7 - magnetism and electromagnetism Flashcards
which direction do magnetic field lines go
north to south
how do compasses show the dircetion of a magentic field
- A compass contains a small bar magnet on a pivot so that it can rotate.
- The compass needle points in the direction of the Earth’s magnetic field, or magentic field of the magnet
difference between a permanent or induced magnetic
PERMANENT
- produce their own magnetic field
INDUCED
- magnetic material that turns into a magnet when in a magnetic field.
whats the thumb rule for direction of magnetic field
RIGHT HAND thumb points in direction of current and fingers curl round to see direction of magnetic field
how to increase the stength of a magentic field
- add more coils of wire
- increase the current
- ad an iron core - because it becomes sn induced magnet whenever current flows through
example of an electromagnet
- a solenoid with an iron core
- its magantic field can be turned on an off with an electric current
uses of electromagnets
- attract and pick up things from scrap yards and then drop them in a dumb
- in circuits as switches
whats happens during the motor effect when the force is parrallel VS perpendicular
it will experience a full force when the wire perpendicular
but wont get any force at all if its parrallel
what is the motor effect
- when a wire carrying a current creates a magnetic field
- This interacts with another magnetic field
- causing a FORCE that pushes the wire at right angles
flemmings left hand rule - 3 fingers
thumb - FORCE
first finger - MAGNETIC FIELD DIRECTION
second finger - CURRENT DIRCETION
whats the role of a split-ring commutator in the motor effect
allows the current to change direction through the coil every half turn so that the coil rotates in the same direction.
whats the definition of the generator effect
- when PD is created in a wire due to a change in the magnetic field
- if the wire is a full circuit then a current will flow through the wire
alternators (AC)
- generator rotates coil in mag field
- when coil spins current is induced in coil making current change direction every half turn
- have slips and brushes (same as split ring commuter) so they spin 360
- produce AC
dynamos (DC)
- same as alternators
- but instead of slips and brushes they use SPIT RING COMMUTATORS
- produces DC
whats an oscilloscope and what happens when you increase the frequency of revolutions
- show how PD generated in a coil, changes
- when you increase the frequency of revolutions you increase the overal pd but also creates more peaks
what does a oscilloscope graph look like for AC and DC
AC
- up and down with large curves over the x - axis
DC
- smaller curves up and down ABOVE the x - axis
step up transformers VS step down tranformers
STEP UP
- increase the PD
- have more turns in secondary coil
STEP DOWN
- decrease the pd
- more turns in primary coil
link loud speaks and microphones
- loudspeakers - take electrical signals and use MOTOR effect to convert them to vibrations generating sound waves
- microphones - detect vibrations from sound waves and use GENERATOR effect to convert them to electrical signals
how do loud speakers work
- ac flows through wire creates mag field
- mag field of coil interacts w mag field of permanent mag
- interacting meg fields exert a force (motor effect) - move back and forth
- causes cone to change shape - vibrations
- vibrations in air - sound waves
how do microphones work
- sound waves hit diaphram
- diaphram and coil of wire move bc of this
- wire moves in mag field of permanent mag - current produced (generator effect)
- freq and amplitude of sound waves determines the fre and amp of current
how to increase the size of the induced pd in the GENERATOR EFFECT
- change strength of mag field (stronger field - larger pd)
- more wire faster (larger pd)
- shape wire into coil (more turns larger pd)
