motors, generators and transformers Flashcards
what do magnetic field lines show
magnetic field lines are used to show the size and direction of magnetic fields.
which way magnetic field lines point in diagrams of magnetic fields
magnetic field lines always point from north to south
how can you look at magnetic field patterns
you can use compasses and iron fillings as they align themselves with magnetic fields
magnets ……….. and …………. other magnets
magnets ATTRACT and REPEL other magnets
how do you produce a uniform magnetic field using two bar magnets
placing the north and south poles of two permanent bar magnets near each other creates a uniform field between the two magnets
what happens when a current flows through a wire
when a current flows through a wire a magnetic field is produced around the wire
what can the shape of the magnetic field be seen as
the shape of the magnetic field can be seen as a series of concentric circles in a plane, perpendicular to the wire
what do the direction of magnetic field lines depend on
the direction of the field lines depends on the direction of the current
what is an electromagnet
a magnet whose magnetic field can be turned on and off with an electric current
how can you increase the strength of the magnetic field around a solenoid
you can increase the strength of the magnetic field around a solenoid by adding a magnetically ‘soft’ iron core through the middle of the coil
what is a magnetically soft material
a magnetically soft material magnetises and demagnetises very easily. so, as soon as you turn off the current, the magnetic field disappears - the iron doesn’t stay magnetised.
this is what makes it useful for something that needs to be able to switch its magnetism on and off
how are electromagnets used in scrapyard cranes
electromagnets are used in some cranes to attract and pick up things made from magnetic materials like iron and steel, when the current passes through the coil of the electromagnet .
when the current is switched off, the magnetic material drops off the electromagnet as the electromagnet is demagnetised
how can electromagnets used in a circuit breaker
in a circuit breaker, there is a switch in series with an electromagnet.
the switch is normally held closed by a spring. when too much current passes through the electromagnet, the switch is pulled open by the electromagnet.
the switch stays open when it has been opened until it is reset manually
what is the left hand rule
you can use Fleming’s left-hand rule to remember the direction of motion in an electric motor.
First finger points in the direction of the magnetic Field. (N to S)
The seCond finger points in the direction of the Current and is at right angle to the field. (+ to -)
The thuMb points in the direction of the Motion.
what does an electromagnet consist of
an electromagnet consists of insulated wire wrapped round an iron bar (the core)
what happens when a current is passed along the wire in an electromagnet
when current is passed along a wire, a magnetic field is created around the wire. as a result, the magnetic field of the wire magnetises the iron bar. when the current is switched off, the iron bar loses most of its magnetism.
what can electromagnets be used in
- scrapyard cranes
- circuit breakers
- electric bells
- relays
how are electromagnets used in electric bells
when the bell is connected to the battery, an iron amateur is pulled onto the electromagnet.
this opens the make-and-break switch, and the electromagnet is switch off.
as a result, the armature springs back and the make-and-break switch closes again so the whole cycle repeats itself
how are electromagnets used in relays
a relay is used to switch an electrical machine such as a motor on and off.
when current passes through the electromagnet, the armature is pulled onto the electromagnet. as a result, the armature turns about the pivot and closes the switch gap.
in this way, a small current (through the electromagnet) is used to switch on a much larger current.
what is the motor effect
the force that a wire carrying a current experiences when it is placed in a magnetic field
what will happen when the conductor is parallel to the magnetic field
the conductor will not experience a force if it s parallel to the magnetic field
what can the size of the force be increased by
- increasing the strength of the magnetic field
- increasing the size of the current
what will reverse the direction of the force
by either:
- reversing the direction of the current
- reversing the direction of the magnetic field
what does a simple motor consist of
the simple motor consists of a rectangular coil of insulated wire (the armature coil) that is forced to rotate.
the coil is connected via two metal or graphite ‘brushes’ to the battery.
the brushes press onto a metal split ring commutator fixed to the coil.
why does the coil in an electric motor spin when current is passed through the coil
- a force acts on each side of the coil due to the motor effect
- the force on one side is in the opposite direction to the force on the other side
what does the split ring commutator do
the split ring commutator reverses the current round the coil every half-turn of the coil.
because the sides swap over each half-turn, the coil is pushed in the same direction every half-turn
what is a loudspeaker designed to do
a loudspeaker is designed to make a diaphragm attached to a coil vibrate when alternating current passes through the coil
what happens in a loudspeaker
when a current passes through the coil, a force caused by the motor effect makes the coil move.
each time the alternating current changes direction, the force reverses its direction. so the coil is repeatedly forced backwards and forwards. this motion makes the diaphragm vibrate so that sound waves are created
what happens if an electrical conductor ‘cuts’ through a magnetic field
if an electrical conductor ‘cuts’ through a magnetic field a potential difference is induced across the ends of the conductor
what is the generator effect
the generator effect is the effect of inducing a potential difference using a magnetic field
what happens if a magnet is moved into a coil of wire
if a magnet is moved into a coil of wire a potential difference is induced across the ends of the coil
when does the generator effect also occur
if the magnetic field is stationary and the coil is moved
the faster a wire cuts across the lines of a magnetic field, the ……………. the induced potential difference
the faster a wire cuts across the lines of a magnetic field, the greater the induced potential difference
what needs to be done to induce a potential difference when a direct current electromagnet is used
when a direct current electromagnet is used, it needs to be switched on or off to induce a potential difference
what happens if the coil of wire is part of a complete circuit
if the coil of wire is part of a complete circuit, a current is induced in the wire
what happens if the direction of motion or the polarity of the magnet is reversed
if the direction of motion or the polarity of the magnet is reversed, the direction of the induced potential difference and any induced current is reversed
when does the size of the induced potential difference increase
- when the speed of the movement increases
- when the strength of the magnetic field increases
- when the number of turns on the coil increases
- when the area of the coil increases
what does a simple ac generator consist of
the simple ac generator consists of a rectangular coil that is forced to spin in a uniform magnetic field.
