magnetism (p7) Flashcards
what is a magnet?
any material/object that produces a magnetic field
- e.g. a bar magnet, earth, horseshoe magnet
what is a magnetic material?
any object that can be influenced by a magnetic field, and has the potential to become a magnet
what do all magnets have in common?
- have two poles - north and south
- surrounded by a magnetic field, represented by field lines
what are field lines?
arrows drawn around a magnet, from the north pole to the south pole
- not only shows where the magnetic field is
- also shows the strength of the field. the more dense the field lines are, the stronger the field in that area
- shows whether two magnets would attract/repel
what is the rule for the direction field lines point?
IN to the south pole
AWAY FROM the north pole
how can we find the north and south poles of a magnet?
- use a compass. the arrow of the compass is a tiny bar magnet, so lines up with the field lines of whichever magnetic field it’s put in
- it will always point towards the south pole of the magnet
describe the interaction between 2 bar magnets:
- pushing the same poles together makes them repel. it’s the interaction of the magnetic fields that creates this force of propulsion
- pushing opposite poles together means they attract. all the field lines would still be going from north to south, so it would be permitted
what common magnetic materials/elements are there?
- nickel
- cobalt
- iron
- the alloys of these elements, e.g. steel, also count as magnetic materials
what are permanent magnets?
objects that produce their own magnetic field all the time (e.g. bar magnets)
what are induced/temporary magnets, and how are they created?
- only have a magnetic field temporarily
- made when a magnetic material is placed in the field of a permanent magnet. this induces the magnetic material to develop its own magnetic field, with its own north and south pole
can induced magnets both attract and repel?
induced magnets can only attract
how does an induced magnet lose its magnetism?
remove it from the magnetic field it was placed in
- nickel and iron lose their magnetism very quickly. they’re ‘magnetically soft’ materials
- steel loses its magnetism more slowly, so it’s ‘magnetically hard’
what is electromagnetism?
the phenomenon where electric currents produce their own magnetic field
describe the magnetic field produced by a wire:
- imagine a thin wire with current flowing through it, from bottom to top
- this current would create a magnetic field around the wire, shown by field lines. these lines would be concentric circles around the wire, and they’d be closest together near to the wire, as that’s where the magnetic field is strongest
what is the right hand grip rule?
- the direction of the magnetic field depends on the direction of the current flow
- use the right hand grip rule. point your right thumb in the direction of the current flow, and the way your fingers curl will be the the direction of the field
what is a solenoid?
- a circular coil. contains many turns, all next to each other, in one long piece of wire
- the magnetic field inside the solenoid is strong and uniform, and flows away from N and towards S
- the field lines around the solenoid are similar to those found around a bar magnet (loops)
is a solenoid an electromagnet, and what is useful about this?
a solenoid is an electromagnet, as we have used electricity to create the magnet
- useful as they’re only magnetic as long as we keep the current flowing through the wire. turning off the current means the magnetic field disappears
how do you reverse the field direction in an electromagnet?
reverse the direction in which the current is flowing
- show this by reversing the direction of the current arrows, and flipping the poles
what are the 4 ways to increase an electromagnet’s strength?
- increase the current flowing through
- increase the number of turns in the coil, whilst keeping the length of the solenoid the same
- decrease the length of the solenoid, whilst keeping the number of turns the same
- solenoids with densely packed coils are the strongest - add a soft iron core to the inside of the solenoid
how is adding a soft iron core to the solenoid going to make it stronger?
iron will become an induced magnet when the solenoid is switched on, increasing the strength of the electromagnet’s magnetic field
what is the motor effect?
the idea that a current carrying wire in the presence of a magnetic field will experience a force
describe a current-carrying wire passing between two bar magnets:
- the wire will produce its own magnetic field
- placing the wire between the north and south poles of 2 magnets (with their own magnetic field) means the two fields will interact
- this interaction results in a force on the wire to basically ‘push it out’ of the field
how can we ensure the wire experiences the full force caused by the motor effect?
must be at exactly 90 degrees to the magnetic field, whereas if it’s at a slight angle, it will feel slightly less force. if the wire was running in the same direction as the field (from one bar magnet to the other), it’d feel no force at all
how do we find the direction of the force during the motor effect (Fleming’s left hand rule)
- must know the direction of the magnetic field and the direction of the current in the wire
- use Fleming’s left hand rule
- point your index finger in the direction of the magnetic field. point your second finger in the direction of the current. whichever way your thumb now points is the direction of the force experienced by the wire
how do you calculate the strength of the force acting on a wire during the motor effect?
