electromagnetism Flashcards
at which part of a magnet are the magnetic forces strongest
the poles of the magnet
What happens when two magnets are brought close to each other
they exert a force on each other
what type of force is exerted if two like poles of a magnet are brought near each other
repel , non contact force
what type of force is exerted if two unlike poles of a magnet are brought near each other
They will attract , non contact force
why is repulsion and attraction non contact forces
because the magnets don’t have to touch to experience the force
what is the difference between a permanent magnet and an induced magnet
- permanent: produces its own magnetic field
- induces magnet : becomes magnetic when placed in a magnetic field
what type of force does induced magnetism always cause
a force of attraction
what happens when an induced magnet is removed from a magnetic field
the induced magnet loses most/ all of its magnetism
what is an induced manget
a material that becomes a magnet when it is placed in a magnetic field
what is a magnetic field
the region surrounding a magnet when another magnet or magnetic material experiences a non - contact force
give 4 examples of magnetic materials
- iron
-steel
-cobalt
-nickel
what can always be said about the force between a magnet and a magnetic material
it is always attractive
how does the strength of a magnetic field change as you move further away from the magnet producing it
the magnetic field decreases the further you move away
in what direction does a magnetic field point
- in the direction that a north pole would experience a force if place in the field
- from the north seeking pole to the south seeking pole of a magnet
how do you plot the magnetic field pattern of a magnet using a compass
- place the compass near the north pole of the bar magnet
- draw cross at the north pole of the compass
- Now move the compass so that the south pole of the compass is on the cross
- again, draw another cross at the north pole of the compass
- Continue doing till a plotted magnetic field line is completed
-Now connect all the dots with a line - show the direction of the field line using an arrow ( direction always. runs from the north pole to the south pole)
- repeat this starting at different points around the north pole of the bar magnet
using the field lines how do you know where the field is strongest
where the lines are closer together
what does a magnetic compass contain
small bar magnet that points in the direction of the Earth’s magnetic field
how can we determine that Earth has its own magnetic field
if you hokld the compass away frmo any magnets the needle always points in the North - South direction
what is the earth’s magnetic field due to
the earth’s core
what is produced when a current flows through a conducting wire
a magnetic field around the wire
what determines the strength of the magnetic field around a current - carrying wire
the magnitude of the current flowing through the wire ( a larger current produces a stronger magnetic field )
the distance from the wire ( magnetic field is strongest closer to the wire ) - further away from the wire = strength of magnetic field decrease
what is a solenoid
the coil of wire which when current passes through creates a strong magnetic field
how can we prove there is a magnetic field around the conducting wire
using a compass
- When current is turned off, the compass needle lines up with the earth’s magnetic field
- but when the current is turned on the compass needle deflects = proves that there is a magnetic field around the wire
what happens if you change the direction of the current
changes the direction of the magnetic field
how to work out the direction of the magnetic field that is produced by a wire
- use the right hand grip rule
- place right hand so that the thumb is pointing in the direction of the conventional current
- now the fingers are pointing in the direction of the magnetic field
what is another way to increase the strength of the magnetic field
- increasing the number of turns on the coil.
- increasing the current.
- placing an iron core inside the solenoid.
