magnetic fields Flashcards
what way do the field lines go
north to south
if the field lines are closer what does this mean
the field is stronger
what happens when current flows in a wire or any straight conductor
a magnetic field is induced around the wire
what shape are the field lines around a wire
concentric circles
what is the motor effect
When a current passes along the wire in a magnetic field, a force is exerted on the wire
when is the force greatest during motor effect
*when wire is at a right angle to the magnetic field
*zero when the wire is parallel to the magnetic field
what are concentric circles
circles with the same centre but different radii
how do you work out the direction of the field lines around a current carrying wire
the right hand rule
when using the right hand rule what direction does your thumb point
in the direction of conventional current
if you loop a wire into a coil what shape is the field
doughnut shaped
if you get a solenoid what shape is the field
has a field like a bar magnet
what is a solenoid
coil with a length
if you put a current carrying wire into a magnetic field what happens
external field and the field from the wire are added together resulting in a resultant field causing a force on the wire
the resultant force experienced from the fields is always in what direction to the magnetic field and current
perpendicular to the direction of the magnetic field and the direction of current
if the current is parallel to the field lines the size of the force is
0N
in Flemings left hand rule what direction do the fingers point
thumb points in the direction of the force first finger points in direction magnetic field and second finger points in direction of current
magnetic flux definition
the force on one metre of wire carrying current of one amp at right angles to the magnetic field
what is the force on a wire
F=BIL
1 Tesla =
Wb/m²
what equation do you use to work out the force acting on a wire when it is at an angle to the magnetic field
F=BILsinθ
what is the force acting on a charged particle
BQv
B = magnetic flux density in T,
Q = charge on the particle in C,
v = velocity of the particle in ms–1
what path do charged particle in a magnetic field make
Charged Particles in a Magnetic Field are Deflected in a Circular Path
why do charged particles deflect into a circular path
Fleming’s left-hand rule says that the force on a moving charge in a magnetic field is always perpendicular to its direction of travel.
how do you work out the radius of a charged particle moving in a circular path
r=mv/BQ
where are circular deflections used
cyclotrons
what are cyclotrons used for
Cyclotrons have many uses, for example in medicine. Cyclotrons are used to produce radioactive tracers or high-energy beams of radiation for use in radiotherapy.
what is a cyclotron made of
A cyclotron is made up of two hollow semicircular electrodes
what qualities do cyclotrons have
a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes:
how does the cyclotron work
Charged particles are fired into one of the electrodes. The magnetic field makes them follow a (semi)circular path and then leave the electrode. An applied potential difference between the electrodes accelerates the particles across the gap until they enter the next electrode. Because the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again. The potential difference is reversed so the particle is accelerated again before entering the next electrode. This process repeats as the particle spirals outwards, increasing in speed, before eventually exiting the cyclotron.
what is magnetic flux density a measure of
Magnetic flux density, B, is a measure of the strength of a magnetic field. It helps to think of it as the number of field lines per unit area.
what is magnetic flux
total number of field lines
how do you work out magnetic flux
Φ = BA
when are electromotive forces induced in conductors
Electromotive Forces are Induced in Conductors when they move through Magnetic Field
If there is relative motion between a conducting rod and a magnetic field what happens
If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at one end of the rod.
what is electromotive induction
If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force, which causes them to accumulate at one end of the rod This induces an electromotive force (e.m.f.) across the ends of the rod
how can you induce an e.m.f. in a flat coil or solenoid
- moving the coil towards or away from the poles of a magnet.