- as long as the wire is at a right angle to the magnetic field
F = B x I x L
Force (N) = Magnetic Flux Density (magnetic field strength) (Teslas) x Current (A) x Length of wire (m)
how does a motor work, and what is the role of a commutator?
(explanation is condensed)
- a potential difference is applied to the coil
- this causes a current in the coil which generates a magnetic field around the coil
- the magnetic field of the electromagnet and permanent magnet interact
- the sides of the coil cutting the magnetic field experience a force (motor effect)
- the forces on each side of the coil create equal and opposite moments, meaning the coil turns
- the commutator reverses the current in the circuit every half turn by swapping the wires from one brush to the other
- the current and therefore the forces on each side of the coil reverse and the coil can keep turning
describe an unsuccessful electric motor without a split ring commutator:
- let’s say the left side experiences an upward force and the right side experiences a downward force. the coil will spin 180 degrees clockwise (to the right)
- once it has flipped upside down, the current will be travelling in the opposite direction, meaning the forces acting on each side of the coil have also swapped direction
- the coil will then start turning anti-clockwise instead and flip right back over to where it started
- this means the coil would never turn 360 degrees, so wouldn’t be useful
describe the setup of an electric motor (minus the split ring communicator):
- have two magnets, with a magnetic field between them, and two connected current-carrying wires running down the middle of them, so they form a coil. current flows in from the positive terminal, and flows out from the negative terminal
- the wires have currents flowing in opposite directions, and so due to Fleming’s left hand rule, the forces acting on them will also be in opposite directions (e.g. one up, one down)
what does a split-ring commutator do?
- attached to the positive and negative terminals of the coil
- swaps the positive and negative connections every half turn
- the direction of the current also swaps every half turn
- the forces acting on the coil will always act in the same direction, so the coil will continue to rotate in the same direction
where are split-ring commutators used?
in almost all motors that we see, e.g. fans, vehicles, hard-drives, as it all rests on the idea that we can create a spinning motion using electricity
how do we increase the speed of the rotations in an electric motor?
- increase the current passing through the wire
- add more turns to the coil
- increase the magnetic flux density (magnetic field strength) by using more powerful magnets
describe the generator effect (electromagnetic induction), and the 2 important things to note:
imagine 2 magnets with a magnetic field between them. taking a piece of wire bent into a coil shape and moving it through the magnetic field (up and down) will induce a potential difference in the wire
- joining the two ends together forms a complete circuit. a current would be created by the induced potential difference, as electrons are now able to flow around the circuit
- every time the wire stops moving (when it reaches the top/bottom), the potential difference disappears, as the change in the magnetic field as the wire moves through it is what creates the potential difference
- the direction of the potential difference swaps each time we change the direction (up or down)
so what actually is the generator effect?
the idea that we generate an electric current by moving a wire relative to a magnetic field
would the generator effect still work if we kept the wire still and instead moved the magnets?
yes, it would, as the wire is still experiencing a change in the magnetic field, which is the key idea in electromagnetic induction
- however, moving the wire back and forth (along the magnetic field, from magnet to magnet) would induce no potential difference, as the wire isn’t experiencing a change in the magnetic field
how would we change the size of the induced potential difference/current in the generator effect?
- change the magnetic field strength (e.g. using stronger magnets, producing a stronger magnetic field). this would produce a larger p.d./current
- move the wire/magnets more quickly. the faster they move, the faster the magnetic field will change, so the bigger the p.d./current
- shape the wire into a proper coil (more turns). the more turns, the bigger the induced p.d./current
what happens when we move a single magnet in and out of a coil of wire (a solenoid)?
this movement of the magnetic field relative to the coil induces a potential difference in the coil. as the circuit is complete, it also produces a current
- whenever we change the direction of the magnet, we change the direction of the current. this can also be done by swapping the poles of the magnet (turning it around the other way)
where do we use the generator effect?
in devices called ‘generators’. these devices generate electricity from rotational motion (e.g. rotating a coil of wire)
- the two types of generator are alternators and dynamos
what is the main difference between alternators and dynamos?