the coil is connected to a centre-reading meter via metal ‘brushes’ that press on two metal slip rings.
the slip rings and brushes provide a continuous connection between the coil and the meter
what happens in a simple ac generator
as the coil (or magnet) spins, a current is induced in the coil. this changes direction every half turn.
instead of a split-ring commutator, AC generators have slip rings and brushes so the contacts don’t swap every half turn, therefore producing alternating current voltage.
what do generators do
generators rotate a coil in a magnetic field (or a magnet in a coil)
what happens in a simple dc generator
dc generators also rotate a coil in a magnetic field (or a magnet in a coil).
as the coil (or magnet) spins, a current is induced in the coil. this current changes direction every half turn.
dc generators have a split-ring commutator, which reconnects the coil the opposite way round the circuit every half turn. this happens each time the coil is perpendicular to the magnetic field lines.
as a result, the induced potential difference does not reverse its direction as it does in the ac generator. current flowing in the same direction (i.e. its DC)
this means they produce DC voltage.
when is the size of the induced potential difference greatest in a simple AC generator
the size of the induced potential difference is greatest when the plane of the coil is parallel to the direction of the magnetic field
(as the sides of the coil parallel to the axis of rotation cut directly across the magnetic field lines)
when is the size of the induced potential difference zero in a simple AC generator
the size of the induced potential difference is zero when the plane of the coil is perpendicular to the magnetic field lines.
(at this position, the sides of the coil move parallel to the field lines and do not cut through them)
in a simple AC generator, the faster the coil rotates, the ……… the frequency of the alternating current
the faster the coil rotates, the GREATER the frequency (i.e the number of cycles per second) of the alternating current.
(this is because each full cycle of the alternating potential difference takes the same time as one full rotation of the coil)
in a simple AC generator, the faster the coil rotates, the …………. the peak value of the alternating current
the faster the coil rotates, the LARGER the peak value of the alternating current.
(this is because the sides of the coil move faster and therefore cut the field lines at a faster rate, so the peak value of the induced potential difference is greater)
in a simple AC generator, what effects does rotating the coil faster have
the faster the coil rotates:
- the greater the frequency of the alternating current
- the larger the peak value of the alternating current
what happens to the value of the induced potential difference in one complete rotation of the coil (or ‘one full cycle’) in a simple ac generator
In one complete rotation of the coil, the induced potential difference increases from zero to maximum value, then decreases to zero, reverses and increases to a negative maximum and then becomes zero again.
what is the peak value
both the positive and negative maximum values are called the peak value
what happens to the value of the induced potential difference in one complete rotation of the coil in a simple dc generator
the induced potential difference varies from zero to maximum value twice each cycle, and never changes polarity
what does a basic transformer consist of
a basic transformer consists of a primary coil and a secondary coil wound on a soft iron core.
what does an alternating current in the primary coil of a transformer produce
an alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil.
this induces an alternating potential difference across the ends of the secondary coil
what does a transformer consist of
a transformer has two coils of insulated wire, both wound round the same soft iron core. this core is easily magnetised and demagnetised.
the primary coil is connected to an alternating current supply.
what happens when alternating current passes through the primary coil of a transformer
when alternating current passes through the primary coil, an alternating potential difference is induced in the secondary coil
why is an alternating potential difference is induced in the secondary coil of a transformer when alternating current passes through the primary coil
- alternating current passing through the primary coil produces an alternating magnetic field
- the lines of the alternating magnetic field pass through the secondary coil
- the magnetic field is changing
what happens if a bulb is connected across the secondary coil of a transformer
if a bulb is connected across the secondary coil, the induced potential difference causes an alternating current in the secondary circuit, and so the bulb lights up.
this proves that electrical energy is transferred from the primary coil to the secondary coil EVEN THOUGH they are not electrically connected in the same circuit
what is proved by the fact that a bulb that is connected across the secondary coil of a transformer lights up
this proves that electrical energy is transferred from the primary coil to the secondary coil EVEN THOUGH they are not electrically connected in the same circuit
what does a step-up transformer do
a step-up transformer makes the potential difference across the secondary coil greater than the potential difference across the primary coil.
its secondary coil has more turns than its primary coil
what does a step-down transformer do
a step-down transformer makes the potential difference across the secondary coil less than the potential difference across the primary coil.
its secondary coil has fewer turns than its primary coil
what are the potential differences across the primary and secondary coils of a transformer related to the number of turns on the coils by
Vp np
—- = —–
Vs ns
what does Vp and Vs mean
the potential differences across the primary coil of a transformer and the potential differences across the secondary coil of a transformer, respectively
what does np and ns mean
the number of turns on the primary coil of a transformer and the number of turns on the secondary coil of a transformer, respectively
what is to be the case if transformers are assumed to be 100% efficient
if transformers are assumed to be 100% efficient, the electrical power output would equal the electrical power input
what is the relationship between the power input of the primary and secondary coils and the power output of the primary and secondary coils in a 100% efficient transformer
Vp x Ip = Vs x Is
what are Vp and Ip
power input (primary coil)
what are Vs and Is
power output (secondary coil)
what is the input to a transformer determined by
the input to a transformer is determined by the required output
switch mode transformers are transformers that:
- operate at a high frequency, often between 50 kHz and 200 kHz
- are much lighter and smaller than traditional transformers that work from a 50 Hz mains supply, making them useful for applications such as mobile phone chargers
- use very little power when they are switched on but no load is applied
