magnetic feilds (everything - detailed) Flashcards
what is a magnetic feild
a region where force is exerted on magnetic meterial
what is another word for magnetic field lines
flux lines
field lines go form _____ to _____ pole
north to south
a _____ ____ is generated by a _____ _____ ___
what rule do you use to find the direction of these flux lines
magnetic field, current carrying wire
fleming right hand rule
explain why there is a resultnent force when a current carrying wire is incedent perpendicular between to 2 fixed magnets
- because the current carring wire generats a magnetic field perpendicular to the direction of the conventinal current
- this magnetic field is then parrelel to the magnetic field generated by the fixed magents
- the magnetic field then adds where the flux lines are going in the same direction and minus were they are going in opposing directions
- this then means there is a greater magnetic flux density above or below the wire depending on the direction of conventinal current
- this means there is a resultent force ‘pushing’ the wire down or up depending on the direction of conventinal current
(note the reletive high magnetic flux density region also pushes the opposit way as well)
if conventinal current is parallel to field lines of a fixed magnet what is the size of the force generated
why…
0N
because there is no component of the magnetic field perpendicular to the current
force in a current carrying wire is preportinal to…
F∝
magnetic flux density
F∝B
magnetic flux density (B) is defined as …
therefore one of B si units are…
the other is… wich is…
the force on 1m of wire carrying a current 1A perpendicular to the magnetic field
Nm^-1A^-1
Tesla(T) = WB/m^2 (as B=Φ/A)
when current and flux lines are perpendicuar what is the equation for F
if they are not perpendicular then the equation for F is…
F=BIL
F=BILcos(θ)
where θ is the angle bwetween the normal to the wire and the magnetic flux lines
what is L in BIL equation
the lenth of wire that is cutting the flux lines (lenth if wire in the magnetic field)
why does a current carring wire experince a force when in a magnetic field (moleculer explenation)
- there are charged particals (e-) moving within a current carring wire
- since the current carrying wire is in a magnetic field the charged particals are also moving within a magnetic field
- this induces a force on the charged particals (e-)
- this force is then by extension experienced by the wire as a whole
what is quation for F experinced by a partical in a magnetic feild…
derive this equation
F=BQV
F=BIL
I=Q/t, s=d/t d=sxt l=vxt
F=BQ/tvt
F=BQv
charged paricals in a magnetic feild are deflected in a ______ path
circular
force on a moving charged partical in a magnetic field is always _______ to its direction of motion
perpendicular
a charged partical moving in magnetic field travel with ______ motion
this is because…
circular
- the magnetic force acting on the partical is perpendicular to its direction of travel
- this magnetic force acts as cetripetal acceleration
- thereofre causing circular motion
what is the equation for the radius of the circle a moving charged partical creats in a magnetic field
derive this equation
r=mv/BQ
circular motion equation = F=mv^2/r
F equation for a moving charged partical in a magnetic feild = F=BQv
F=F
mv^2/r=BQv
1/r=BQv/mv^2
1/r=BQ/mv
r=mv/BQ
when is F=BQv true
(not sure about this one)
when the partical is moving perpendicular to the flux lines (magnetic feild)
explain how a cyclitron works
see pg 143 for diagram
- charged particals are fired into one of the electrodes from the center of the cyclitron
- the magnetic field within the electrode induces a magnetic force that acts a centripetal acceeration thereofre making the particals follow a semi circular path
- as the charged particals leave the electrode an applied p.d between the electrodes accelerates the charged partical invreseasing its velocity
- because the particals speed is slightly high it follows a circular path with a slightly higher raduis within the other electrode
- as the chrages partical leaves the elecrode again the p.d is reversed accelerating the charged partical again before entering the next electrode
- this process repeats as the charged partical spirals outwards, increaseing in speed, before eventually exiting the cyclitron
in a cyclitron T is…
constent
what is magnetic flux
magnetic flux equarion
the total number of magnetic flux lines (magnetic field lines) passing through defined region of space (area) (magnetic flux density x area)
magnetic flux = magnetic flux density x area
Φ=BA
when is emf induced (assuming there is a changeing magnetic field)
when conductors cut magnetic flux lines
in electromagnetic induction:
if there is a _____ ______ between a ________ ____ and a magnetic field, the electrons in the rod will experince a ____. wich cause them to…
what does this do…
reletive motion, conducting rod, force
accumilate at one end of the rod