- dynamos have a split ring commutator, so produce a direct current
- alternators have slip rings and brushes, so produce an alternating current
how do alternators work?
- the coil of wire moves relative to the magnets (like in an electric motor), and this induces a magnetic field in the coil, which then induces a voltage and current in the coil (generator effect)
- the slip rings and brushes means the contacts don’t swap every half turn (like they do in a motor/dynamo)
- this means that they produce an alternating current (a.c.)/alternating p.d.
- 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, hence a voltage and current, is induced in the coil
- the split-ring commutator means that the contacts swap every half turn
- therefore they produce a direct potential difference and therefore a direct current (always flowing in the same direction)
- this can be shown on an oscilloscope as lots of bounces on the x-axis. as the coil rotates faster, the peaks of the oscillations get higher and more frequent
how are microphones and loudspeakers similar?
- both convert between sound waves and electrical signals
- for both, the electrical signals involved are alternating currents (or voltage)
how do loudspeakers work?
- an alternating current flows through the wire and creates a magnetic field in the coil
- the magnetic field of the coil interacts with the magnetic field of the magnet
- the interacting magnetic fields will exert a force on the coil (the motor effect), causing it to move back or forth along the permanent magnet
- this causes the cone to change shape
- as the current is alternating, its magnetic field and the force it experiences will also alternate. this means that the coil of wire and cone will move rapidly back and forth, meaning they vibrate
- these vibrations are so fast that the cone vibrations cause pressure variations in the air, which are sound waves
what does a loudspeaker consist of?
a coil of wire is wrapped around one pole of a permanent magnet and connected to the cone. the coil of wire is permanently attached to the cone, but the coil can slide back and forth over the magnet.
what is the structure of a microphone?
similar structure to a loudspeaker, but instead of a cone at the front, it’s a diaphragm that moves, attached to the coil, which also moves.
how do microphones work?
- sound waves hit the diaphragm
- this causes the diaphragm and coil of wire to move
- as the wire is moving within the magnetic field of the permanent magnet, it will generate a current (generator effect)
- the frequency and amplitude of the sound waves will determine how much the diaphragm vibrates, and therefore determine the frequency and amplitude of the current
how are transformers used in the national grid?
- most of our electricity is produced in power stations across the country
- this electricity must now be passed through step-up transformers, which increase the voltage to around 400,000 volts. this minimises energy losses as the electricity is sent across the country in a huge network of wires and pylons
- once the electricity reaches its destination, it passes through a step-down transformer, decreasing its voltage to around 230 volts, so it’s safe to use domestically
what is the role of a transformer?
increases or decreases the voltage and current of electricity
- e.g. in a step-up transformer, voltage is increased, current is decreased. vice versa for a step-down transformer
how do transformers work?
- an alternating potential difference is applied across the primary coil, causing a current to flow
- this generates a magnetic field around the coil. however, as the potential difference is alternating, the direction of the current, and thus the magnetic field, also alternates
- this alternating magnetic field in the primary coil induces an alternating magnetic field in the iron core, as iron is a magnetic material, so becomes an induced magnet
- the iron core’s magnetic field induces a potential difference across the secondary coil. this causes a current to flow around the secondary coil
describe the structure of a transformer:
chunk of iron in the centre (iron core). hollowed out rectangle shape.
- connects primary coil on one side to the secondary coil (much tighter) on the other
why can’t the electricity simply conduct straight across from the primary to the secondary coil in a transformer?
- uses induced magnetic currents to conduct across
- the wires are insulated in plastic, so the electricity can’t pass directly between the wire and the iron core
how do transformers change the potential difference between the two coils?
- use a step-up transformer as an example
there are twice as many turns in the secondary coil than in the primary coil, so the voltage is doubled
- step down transformers have less turns on the secondary coil, so they decrease the potential difference
what equations allow us to figure out how the potential difference and current vary with the number of turns on each coil?
equation 1: Vp/Vs = np/ns
primary p.d. / secondary p.d. = primary no of coils / secondary no of coils
equation 2: Vp x Ip = Vs x Is
primary p.d. x primary current = secondary p.d. x secondary current
(the power in the primary coil is equal to the power in the secondary coil, as P=IV)
V = potential difference
n = number of turns of the coil
I = current
p = primary coil
s = secondary coil
