Physics - Magnetism Flashcards
how many poles do permanent magnets have
two - north and south
what is the north pole of the magnet attracted to
‘north seeking’
geographical north
what can iron filings be used for
to depict the magnetic field lines
soft magnetic materials characteristics
easily magnetised but also lose their magnetism easily
hard magnetic materials characteristics
difficult to magnetise but once magnetised they are difficult to be demagnetised
what do electromagnets usually consist of
coil wrapped around a soft iron core
why is soft iron used in electromagnets
gains and loses magnetism quickly
- can be turned on and off quickly
inducing magnetism
pole of bar magnet near an unmagnetised magnetic material, there is a force of attraction experienced (never repulsion)
what is induced magnetism used for
making permanent magnets by placing a hard magnetic field in a strong magnetic field as a solenoid
what links electric current and magnetism
as current flows through a wire, it creates a magnetic field in the surrounding area
what does reversing current do to the magnetic field created
reverses magnetic field
what does increasing current do to the magnetic field
increases magnetic strength
what causes the magnetic field in wires
charge carriers
how can a magnetic field be created in a vacuum
beam of charged particles
magnetic field around wire characteristics
consists of concentric circles
circles get further apart and further from wire
have a direction predicted by right hand grip rule
right hand grip rule
thumb in direction of current
fingers curl in direction of magnetic field lines
what happens to the magnetic field when the wire is wrapped in a tight coil
strong field running through cores centre
what is a solenoid
long coil of electrical wire with a magnetic field
narrow coils wound together
magnetic field in a solenoid
resultant = very uniform
field direction can be predicted by right hand grip rule
field is comparable to a bar magnet with poles at each end of solenoid
weak field at sides of solenoid and opposite in direction to the field inside solenoid
field strength and direction controlled by current
what pole is created by coils turning clockwise
north pole
what pole is created by coils turning anticlockwise
south pole
magnetic field strength is dependent on
current
distance
surroundings (magnetic materials surrounding the wire can increase field strength)
iron is ferromagnetic, what does that mean
each iron atom creates a magnetic dipole
what happens to iron atoms when placed in an external magnetic field
atomic dipoles line with this field
how to increase strength of magnetic field of solenoid
increasing number of turns per unit length
using a soft iron core inside coil
increasing current in coil
key differences between permanent magnets and electromagnets
- field shape
- electromagnets can be turned on/off
- electromagnets field strength can be varied whereas a permanent magnet’s field strength is largely constant
- polarity of electromagnet can be reversed by reversing the current whereas a permanent magnet has a constant dipole
how are permanent magnets made
placing a hard magnetic material (iron alloy, neodymium) in a strong external magnetic field
permanent magnets field strength
magnets retain their strength for a long time but are weakened by impact and can be demagnetised by heating above their Curie temperature
why are superconducting coils useful
current can only be increased so much before the wire gets too hot
superconducting coils (zero resistance) which require a very low temperature to function
motor effect
force perpendicular to direction of current and direction of magnetic field
current carrying wire is parallel to the magnetic field, no force is experienced by the wire
current carrying wire is perpendicular to the magnetic field, then the force is at maximum
force occurs as magnetic field created by current carrying wire interacts with magnetic field of the permanent magnet creating a force on both the wire and the magnet
flemings left hand rule
middle = direction of curent
index = direction of magnetic field
thumb = direction of motor force
what happens to motor effect if you reverse current OR magnetic field
motor effect reversed
what happens to motor effect if you reverse current AND magnetic field
motor effect unaffected
what is the strength of the motor effect force determined by
current - increasing the current increases the force
magnetic field - increasing the field strength increases the force
length - increasing length of wire in field increases the force
angle - between current and magnetic field: greatest at 90 degrees and zero when parallel
equation to determine the magnitude of the motor effect force
F = BIL
F = motor effect force in Newtons (N)
B = magnetic field strength in Tesla (T)
I = current in amperes (A)
L = length of wire in metres (M)
what is a DC motor
motor effect used to create a turning effect on a current carrying coil arising from a pair of motor effect forces acting in opposite directions
how does a DC motor work
current carrying rectangular coil is placed in a permanent magnetic field
motor effect forces on either side of the coil can produce a turning effect
maximum force occurs when the coil is in the plane of the field and the current is perpendicular to the field
force is zero when the coil is perpendicular to the field and the current is parallel to the field
what must happen for a DC motor to rotate in the same direction and how is it achieved
current must be reversed every time the coil passes the vertical
using a SPLIT RING COMMUTATOR which rotates with the coil and connects to the DC power supply by two brushes and acts as a rotating switch
split ring commutator
rotates with the cpil in DC motor and connects to the DC power supply by two brushes and acts as a rotating switch
made from brass or coper and with graphite brushes which are low friction but make good electrical contact (brushes need replacing when worn down)
turning effect of coil determined by
- current
- size of coil
- number of turns
- angle of coil in field
- winding the coil onto soft iron core
- magnetic field strength
how can voltage be induced in a conductor
- passing conductor across lines of magnetic field
- magnetic field line changes across wire
doesnt matter whether coil or magnetic field moves, if one crosses the other a voltage is induced in the wire or coil
when does electromagnetic induction result in an induced voltage and induced current
induced voltage = always
induced current = only in closed circuit
magnitude of induced voltage is directly proportional to ..
- rate at which wire cuts magnetic field lines
- rate at which the magnetic field through a conductor changes
how does stronger magnet affect induced voltage
stronger magnet = greater voltage induced
if electromagnet is used to generate a magnetic field, the induced voltage can be increased by
- increasing frequency of AC supply
- increasing amplitude of AC supply
- increasing number of coils on electromagnet
why does induced voltage create a dipole
induced voltage opposes the change that makes it in order to obey the principle of energy conservation
dipole created to repel the magnetic field that created voltage
use this rule to predict the current direction in coil
what can induced voltage be reversed by
- reversing direction of cutting the magnetic field lines
- decreasing magnetic field strength instead of increasing it
AC generators
consists of coil rotating in magnetic field causing coil to continuously cut the magnetic field and induce a voltage
voltage induced alternates as the coil rotates and cuts the magnetic field in the opposite direction, therefore generator produces AC out put
how to increase amplitude of output of AC
- rotate coil quicker
- increase magnetic field strength
- increase area of coil in magnetic field
- increase number of turns of coil in magnetic field
all factors increase number of field lines being cut per unit time
why are generators important
transform mechanical energy n rotating cooil into an electrical output
source of mechanical energy can be from
- chemical/nuclear
- kinetic
why are transformers important
convert AC at one voltage to AC of a different voltage
This is useful as different voltages of electricity have different uses
