Electromagnetism Flashcards
what is magnetism caused by?
movement of electric charges
what is a stationary charge surrounded by
an electric field
what is a moving charge surrounded by
a magnetic field
what is the cause of magnetism in a permanent magnet
electrons orbiting and spinning around the nucleus
what does the medium surrounding the moving charge have an influence on?
has an influence on the effects of the magnetism
what is the magnetism caused by in a non-magnetic object
the different electrons in each atom cancel each other out
does the cancellation happen in a permanent magnet?
no
what forms magnetic domains
magnetic fields due to moving electrons align themselves
what happens when the magnetic domains are aligned?
you have a permanent magnet
poles in magnets
permanent magnets have a north and south pole
if a permanent magnet is broken in two
each piece has a north and south pole
earths magnetic field is partially due to
currents circulating in molten lava within the earth
what does the earths magnetic field resemble
as if there was a large bar magnet within the middle of the earth
do the earths magnetic poles vary
they vary from time to time
how to plot the magnetic field of a bar magnet
place it on a piece of paper and sprinkle iron filings around it. tap the paper gently and the iron filings align themselves
what is a solenoid
a coil of wire used as an electromagnet
describe the apparatus used to plot the magnetic field of a solenoid
it consists of a solenoid with a piece of plastic passing midway through it
how to obtain the magnetic field of a solenoid
sprinkle iron filings on the sheet of plastic and tap gently
looking end on, a solenoid, current flowing clockwise
south pole
looking end on, a solenoid, current flowing anti clockwise
north pole
describe set up of apparatus when plotting the magnetic field of a straight wire
a straight conducting wire is passed through a piece of paper held horizontally
how do you find the magnetic field of a straight wire
sprinkle iron filings on the piece of paper and tap gently, they will arrange themselves in the magnetic field
how to find the direction of magnetic field lines of a straight wire
the right hand thumb rule
describe the right hand thumb rule
when you hold the wire in your right hand, the conventional current is flowing in the direction of your thumb, then the electric field is flowing in the direction of your fingers
how do you show that a current carrying conductor in a magnetic field experiences a force
put a wire connected to a switch and battery inside a folded magnet. close the switch. if the force it experiences is big enough, it will move.
the magnitude of a magnetic force equation
F = I l B sinθ
I (letter i)
Current
l (letter L)
length of a conductor in a magnetic field
B
magnetic flux density
θ
angle between direction of current and magnetic field
if there is no angle given and you want to find the magnetic force
F = I l B
Definition of magnetic flux density
the force acting on 1 m of a conductor carrying 1 A of current, perpendicular to the direction of the magnetic field
symbol of magnetic flux density
B
unit of magnetic flux density
T
tesla
is magnetic flux density a vector or scalar quantity
scalar
how is the force obtained
flemings left hand rule
flemings left hand rule is the principle of what
main principle behind electric motors, loudspeakers and electric meters (analog)
force is perpendicular to what? (2)
force is perpendicular to the direction of the magnetic field and perpendiculoar to the direction of motion of a charged particle
what does the fact that “force is perpendicular to the direction of the magnetic field and perpendiculoar to the direction of motion of a charged particle” cause?
circular motion
wht is perpendicular in circular motion?
force and velocity
force on a charged particle moving in a magnetic field equation
F = q v B sinθ
F
force
q
charge
v
velocity
B
magnetic flux density
θ
angle between direction of moving charge and direction of magnetic field
how can two parallel conductors with current flowing the the same direction, attract each other?
the magnetic fields cancel due to the fact that they are opposite in this region. weak magnetic field between them, strong magnetic fields on the other sides. they get pushed into the weak magnetic field. They seem like they attract
how can two parallel conductors with current flowing the opposite direction, repel each other?
the magnetic fields add due to the fact that they are the same in this region. strong magnetic field between them, weak magnetic fields on the other sides. they get pushed into the weak magnetic fields. They seem like they repel
2 equations for magnetic force
F = I l B F = q v B
if a magnetic force causes circular movemet, what do we do in maths questions?
let magnetic force equal centripetal force
magnetic force
q v B
centripetal force
r
set up to show how a motor works
iron cylinder in the middle
bent magnet around it
wire with current flowing in it around top and bottom of cylinder
what happens when you close the switch on a motor
left hand side of the coil is forced inwards and right hand side of the coil is forced outwards
if the magnetic field is uniform for a motor
then the two forces are equal in magnitude, opposite in direction and do not have a common line of action
ie. a couple
what happens when you move a coil relative to a magnet
current starts to flow
what causes the current to flow when you move a coil relative to a magnet
the change in magnetic flux passing through the coil made a voltage which caused the current to flow
how does alternating current work with a lamp?
current flow from terminal A of the generator, through the lamp and to terminal B of the generator and then from terminal B to terminal A. it switches 50 times a second
explain the graphs of both voltage and current over time with ac
the magnitude of both current and voltage starts at zero, rises rapidly to a peak value, decreases rapidly to zero. then the direction changes and magnitude of current and voltage start at zero, rise rapidly to a peak value, decrease rapidly to zero
how long does the ac cycle usually take
0.02 seconds
equation for root mean square voltage of ac
Vo
Vrms = —-
√2
equation for root mean square current of ac
Io
Irms = —-
√2
Vrms
root mean sqaure voltage
Vo
peak voltage
Io
peak current
Irms
room mean square current
what is the primary circuit of a conductor connect to?
an ac source which give a changing magnetic flux in the iron frame
what’s happening to the north and south poles on both sides of the iron fram
they are switching places at the same frequency as the a.c source
what does the changing magnetic flux cause
it causes an emf in the secondary circuit
what happens in the second circuit?
the bulb lights/ammeter detects current
what is the value of the voltage of the secondary circuit compared to the first?
it could be smaller, the same or larger
what does the voltage of the secondary circuit depend on?
the ration of number of turns of wire in the two circuits
step up transformer (turns in wires)
larger number of turns in the secondary circuit
step down transformer (turns in wires)
smaller number of turns in the secondary circuit
to obey the law of conservation of energy, a larger voltage in the secondary circuit results in?
an smaller current in the secondary circuit
to obey the law of conservation of energy, a smaller voltage in the secondary circuit results in?
an larger current in the secondary circuit
Vi
—- = (2)
Vo
Np Io
—– = —–
Ns Ii
Vi
input voltage
Vo
output voltage
Np
number of turns in pri
Ii
input current
Io
output current
2 energy losses in transformer
laminated iron slices get loose and vibrate and generate the sound of a hum
ohmic resistance in the coils of a wire
1 use of step down transformers
mobile phone rechargers
1 use of step up transformers
older TV sets
2 reasons why ESB uses tranformers
- energy transmitted at very high voltages and low currents results in reduction in heat energy loss
- ac supply very versatile because you can change the voltage of the ac
