating potential difference

• the alternating potential difference will have the same frequency as the alternating current supplied to the primary coil

• if the secondary coil is part of the complete circuit, it will cause an alternating current to flow

describe how the ratio of the potential differences across the two coils depends on the number of turns on each coil

• the higher the number of turns

• the higher the potential difference induced will be

state the role of transformers

• increases p.d. of electricity before it is transmitted across the national grid

• lowers high voltage electricity used in power lines to the lower voltages used domestically

• used in adapters to lower mains voltage to the lower voltages used by many electronic devices

explain the advantages of high voltage transmission

• when electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss

• to transmit the same amount of power as the input power, the p.d. at which electricity is transmitted should be increased

• resulting in a smaller current being transmitted through the power lines

• because P = I x V, so if V increases, I must decrease to transmit the same power

• a smaller current flowing through the power lines results in less heat being produced in the wire

• resulting in a reduction of energy loss in the power lines