- moving a magnet towards or away from the coil.
what is needed for emf to be induced
magnetic field or magnetic flux that passes through the coil has to change
what happens if a coil is part of a circuit and emf is induced
an induced current will flow through it
what does more turns mean for the induced emf
More Turns in a Coil of Wire Mean a Bigger e.m.f. will be Induced
what is the flux linkage equation
NΦ = BAN
what does the rate of change in flux linkage tell you
The rate of change in flux linkage tells you how strong the electromotive force will be in volts
A change in flux linkage of one weber per second will
A change in flux linkage of one weber per second will
induce an electromotive force of 1 volt in a loop of wire.
what is faradays law
The induced e.m.f. is directly proportional to the rate of change of flux linkage.
faradays law
ε = flux linkage change/time taken = NΦ/t
for a graph of NΦ against time what is the gradient
the induced emf
for a graph of emf against time what is the area under the graph
the flux linkage change
what is Lenz law
the induced emf is always in such a direction as to oppose the change that caused it
Lenz law equation
emf= -flux linkage change/ time taken = -NΦ/t
how does the idea that a induced e.m.f. ina conductor will oppose the change that caused it, agree with the principle of the conservation of energy
the energy used to pull a conductor through a magnetic field, against the resistance caused by magnetic attraction, is what produces the induced current.
what is a alternator
An Alternator is a Generator of Alternating Current
what do generators do
Generators, or dynamos, convert kinetic energy into electrical energy —
they induce an electric current by rotating a coil in a magnetic field.
how many degrees are flux linkage and induced voltage out of phase
90 degrees
write flux linkage in terms of angular speed
flux linkage = NF = BAN cos wt
write induced emf in term of angular speed and time
e = BAN w sin wt
what does alternating current or voltage change with
time
what can you use oscilloscope for
You can use an oscilloscope to display the voltage of an alternating current
what does the vertical height on oscilloscope show
input voltage at that point
what pattern does alternating current give you on a oscilloscope
An alternating current (ac) source gives a regularly repeating sinusoidal waveform.
what pattern does direct current give you on a oscilloscope
horsizontal line
if you turn the time base on a oscilloscope what pattern does it give you for alternating current
vertical line
if you turn the time base on a oscilloscope what pattern does it give you for direct current
a dot
what is the root mean square voltage useful for
can compare direct and alternating current supply
what is the root mean square voltage equation
vrms= v0/√2
what is root mean square current
irms=I0/√2
how to work out average power for ac supply
irms x vrms
is mains electricity as or dc
ac
what is the uk’s mains electricity supply
230v
what do transformers do
Transformers are devices that make use of electromagnetic induction
to change the size of the voltage for an alternating current.
what does alternating current flowing in the primary coil do in a transformer
produce magnetic flux
when magnetic field is changed because of the first coil what happens next
The changing magnetic field is passed through the iron core to the secondary (or output) coil, where it induces an alternating voltage of the same frequency as the input voltage.
what is the transformer equation for a ideal transformer
Ns/Np = Vs/Vp
what o step up tarnsformers do
Step-up transformers increase the voltage by having more turns on the secondary coil than the primary.
what do step down transformers do
Step-down transformers reduce the voltage by having fewer turns on the secondary coil.
if a transformer was 100% efficient what would be true
power in = power out
why will there be a loss in power in transformers
because of eddy currents in the transformers core
what are eddy currents
Eddy currents are looping currents induced by the changing magnetic flux in the core. They create a magnetic field that acts against the field that induced them, reducing the field strength. They also dissipate energy by generating heat.
how can you reduce the effect of eddy currents
laminating the core with layers of insulation
heat is also generated by resistance in the coils how can you minimise this
Heat is also generated by resistance in the coils — to minimise this,
thick copper wire is used, which has a low resistance.
how do you work out the efficiency
the ratio of power out to power in
IsVs/IpVp
how is electricity from power stations sent round the county
sent round the country by the national grid
why is electricity sent round thew country at the lowest possible current
because the power losses due to the resistance of the cables is equal to P = I²R
— so if you double the transmitted current, you quadruple the power lost.
Transformers allow us to step up the voltage to around 400 000 V for transmission through the national grid and then
and then reduce it again to 230 V for domestic use.
What is a generator
turns kinetic enrgy into electrical energy
What does a alternator contain
A rectangular coil that spins in a uniform magnetic field, slip rings and brushes to connect the coil to an external circuit
What happens when the coil in a alternator spins
The flux linkage changes continuously
In a alternator why is there alternating current
Because the output voltage and current change direction with every half rotation of the coil producing an alternating current
