magnetism- induced potential and transformers + motor effect Flashcards
transformers
a transformer consists of two coils of wire:
- primary coil left hand side
- secondary coil right hand side
the two coils are completely separate
there is no way that electrical current can pass directly from one coil to another
the two coils are wrapped around an iron core
why is an iron core used
because its easily magnetised
primary coil is connected to an alternating current
as the current flows through the primary coil , it generates a changing magnetic field
this magnetic field is transmitted along iron core and passes through secondary coil
what happens when a changing magnetic field is passed through the secondary coil
a potential difference is induced
what does the iron core do
increases the strength of the magnetic field
why do transformers only work with an alternating current and not a direct current
because we need a changing magnetic field to induce a potential difference
a direct current produces a constant magnetic field and that does not induce a potential difference therefore it does not work in transformers.
if the primary and secondary coil both have the same number of turns
the potential difference in the primary coil will be the same as the potential difference in the secondary coil but it is only the case if the transformer is 100 percent efficient( if there is no energy wasted)
what if there are more turns in the secondary coil than in the primary coil
the potential difference induced in the secondary coil will be greater than the potential difference induced in the primary coil this is called a step up transformer as it steps up (increases) the potential difference
what if there are twice the number of turns in the secondary coil than in the primary coil
the potential difference in the secondary coil will be twice the potential difference in the primary coil
in a step down transformer, there are fewer turns in the secondary coil than in the primary coil
in a step down transformer, potential difference in secondary coil is less than potential difference in the primary coil
what if the number of turns in the secondary coil is halve the number of turns in the primary coil
the potential difference in the secondary coil will be half the potential difference in the primary coil
potential difference in primary coil/ potential difference in secondary coil=
number of turns in primary coil/ number of turns in secondary coil
a transformer has 200 turns in the primary coil and 50 turns in the secondary coil . a potential difference of 48v is induced in the secondary coil . calculate potential difference in the primary coil
200/50 = 4
48x4=192
192 V
a transformer has 100 turns in the secondary coil. it steps up the potential difference from 50v to 250v . calculate number of turns in primary coil
20 turns
50/250 x 100
power must be conserved
power of primary coil= power of secondary coil
power= potential difference in primary coil x current in primary coil
potential difference in secondary x current in secondary coil
it only applies if transformer is 100 percent efficient
a trasnformer has a potential difference of 200v and a current of 0.1A in the secondary coil the potential difference of the primary coil is 50v calculate current in primary coil
0.4A
when a current is produced around a conducting wire
a magnetic field is produced around the wire
how can we prove that there is a magnetic field around the wire
we can use a compass when the current is turned off the compass needle lines up with the Earths magnetic field.
if we turn the current on again, the compass needle deflects this proves that there is a magnetic field around the wire
the strength of the magnetic field depends on teh size of the
current
a stronger magnetic field is produced by
a larger current
where is the magnetic field strongest
closer to the wire
as we move further from the wire the strength of the magnetic field decreases
what happens to the strength of the magnetic field as we move further from the wire
the strength of the magnetic field decreases
what happens if we change the direction of the current
the direction of the ,magnetic field changes
direction of magnetic field
use right hand grip rule
https://electrical4dummies.blogspot.com/2016/06/flemings-right-hand-rule-and-right-hand.html
another way to increase the strength of the magnetic field other than increasing the current
is by coiling the wire this hsape is called a solenoid
what kind of current does a solenoid produce
a strong uniformed current inside the solenoid
what does the magnetic field around a solenoid has a similar magnetic field to
a bar magnet
three ways to increase the strength of a magnetic field produced by a solenoid
-increase size of current increases strength of magnetic field
-increase number of turns in coil
-place an iron core inside the solenoid
a solenoid wrapped around an iron core is called an electromagnet
why are electromagnets really useful
we can change the strength of the magentic field by changing teh size of teh current for example we can increase teh strength of teh magnetic field by increasing teh size of the current or we can decrease teh strength of a magnetic field by decreasing size of current
we can turn electromagnets on or off
what is an electromagnet
a solenoid with an iron core
using a switch to turn high voltage circuits on or off can be dangerous
what are the risks of using a switch to turn these circuits on or off
- sparking
- electrocution
what do we use to turn circuits on or off
a relay what is a relayw
relay contains two separate circuits a low voltage circuit containing an electromagnet because it has an electromagnet the circuit is safe to be switched on or off
now on the high voltage circuit teh switch has been replaced by 2 metal contacts
one contact is connected to a spring which keeps the contacts apart
there is also an iron block next to the spring
so the low voltage circuit is turned off so no current flows through the elctromagnet so there is no magnetoc field
the high voltage circuit is alsp turned off because the contacts are not touching
when the low voltage circuit is switched on a current flows through the electromagnet and a magnetic field is produced around the electromagnet
the magnetic field attracts the iron block attached to the contact and causes the contacts to close and switches on high voltage circuit
when we switch off low voltage circuit there is no magnetci field anymore
the contacts now spring apart and the high voltage circuit is switched off
doorbell also uses electromagnet how
when doorbell is pressed, switch is closed causing a current to pass through the circuit
a magnetic field is now produced by the electromagnet
the iron contact is attracted towards magneti field it moves towards magnetic field and clapper now hits the bell, breaking teh circuit
there is now no current flowing so no magnetic field, iron contact springs back to its original position
now circuit is complete again so a current flows around the circuit and the process is repeated
doorbell -electromagnet
