Electromagnetic Induction Flashcards
what is electromagnetic induction
the induction of a potential difference (and current if there is a complete circuit) across a conductor which is experiencing a change in an external magnetic field.
when does electromagnetic induction happen
when a conductor cuts through the magnetic field lines
how to induce a potential difference across the ends of a conductor
by moving the electrical conductor in a magnetic field
by moving or changing a magnetic field (e.g. moving a magnet) relative to the electrical conductor
what happens to the pd/current if you move the magnet in the opposite direction
the potential difference/current will be reversed
what happens to the pd/current if you keep the magnetic field moving backwards and forwards
you produce a potential difference/current that keeps swapping direction which is called alternating current
when is current induced
when a wire is moved in a magnetic field
how do you increase the induced potential difference
increasing the speed of the movement
increasing the strength of the magnetic field
having more turns per unit length on the coil of wire
how does increasing the speed of movement increase the induced potential difference
the field through the coil is changing quicker, so more field lines are being cut in a given time
how does increasing the strength of the magnetic field increase the induced potential difference
there will be more field lines that can be cut
how does having more turns increase the induced potential difference
there are more wires to cut the field lines
how is the second magnetic field produced
when a current flows through a wire, a magnetic field is produced around the wires
how does the magnetic field produced oppose the original change
the magnetic field created by an induced current always acts against the change that made it, whether it is the movement of a wire or a change in the field its in to return things to the way they were
what do alternators produce
alternating current
how do you produce an electric current by the relative movement of a magnet on a small scale
an electrical conductor that is moving relative to a magnetic field can induce a pd across the conductor
how do you produce an electric current by a conductor on a small scale
if there is a change in an external magnetic field around a conductor and if the conductor is part fo a complete circuit, current will flow
how do alternators produce AC current
as the coils spins, a pd and a current is induced in the coil
every half turn, these change direction
Alternators use slip rings and brushes so that the contacts don’t swap every half turn - the positive and negative ends keep swapping
this maintains the alternating potential difference
what do dynamos generate
direct current / DC
how are dynamos and alternators different
dynamos use a split ring commutator as their electrical contact but alternators use brushes
how do dynamos produce DC current
As the coil rotates, it cuts through the magnetic field lines, inducing a current in the coil.
A split ring commutator then ensures that the current flows in the same direction, as it swaps the connection every half turn resulting in a DC output.
how do you produce an electric current by the relative movement of a magnet on a large scale
the boilers heat water to convert it to steam, which turns a turbine
the turbine is connected to a powerful magnet inside a generator
as the turbine spins, the magnet spins with it
this induces a large pd and AC in the coils of the generator
how do microphones generate current from sound waves
sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
This causes the coil of wire to move back and forth in the magnetic field
this generates a current
how do microphones convert the pressure variations of a sound wave into variations in current
the movement of coil depends on the properties of the sound wave
what are transformers
devices that can change the ac/potential difference (and so current) of an electrical supply using electromagnetic induction
how does a transformer work
when an alternating current flows through the primary coil in a transformer it produces a magnetic field which magnetises the iron core
because there is alternating current in the primary coil, the magnetic field in the iron core is alternating too
this constantly changing magnetic field cuts through the secondary coil
the changing field induces an alternating potential difference across the ends of the secondary coil
if the secondary coil is part of a complete circuit, this pd causes an alternating current to flow
what does the size of the potential difference induced across the secondary coil depend on
the size of the potential difference across the primary coil and the number of turns on each coil
what does the iron core do
it is purely for transferring the changing magnetic field from the primary coil to the secondary. NO electricity flows round the iron core
what would happen if you supplied DC to the primary coil instead of AC
you would get nothing out of the secondary coil at all
why would there be nothing out of the secondary coil if we used DC
there would still be a magnetic field in the iron core but it wouldnt be constantly changing, so there would be no induction in the secondary coil because you need a changing field to induce a potential difference.
what is the ratio between the primary and secondary potential differences
the same as the ratio between the number of turns on the primary and secondary coil
what happens in a step up transformer
the output pd is larger than the input pd
what happens in a step down transformer
the output pd is smaller than the input pd
what happens to the number of turns in a step up tranformer
number of turns on the secondary coil and size of pd across it are greater than across the primary pd
what happens to the number of turns and pd in a step down transformer
the number of turns on the secondary coil and size of pd across it are smaller than across the primary coil
pd across primary coil / pd across secondary coil =
number of turns on primary coil / number of turns on secondary coil
electrical power =
current x potential difference
pd across primary coil x current in primary coil =
pd in secondary coil x current in secondary coil
what is the national grid
a network of wires and transformers across the UK
what does the national grid do
it transfers electrical energy from power station anywhere on the grid to anywhere else on the grid where it is needed
what do you need to do to transmit the huge amount of electrical power needed
you need to transmit the electricity at either a high voltage or high current
what is the problem with transmitting electricity at a high current
you lose loads of energy as the cables heat up
why do cables heat up when transmitting electricity
because the electrical charges have to do work against the resistance of the cables and doing work causes energy to be transferred to the thermal energy stores of the cables and surroundings
the lower the current through the cables …..
the lower the resistance of the cables so less energy is wasted
how do you make the national grid more efficient, high voltage and low resistance
use transformers
what do step up transformers do at power stations
they boost the pd up really high and keep the current low which decreases the energy lost by heating
what do step down transformers do in power stations
they bring the pd back down to safe,usable levels at the consumers end
what does the equation vp x ip = vs x is show
it shows that increasing the pd across the secondary coil decreases the current through the secondary coil so the pd must be increased before transmission to lower the current
