P15 - Motor/ Generator effects Flashcards
why does an electric motor work
a force acts on a wire in a mg field as a current passes through
size of the force can be increased by
- increasing current
- using a stronger magnet
force is greatest
when wire is perpendicular to mg field
force is 0 when
wire is parallel to mg field
direction of force is always at a right angle to
wire and mg field lines
direction of force is reversed if
either current or mg field lines are reversed
Fleming’s left hand rule
Forefinger = field
-second finger = current
thumb = movement of force
magnetic flux density - symbol and unit
measure of the strength of the mg field
-symbol B
unit tesla T
force =
BIL
electric motor uses
motor effect
electric motor changes speed by
changing current
Motor effect process
- coil of insulated wire is forced to rotate
- coil is connected to battery by 2 graphite brushes
- brushes press onto metal split ring commutator fixed to coil
why graphite brushes
conducts electricity
- causes little friction with rotating commutator
as the current passes through - coil spins because:
-force acts on each side of coil (motor effect)
- force on one side is in the opposite direction to the force on the other side
split ring commutator and direction of current
reverses current every half turn
- as sides swap over each half turn the coil is pushed in the same direction every half turn
generator
- contains coils of wire hat spin in a mg field
-pd is induced
why is pd induced in a generator
crosses the mg field lines
electromagnetic induction
happens when any conductor crosses mg field
if conductor is a part of a complete circuit
- induced pd induces current pass through
-aka generator effect
generator test
coil of insulated wire
-connected to centre reading ammeter
- when end of bar magnet is pushed into coil
= ammeter pointer deflects
Generator effect happens due to
-movement of bar magnet = induces pd
- induces pd = current as apart of circuit
induced magnetic field always opposes
original change
-so work done by person moving the magnet
in generator effect stronger magnet =
pd and current increases
faster a conductor crosses mg field
bigger pd induced
the electricity generated is a result of
work done by a person moving the magnet
when a dc EM is used
must be switched on/off to induce pd
direction of induced current always
opposes og change that caused it
alternator / ac generator
rectangular coil is forced to spin in a uniform mg field
-attached to centre reading meter by metal brushes
-that press onto 2 metal slip rings
ac generator purpose of having brushes press onto slip rings
provides a constant connection between coil/meter
alternator meter shows
coil turns in one direction = meter deflects one way then the opposite
ac generator current always changes
-as pd changes
- both alternate as they change direction
alternator - pd is greatest when
plane of coil is parallel to direction of mg field
-coil crosses directly
alternator - pd is 0 when
perpendicular
as sides of coil moves parallel to mg field
the faster the coil rotates in an alternator
bigger frequency of AC
- each full cycle of alternating pd = smae time as one full rotation of coil
alternator - bigger peak value
crosses field at a faster rate
to increase peak value of ac generator
-use a stronger magnet
- coil with a bigger area
- more turns in coil
AC generator waveform on oscilloscope of PD
- peaks when side of coil crosses
- rotated faster = more and taller waves
DC dynamo/ dc generator diff
split ring commutator instead of 2 slip rings
dc process - as coil spins
- split ring reconnects to coil the opposite way around every half turn
- so every time coil is perpendicular
- induced pd doesn’t reverse
dc values vary from
0 to max value twice each cycle
moving coil sound devices
-microphone generates alternating PD
- as sound waves make coil vibrate
microphone process
- coil is attached to small diaphragm + between poles
- pressure variations of sound waves on diaphragm make it vibrate
so coil vibrates in mg field - alternating PD induced in coil
- alternating PD induced in coil has the same frequency as
sound waves
Loudspeaker
creates sound waves when alternating PD is applied to coil
loudspeaker process
- current in coil causes a force due to motor effect
-current alternates so force reverses
-making coil/diaphragm vibrate
-creates sound waves - same frequency as alternating PD