describe the magnetic field found inside the solenoid when the current is switched on
strong , uniform
what is a electromagnet
- a solenoid with an added iron core
what is the motor effect
when a force is exerted between a magnetic field and a current-carrying conductor placed in that field
what are the 3 ways to increase the strength of the magnetic field produced by a solenoid
- increasing the size of current = increases the strength of magnetic field
- increases turns of coil
- if a piece of iron is placed into solenoid ( iron core )
how are electromagnets useful
- can change the strength of the magnetic field by changing size of current
- can be turned on or off
how are high voltage circuits dangerous
- sparking and risk of electrocution
so how are voltage circuits turned on and off
- A relay is used ( relay contains two separate circuits , there is a low voltage circuit containing an electromagnet)
the low voltage circuit is safe to be switched on / off - on high voltage circuit the switch is replaced with 2 metal contacts
- one of the contacts is connected to a spring which keeps the contacts apart
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how to calculate the size of the force
force = magnetic flux density ( t) x current x length
what criteria must be met for the equation of force
the conductor must be at right angles to the magnetic field it is placed in
what measure is the magnetic flux density
measure of strength of magnetic field
what rule is used to determine the force experienced due to the motor effect
fleming’s left hand rule
when using fleming’s left hand rule what does the forefinger represent
points in direction of magnetic field ( from north to south)
when using fleming’s left hand rule what does the second finger represent
points in the direction of current flow in the conductor ( + to - )
when using fleming’s left hand rule what does the thumb represent
points in the direction of the motion ( force)
what will happen to the force if the conductor is parallel to the magnetic field
it will not experience a force
how does an electric motor work
- a coil of wire carrying a current is placed in a magnetic field
- the forces on the two sides perpendicular to the field experience forces in opposite directions
- this causes a rotational effect
how do loudspeakers make use of the motor effect
the motor effect is used to convert variation in the current of an electrical circuit into the pressure variations which produce audible sound
explain how a loudspeaker works
- a cone with a coil of wire wrapped around it is connected to an a.c supply and is placed in a permanent magnetic field
- When current flows through the wire, it creates a second magnetic field, which interacts with the permanent field
- This produces a resultant force which causes the cone to vibrate
- When the current switches direction, the direction of the force on the cone reverses
- this causes the cone to move in and out generating sound waves
how is the pitch of the sound from a loudspeaker changed
- the frequency of the a.c current is altered
- this creates a different frequency of vibration in the cone
what pitch will a higher frequency produce
high pitch
what pitch will a lower frequency produce
lower pitch
what will happen to the amplitude if the size of the current increases
the amplitude of the vibration increases = increases the volume of sound
What happens when an electrical conductor moves relative to a magnetic field
a potential difference is induced across the ends of the conductor
what happens to an electrical conductor when there is a change to the magnetic field that it is placed in
a potential difference is induces across the ends of the conductor
what is the requirement for an induced potential differece to cause a current flow
- if a stronger magnetic field is used
- if the wires are moved rapidly
- if the shape of the wire turns into a coil ( the greater the numbers of turn = greater the induced potential difference and current )
What can be said about the direction of the magnetic field produced by an induced current
- the direction of the field is such that it opposes the change that induced the current
- this change is either the movement of the conductor or a change to the field
what are two ways that the generator effect is used to generate different types of current
- in an alternator to produce alternating current
- in a dynamo to produce direct-current
what electromagnet effect does a microphone take advantage of and how
- the generator effect
- it converts the pressure variations in sound waves into alternating current in a circuit
describe the makeup of a basic transformer
a primary coil and a secondary coil of wire wrapped around an iron core
why is iron used as the core for a transformer
it is easily magnetised
explain how a transformer works
- An alternating current flows through the primary coil
- as current flows through the primary coil, it generates a changing magnetic field in the core
-this changing magnetic is transmitted along the iron core and passes through the secondary coil
-when the changing magnetic field passes through the secondary coil, it induces a potential difference
why must the current flowing through the primary coil of a transformer be alternating
- for current to be induced in the secondary coil , the magnetic field in the core must be continually changing
- for the magnetic field to be changing, the current in the primary coil must be alternating
what can be said about the electrical power input and output of a 100% efficient transformer
the electrical power input is equal to the electrical power output
what does a split ring commutator do
changes the direction of the positive and negative connections every half turn so that the direction of the current swaps every half turn so that they forces acting on the coil will always be acting in same direction
what is an alternator
a coil of wire rotating in a magnetic field
how can the speed of rotations in an electric motor be increased
- increase current passing through wire
- add more turns to the coil
-increase the magnetic flux density by using stronger and powerful magnets
what happens when a wire passes through a magnetic field
- a potential difference is induced in the wire
how can the size of the induced potential difference OR induced current be changed
- change the strength of the magnetic field - eg using stronger magnetic field
- move the magnets quickly - the faster the movement= faster the magnetic field will change = bigger potential difference
- shape wire into a coil with multiple and more turns = bigger potential difference
what happens when the wire stops moving in the generator effect
- a potential difference disappears ( this happens at the top and bottom of the wire ) because of a change in the magnetic field that the wire experiences as it moves through the magnetic field
what happens to the current when magnets are moved up and down but the wire remains still
the induced potential difference will generate a current because the wire is still experiencing a change in the magnetic field
what will happen if the wire is moved back and forth
no induce potential difference or current because the wire isn’t experiencing a change in the magnetic field
what happens to the potential difference when the direction changes
the direction of the potential difference swaps each time
what are electromagnets most commonly used for
- electric motors
- loudspeaker
what happens to the current when a wire becomes a complete circuit
the induced potential difference will generate a current because electrons are able to flow around the circuit
how is the generator effect used
to generate an alternating current and a direct current
what position of the coil allows for maximum potential difference
when the coil is horizontal
what happens to the potential difference when the coil is vertical
the potential difference falls to zero becayse the coil is now moving parallel to the field
how do alternators work
- coil of wire rotates relative to the magnets = induces a magnetic field in coil = then induces a voltage and current in coil
- slip ring and brushes mean that contacts dont swap every half turn = alternators produce an alternating potential difference and alternating current
- As the coil rotates faster, the peaks of the oscillations get larger and more frequent.