this induces an emf across the ends of the rod as there is a p.d across the rod
emf is induced in a coil if… (think soliniod)
if this coil is part of a complete circiut…
if magnetic flux (magnetc field) that passes through the coil changes
induced current will flow (AC)
the more turns in a coild means…
bigger emf will be induced
magnetic flux linkage equation
magnetic flux linkage equation if the loop of wire is not perpendicular to the flux lines
NΦ=BAN
NΦ=BANcos(θ)
a change in flux linkage of 1 Wbs^-1 will induce an emf of ____ in a loop wire
1v
give faradays law
give the equaition
therefore
draw NΦ/t graph
what is the gradient of this graph
draw ε/t graph
what is the area
the induced emf is directly preportinal to the rate of change of flux linkage
ε=ΔNΦ/Δt
see pg 147
gradient = ε
see pg 147
area = ΔNΦ
ε∝…
ε∝rate of charnge of flux linkage
ε∝ΔNΦ
what is lenzes law
this is in agreement with the…
the induced emf is always in such a direction as to oppose the change that causes it
principle of conservation of enegy
when trying to find the direction of current in a moving rod within a magnetic feild what is the force (the thimb) when using fleming left hand rule
the force (thumb) is the resistence force porduced by the iduced emf
thereofre the thumb force is in the opposit direction to the motion of the rod within the magnetic field
what is the equation that combines lenses law and faradyas law
ε=-ΔNΦ/Δt
what is a alternator
a generator of alternating current (AC)
what is another word for generators
dynamos
in an alternator the V and I change direction every…
half rotation
when dealing with alternators:
give the flux linkage equation
give the induced ε equation
give a extra equation for ε (this one is not on the silibis but can help)
NΦ=BANcos(θ)
but we are dealing with circular motion of the alternator so
ω=θ/t
θ=ωt
NΦ=BANcos(ωt)
ε=BANωsin(ωt)
(ε = rate of change of BANcos(ωt)) so i assume the extra ω came from diffrentiating BANcos(ωt))
ε=BLv
as
ΔΦ=ΔBA
A=lenth x distentce
distence =vt
ΔΦ=ΔBLvt
ε=NΔBLvt/Δt
ε=NBLv
when dealing with alternators:
draw the graphs for NΦ (flux linkage) over t
drwa the graph for induced ε
what degrees is ε out of phase with NΦ
see pg 147
90 degrees
oscilloscope graphs:
draw a graph of AC with time peried
draw a graph of AC withought time base
draw a graph of dc with time base
see pg 148
will an ac power supply of 2V be 2V all of the time
what has a higher power supply 2V ac or 2V dc
no 2V would be the max voltage
the voltage will follow a sin graph with a max and min V of 2V
2V dc as it is contently at 2V
rms equations
thereofre avarge power of an ac supply will be
Vrms=V(o)/root2
V(0) is max voltage
Irms=I(0)/root2
I(o) is max current
avarge power = Irms x Vrms
what is the UK mains Vrms electricity supply
what is the max V(o)
Vrms=230V
V(o) = 330V
what is the uk main frequenncy
50Hz
transformers make use of ________ ________ to change the size of the ________ for an ________ ______.
electromagnetic induction, voltage, alternating current
how does a tranformer work (simplistic version)
- an alternating current flowing in the primary coil produces a changing magnetic flux (acting as a solinoid)
- this changing magnetic flux (field) is then passed through the iron core of the transformer to the secondary coil
- the secondary coil then experinces the changing magnetic flux linkage inducing an alternating voltage of the same frequency as the input voltage
(the magnetic flux is changing at the primary coil as the current is changing direction (AC))
when dealing with transformers:
give the induced ε equation for the primary and seconday coils
therefore combine these 2 equations and the 100% power efficiency ideal equation to give the equation for ideal transformers
V(p)=N(p)ΔΦ/Δt
V(s)=N(s)ΔΦ/Δt
N(s)/N(p) = V(s)/V(P) = I(s)/I(p)
step-up transformer ______ the voltage by having _____ _____ on the secodary coil then the primary coil.
step-down transformer ______ the voltage by having _______ _____ on the secondary coil.
increases, more turns
decreases, less turns
(more turns = high Voltage)
what are the main ways a transformer losses power
- eddy currents in the transformer core (produces heat in the iron core)
- resistence in the coils (produces heat)
what are the ways to prevent power loss in trensformers for:
eddy currents
resistence in the wires
reduce the effect of eddy currents by laminating the iron core with layers of insolation
use thick wiers with low resistnce
what are eddy currents
- looping currents induced by the changing magnetic flux in the core. they create a magnetic field that acts against the field that produces them, reducing the overall field strenth.
- they also dissipate enegy by generating heat.
give power loss equation
P (power loss) = I^2 x R
what is roughly the transmission voltage used in UK
400KV
emam question:
define Tesla (1mark)
1 Telsa is the magnetic flux density (B) that produces 1N of foce on 1m if wire carrying 1A of current
