Magnetic Fields Flashcards
Magnetic field
A region in which a force acts and is exerted on magnets or magnetically susceptible materials
Field lines
North to south
Closer the lines
The closer the lines the stronger the field
Magnetic field around a wire
Right hand rule
When a current flows in a wire a magnetic field is induced
Stick thumb up in direction of current
Your curled fingers show the direction of the field
Solenoids
If you loop a current carrying wire in one plane the surrounding magnetic field is doughnut shaped
While a coil with length (solenoid) forms a field like a bar magnet
left hand rule
first finger - magnetic field
second finger - current
thumb - force
how to make a wire vibrate
by passing an alternating current through a wire in a magnetic field, the wire can be made to vibrate
as the direction of the force is always perepndicular, when the direction of the crrent alternates, so does the force, from up to down causing the wire to vibrate
magnetic flux density
the force on one meter of wire carrying a current of one amp at right angles to the magnetic filed
measured in telsas
vector
one telsa equal to …
1N per amp per m
1 telsa = 1 N/Am
equation for maximum force a wire could experience
F = BIL
charged particles in a magnetic field
a force acts on charged particles in a magnetic filed
this is why current carrying wire experiences a force in a magnetic filed due to the elctrons
equation for force with charge and velocity
F = BQv
FLHR direction of current for a positive charge
current point direction of motion
FLHR of current for a negative charge
opposite to motion
what is the force indepent of in a magentic filed
force experienced by a particle in a magnetic field is independent to the particles mass although the centripetal acceleration it experiences does depend on the mass
equation for radius
r = mv / BQ
radius curvature increases if…
the mass or velocity of the particle increases
radius curvature decreases if…
strength of magnetic field or if charge on the particle increases
equation for frequency
f = v / 2pi r
r = mv / BQ
f = BQ / 2pi m
so frequency is independent from velocity
what does time taken depend on
frequency is independent from velocity
the time taken for a particle to complete a full circle depend only on magnetic flux density and its mass and charge
increasing the velocity in a magnetic field
make it follow a larger radius but itll take the same amount of time to complete it
cyclotron
made up of 2 hollow semi-circular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes
and alternating p.d. is applied between the electrodes
particle accelerators
charged particles are produced and fired into one of the electrodes, where the magnetic field makes them follow a semi-circular path and then leave the electrode
an applied p.d. between the electrodes then accelerates the particles across the gap until they enter the next electrode
as the speed is slightly higher it will follow a larger radius before leaving the electrode
p.d. will be reversed and it’ll accelerate the particle again before entering the other electrode
this process repeats until the particle exits the cyclotron
total magnetic flux
Φ = BA
only possible if B is normal to A
electromagnetic induction
if there is relative motion between a conducting rod and a magnetic field, the electrons will accumulate at one end of the rod
This induces an emf across the rod
inducing emf in a flat coil or solenoid
happens in the same way
the emf is caused by the magnetic field that passes through the coil changing
if the coil is part of a complete circuit, an induced current will through through it.
magnet moves away from coil
positive
anticlockwise
magnet moves towards the coil
negative
clockwise
flux linkage
when a wire coil is moved in a magnetic field the size of the emf induced depends on the magnetic flux passing through the coil and the number of turns on the coil cutting the flux
flux linkage equation
N Φ = BAN
what does the rate of change in flux linakge tell you
how strong the emf is
a change in flux of one wbber per second will induce
an emf of 1 volt in a loop of wire
faradays law
induced emf is directly proportional to the rate of change of flux linkage
magnitude of emf
rate of change of flux linkage
emf equation
-N △Φ / △t
lenzs law
induced emf is always in such a direction as to oppose the change that caused it
lenzs law and the conservation of energy
the energy used to pull a conductor to pull a conductor through a magnetic field against the resistance caused by the magnetic field against the resistance caused by the magnetic attraction is what produces the induced current
the idea that induced emf opposes the change that caused it agrees with the conservation of energy
lenzs law and motion of a conductor
lenzs law says that induced emf will produce a force that opposes the motion of the conductor (resistance)
induced emf in a rotating coil
when a coil rotate uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced
the amount of flux cut by the coil (flux linkage) is given by
NΦ = BAN cosx
as the coil roates, x changes so it varies sinusoidally between +BAN and -BAN
how fast x changes depends on the angular speed, w, of the coil
x = wt
NΦ = BAN coswt
equation for emf for graph
NΦ = BAN w sinwt
graph for NΦ
cos
thingy perpendicular at +BAN and -BAN
paralell at 0
graph for emf
sin
perpendicular at 0
parallel at +-emf
peak emf equation
2NBlV
BANw
BAN 2pi f
the graph of the induced emf can be altered by
-increasing speed of rotation increasing frequency and increasing max emf
-increasing magnetic field density will increase max emf butll have no effect on frequency
generators
or dynamos, convert kinetic energy into electrical energy
they induce an electric current by rotating a coil in a magnetic field
simple alternator
a generator of alternating current
it has sliprings and brushes to connect to the coil to an external circuit
the output voltage and current and current change direction with every half rotation of the coil producing an alternating coil
alternating current
one that changes with direction with time means the voltage across a resistance goes up and down in a regular pattern
using an oscilloscope to display the alternating current ( and d.c.)