how do dynamos work
- As the coil of wire spins relative to the magnet, a magnetic field, and hence a voltage and current, is induced in the coil
- coil is connected to The split ring commutator = that the contacts swap every half turn.
- This means that they produce a direct potential difference and hence a direct current
-As the coil rotates faster, the peaks of the oscillations get larger and more frequent.
what is a difference between dynamo and alternator
- dynamo have split ring commutator
- alternator have slip rings and brushes
- due to split ring commutator, dynamos produce direct current.
- Due to the slip rings and brushes, alternators produce alternating current.
describe this graph
the maximum potential difference is when the coil is horizontal - at this point the wire is sweeping directly through the magnetic field lines at the fastest possible rate
what is happening at this point
when the coil is vertical, the potential difference falls to zero because the coil is now parallel to the field ( the coil is not cutting through the magnetic field lines
what are the ways the size of the alternating current is increased
- if the strength of the magnetic field is increased
- if we increase the number of turns on the coil or if we increase the area of the coil
what happens if the rotation speed of the coil is increased
the size and frequency of the ac increases
what does a dynamo produce
a direct current
what is a key feature of a dynamo
it has a split - ring commutator
explain how a moving -coil microphone works
- when soundwaves hit the diaphragm, they cause it to vibrate
- the coil of wire moves in and out through the magnetic field = induces a potential difference across the ends of the wire ( the potential difference switches direction as the coil moves backwards and forwards through the magnetic field)
- the frequency of the changing potential difference is the same as the frequency of the sound waves
- the changing pattern of potential difference is now passed through an amplifier and then into a moving coil loudspeaker = increases volume of sound
how do microphones use the generator effect
to convert the pressure variations in sound waves into variations in current in an electrical circuit
how does the iron core affect the magnetic field
it increases the strength of it
what type of current do transformers work with and why
- only work with AC because we need a changing magnetic field to induce a potential difference
why is a direct current not used in transformers
a DC produces a magnetic field so it wont work in a transformer
What happens in the transformer if the primary and secondary coils have the same number of turns
the potential difference in the secondary coil will be the same as the potential difference in the primary coil
What happens in the transformer if the primary coils have more turns than in the secondary coil
the potential difference in the secondary coil will be much less than the potential difference in the primary coil
What happens in the transformer if the secondary coils have more turns than in the primary coil
the potential difference induced in the secondary coil will be greater than the potential difference in the primary coil ( also known as a step up transformer )
what is the potential difference like in the primary and secondary coil in a step down transformer
the potential difference in the secondary coil is less than the potential difference in the primary coil
how do step up transformers affect the potential difference of an electricity supply
it increases the pd
how do step down transformers affect the potential difference of an electricity supply
it decreases the pd
what is the equation to determine the pd of the coil
how is electrical power transmitted from power station to homes
through high - voltage cables
what do we need to transmit a large amount of power
a large current is needed or a large voltage
how is power wasted in transmission cables
power wasted as heat
what is a problem of using a large current in the transmission cables
a large amount of power would be wasted as heat = large pd is better
describe how electricity is transferred from power station to homes
- electricity from power station is first passed through a step up transformer = steps up the pd to around 400,000 v
- electrical power is then transmitted down high - voltage cables
- it is then passed through step down transformers = reduce pd
what are examples of electric motors
fans
vehicles
hardrives
