electromagnetism Flashcards
what is a magnetic field?
an area around a magnet where other magnets or magnetic materials experience a force. non contact
how do you show a magnetic field in a drawing?
magnetic field lines going from north to south. the closer together lines are the stronger the magnetic field
where on a magnet is the magnetic field the strongest?
at the poles.
how do compasses work?
a small bar magnet is attracted to earth’s poles.
what are the 2 types of magnet?
permanent.
induced.
what is a permanent magnet?
a magnet that produces its own magnetic field.
what is a induced magnet?
a magnetic material that turns into a magnet when put in a magnetic field. the force between a permanent and induced magnet is always attractive
how is a magnetic field created around a wire?
when current flows in a wire a magnetic field is created around the wire made up of concentric circles. larger current gives a stronger magnetic field
what is the right hand thumb rule?
point your right thumb in the direction a current is flowing and curl your fingers showing the direction of the magnetic field.
what is a solenoid?
a wire wrapped into a coil.
how does a solenoid increase the strength of a magnetic field in a wire?
the magnetic field around each loop of wire point in the same direction.
what is an electromagnet?
a solenoid with an iron core. the magnetic field can be turned on and off with current
what is a use of an electromagnet?
cranes in scrapyards can pick up metals.
what is the motor effect?
if a current carrying wire is in a magnetic field the wire will experience a force.
how do you get the strongest force out of the motor effect?
the wire is at 90° to the magnetic field.
increase the strength of the magnetic field.
increase current through the wire. or conductor
what direction does force act in the motor effect?
the force always acts at right angles to the magnetic field and direction of current in the wire.
what is the formula for the force acting on a conductor in a magnetic field?
force newtons = magnetic flux density tesla x current amps x length m
magnetic flux density shows the strength of a magnetic field via how many field flux lines there are in a region. a fancy word for magnetic field strength
the length of the conductor in the magnetic field.
formula is only applicable is the current is 90° to the magnetic field.
what is fleming’s left hand rule.
point your left index finger in the direction of the magnetic field your middle finger in the direction of the current and your thumb points in the direction of the force.
what is a split ring commutator?
a component of a dc motor that swaps the positive and negative connections every half turn. therefore swapping the direction of the current
how can the motor effect be shown?
by using a horseshoe magnet with 2 fixed rails inside it and 1 unfixed rail bridging the fixed rails.
when current is supplied the unfixed rail will roll in the direction of the force.
how does a dc motor work?
a loop of wire is placed in a magnetic field.
1 side of the loop experiences upwards force and the other side experiences downwards force causing the wire to rotate.
a split ring commutator swaps the positive and negative power connections every half turn meaning the
forces acting on the wire are always the same direction.
how does a loudspeaker work?
ac is sent through a coil of wire attached to the base of a paper cone.
the coil surrounds one pole of a permanent magnet such as the south pole and is surrounded by the other pole.
when current is supplied there is a force on the coil causing the cone to move.
variations in the current make the cone vibrate which makes the air around the cone vibrate too creating variations in pressure creating sound waves.
the frequency of the sound waves is the same as the frequency of the ac which can be controlled.
what is the generator effect?
when a wire experiences a change in a magnetic field a potential difference is created across the wire.
if the ends of the wire are connected to make a closed circuit a current will flow around the circuit.
how can the strength of the current in the generator effect be increased.
increase the strength of the magnetic field the wire is moving through.
increase the speed the wires moves at.
add more turns to the coil of wire.
what does induced current oppose?
the change that made it.
what is an alternator?
a generator that creates alternating current.
how does an alternator work?
a loop of wire is placed in a magnetic field.
1 side of the loop experiences upwards force and the other side experiences downwards force causing the wire to rotate.
instead of a split ring commutator like a motor alternators have slip rings and brushes so the contacts don’t swap every half turn.
what is a dynamo?
a alternator with split ring commutators to give a direct current.
how can you use an oscilloscope to see generated potential difference?
oscilloscopes show how the potential difference generated in a coil changes over time.
for ac a line goes up and down crossing the horizontal axis. like a sine wave
for dc the line stays above the horizontal axis as the pd is always positive.
the height of the line at any point is the generated pd
how do microphones work?
sound waves hit a flexible diaphragm a thin flexible structure attached to a coil of wire wrapped around a pole of a magnet surrounded by the other pole.
this causes the coil of wire to move in a magnetic field generating a current.
the movement of the coil depends on the properties of the sound wave. louder sounds give more movement
what is a transformer?
a device that changes the voltage of an alternating current.
how does a transformer work?
a transformer has 2 coils primary and secondary joined with an iron core.
when an alternating voltage is applied across the primary coil the iron core magnetises and demagnetises rapidly inducing an alternating voltage in the secondary coil. because of the generator effect
if the secondary coil is part of a circuit current is induced.
the ratio of turns in the primary and secondary coils match the ratio of voltages.
what does a step up transformer do?
increases the voltage. more turns on secondary coil than primary
what does a step down transformer do?
decreases the voltage. more turns on primary coil than secondary
what is the transformer equation?
input voltage ÷ output voltage = primary coil turns ÷ secondary coil turns
why is low current beneficial when transmitting power across the national grid?
low current means less energy is lost through heat. so power is transmitted at high voltage and low current as P = VI