the trace you see is made by an electron beam moving across the screen
you can set this using a dial on the front of the oscilloscope
oscilloscope are basically just voltmeters
the vertical height of the trace at any point shows the input volatge at that point
produces at sinusoidal waveform
d.c = horizontal line
oscilloscopes if you turn off time base
ac voltage = vertical line
dc voltage = dot
three basic pieces of info to get from an oscilloscope
-time period
-peak voltage
-peak-to-peak voltage
root mean square volatge
an ac supply of 2v will be below 2v most of the time, means wont have as high power output as a 2v dc supply
have to average them out, need to sqaure as + and - bit cancel out
Vrms = V0 / root2
root mean current
Irms = I0 / root2
avearge power equation for ac supply
p = VI
Irms x Vrms = V0 / root2 x I0 / root2
uk main electricty supply voltage
230
tranformer
devices that make use of electromagnetic induction to change the size of the voltage for an alternating current
how a transformer work
an ac flowing in the primary coil causes the core to magnetise, demagnetise and remagnetise continuously in opposite directions
this produces a rapidly changing magnetic flux across the core
becuase of this, a magnetically soft material is needed (usually iron)
how a secondary coil works
a rapidly changing magnetic flux in the iron core passes through the secondary coil where it induces an alternating voltage of the same frequency
primary/secondary coil equation
(Vp/s) = (Np/s) x △Φ/△t
Ns / Np = Vs / Vp
step-up transformers
increase the voltage by having more turns on the secondary than the primary
step-down transformer
reduce voltage by having fewer turns on the secondary coil
ineffiency in a transformer
have small loses in power mostly from heat
the metallic core being cut by the continuously changing flux which induced an emf in the core
in a continuous core this causes currents called eddy currents which causes it to heat up and energy to be lost
eddy currents
a looping current induced by the changing magnetic flux in the core of the transformer
how to reduce the effects of eddy currents
laminating the core
involves having layers of the core separated out by thin layers of insulator so a current cant flow
reducing the effect of energy needed for magnetism
using a material easily susceptible to magnetism reducing the energy needed
how to reduce magnetic loss
ideally the magnetic flux created by the primary coil would cut through the secondary coil, but this isnt the case in practice (especially if the coils are far apart)
to reduce this magnetic loss, a core design in which the cores are as close as possible can be used - including winding the coils on top of eachother around the same part of the core rather than round different parts of the core
for an ideal transformer where power in = power out
Ip x Vp = Is x Vs
Vp / Vs = Is / Ip
calculating the effieciency of a transformer
efficiency = Is x Vs / Ip x Vp
transformers on the national grid
electricty from power stations are sent round the country at the lowest possible current because a high current causes greater loss in energy and power loss in cables (P = iiR)
using cables with the lowest possible resistance also reduces energy loss
P = VI so low current = high voltage
volatages for stations for national grid
power station –> step up = 25kV
step up –> plylons / cables = 400kV
cables –> step down –> homes = 230V
investigating relationship between voltage and number of coils
-wrap wire around 2 C-cores
begin with 5 turns in primary and 10 in secondary = 1:2 ratio
-turn on ac supply - use low volatge as transformers increase voltage
-record voltage in each coil
-repeat with different ratios
transformers experiment equation
Ns / Np == Vs / Vp
find relationship betwwen current and voltage of the transformer for a given number of turns in the coil
same set up but add a variable resistor to primary coil and ammeter to both circuits
Ns / Np = Vs / Vp = Is / Ip