when buzzer is pressed the switch is closed a current flows around a circuit. the current flows through the electromagnet creating a magnetic field around the electromagnet. the iron contact is attracted to the magnetic field and as it moves closer to the magnetic field the clapper hits the bell, breaking off the circuit. there is now no current flowing so no magnetic field iron contact springs back to its original position and the current can flow around the circuit again and the process is repeated
when a current moves through a wire what is produced
a magnetic field
example: there is a wire, a current is flowing through a wire so there is a magnetic field around the wire
when this wire is placed between two magnets in a magnetic field then
the magnetic field around the wire interacts with the magnetic field between the magnets
the wire now experiences a force because a force is a push or a pull acting on an object due to interaction with another object. when two objects interact they exert a force. in this case the magnetic field of the wire is interacting with the magnetic field between the two magnets
the force is in the upwards direction so the force would cause wire to move upwards this is called the motor effect
how can we calculate the size of the force acting upwards in the motor effect
give the equation
the equation only applies to a wire at right angles to the magnetic field
force= B X I X L
b=MAGNETIC FLUX DENSITY Tesla (measure of strength of magnetic field)
I=current A
l=length m
a wire has a length of 2,5 meters and is carrying a current of 1.5 amps. the magnetic flux density is0.05T calculate the force acting on the wire
force N= B X I X L
- 05 x 1.5 x 2.5=
- 19 N
a wire has a length of 2,5 meters and is carrying a current of 1.5 amps. the magnetic flux density is0.05T calculate the force acting on the wire
force N= B X I X L
- 05 x 1.5 x 2.5=
- 19 N
what are the factors that affect the size of the force
- magnetic flux density (measure of strength of magnetic field)
current
-length of conductor
now that we can determine the size of the force acting on the wire using the equation we can determine the direction of the force using the left hand rule
first finger=direction of magnetic field from north to south
second finger= direction of conventional current
direction that thumb points = direction of force
if the conductor is parallel to the magnetic field it will not experience a force
to experience a force, conductor must be at right angles to the magnetic field
when a conductor such as a wire is carrying a current, it experiences a force in a magnetic field
this called motor effect
electric motors
imagine a loop of wire carrying a current
the current runs in opposite directions on either side of the loop
on the left hand side current runs back to front
on the right hand side current runs front to back
the wire experiences a force on both the right and left sides
There is a moment on the right hand side and there is a moment on teh left hand side so the loop will rotate in teh clockwise direction
problem-once loop is at 90 it stop srotating because when it rotates beyond 90 degrees the force on the left acts downwards and the force on the right acts upwards
these forces push the loop back to 90 degrees position
however problem can be solved if direction of current is changed when loop rotates 90 degrees
split ring commutator is used ot do taht
the split ring commutator is a split metal ring connected to conducting brushes the brushes allow the electric current to pass onto the ring
the current produces a turning force on the motor
the force makes the motor rotate in the clockwise direction
the current is broken for a tiny fraction of a second but the wire keeps turning due to momentum
the currnet switches direction so once again the force on th eleft hand side still acts upwards and force on the right hand side still acts downwards
by switching teh direction of the current the split ring commutator allows the motor to keep rotating in the same direction
loudspeakers and headphones
cone with coil of wire wrapped around one end
a coil of wire is connected to an alternating current electrical supply
permanenet magnet goes inside coil of wire
as the current passes through, it generates a magnetic field
the magnetic field from the coil interacts with the magnetic field from the permanent magnet
these magnetic fields either attract or repel each other
this produces a resultant force which causes the cone to move
when the current switches direction, the direction of teh force of the cone reverses
causes cone to move in and out generating sound wave
by changing frequency of ac supply, we can chnage the freqeuncy that the cone vobrates
a higher frequency produces a higher pitch
a lower freqeuncy will produce a lower pitch sound
if we increase the size of the current we increase the amplitude of the vibration. this increases the volume of the sound
the cone has a coil of wire wrapped around one end############################?????
generator effect
imagine a wire at right angle to the magentic field between two magnets
if we move the wire through the magnetic field then a potential difference is induced across the ends of the wire
when the wire stops moving the potential difference is lost
if we move the wire back down through the magnetic field we get the potential difference again
potential difference has now reversed direction
scientists call this potential difference the induced potential
if we have a complete circuit then we induce a current
this is called the generator effect
direction of force switches when direction of current switches
when wire stops moving current stops
we can get an induced current and induced potential difference if we keep the wire still but move the ,magnetic field
we only get a generator effect if we move the wire through the magnetic field
if the wire moves along the magnetic field we do not get an induced potential difference or current
the size of the induced current or potential difference depends on three factors
induced potential difference and current are larger if we increase the strength of the magnetic field
if we move the wire more rapidly
if we shape the wire into a coil
the greater the number of turns on the coil the greater the potential difference and current
magnet moving in and out of a coil wire - this also produces an induced current
the direction of current changes when direction of movement changes
we can also switch direction of induced current if we switch the poles of the magnets
generator effect
when we move a magnet into a coil of wire a current is induced in the wire
this induced current creates its own magnetic field
this magnetic field opposes the movement of the magnet
when we insert the north pole in the coil the end of the coil becomes anorth pole
this repels the magnet making it harder to push in
when we pull north pole out, that end of the coil becomes a south pole
this attracts the magnet making it harder to pull out
the induced current makes it harder to move the magnet
this means we are doing work because we are transferring energy from movement of the magnet into th emovement of the current
