23 - Magnetic Fields Flashcards
ways to draw magnet’s magnetic field
dump iron filings or use a plotting compass with a bar magnet
direction of magnetic field lines are always
north to south
direction of earth’s magnetic field
south pole towards north pole (geographically) therefore bar magnet would have north pole and geographic south pole
define magnetic field
the region around a permanent or magnet or a current carrying conductor in which other magnetic objects will experience a force
what is the current carrying conductor responsible for the earth’s magnetic field
the earth’s molten core
where is magnetic field strongest around a magnet and why
at poles due to field lines being closest
what is responsible for creating a magnetic field around a bar magnet
the flowing of free electrons
electromagnetism
when a charges particle moves, it creates a magnetic field so when a current passes through a wire, a magnetic field is produced.
how to find direction of magnetic field around current carrying wire
using right hand grip rule (thumb is current, rest is field direction)
when representing in exam, make sure each field line gets further away from each other from the wire
magnetic field of solenoid is
uniform in the middle
how to find direction of field for solenoid
use right hand grip rule but thumb is field direction and rest are current direction
north and south poles for solenoid
the solenoid produce north and south poles at their opposite faces
how to determine direction of force on current carrying conductor in external magnetic field
Fleming’s left hand rule
force on current carrying wire
F=BILsinϑ
force = magfluxdensity x current x length x sinϑ
ϑ is the angle betwee FIELD and CURRENT
if perpendicular F=BIL
if parallel F=0
what is magnetic flux density
the strength of the magnetic field measured in Tesla T
what is one tesla equal to
the magnetic flux density when a wire of length 1m carrying a current of 1A is perpendicular to the field, experiences a force of 1N
other equations relating to magnetic fields that can be used in qs where factors increase and decrease
R=pL/A and V=IR - V is constant
how are northern lights created
charged particles from the sun spiral down the Earth’s magnetic field to the poles (strongest), colliding with atoms in the atmosphere to emit light
electron flow and flemings left hand rule
electron flow is opposite to conventional current - important in flemings left hand rule
force on electric charge in magnetic field
F=Bqvsinϑ
force = magfluxdensity = charge x velocity x sinϑ
F=Bqv if perpendicular
motion of elec charge in magnetic field
circular motion
radius of circular motion equation and derived
F=mv2/r and F=BQv
cancel out v so
r=mv/BQ
electron motion around field lines - radius of circular motion
electrons spiral around field lines due to constant component velocity in direction of field lines
velocity selector
a device that uses both electric and magnetic fields to select charged particles of specific velocity
Forcemagnetic = Forceelectric
BQV=EQ
so V=E/B
when to use FLHR
motors
when to use FRHR
generators
when wanting to find direction of current, opposite to LHR due to lenz’s law.
how is current induced in a wire
when a wire moves relative to a magnetic field a current is induced due to the cutting of magnetic field lines. the amount of cutting is quantised as magnetic flux
other way to induce emf
moving magnet through coil of wire
define magnetic flux
the product of the component of the magnetic flux density perpendicular to the cross-sectional area
mag flux equation
ɸ=BAcosϑ
flux = flux density x cross sectional area
ϑ - angle measured to normal of surface
ɸ=BA if perpendicular
unit of magnetic flux
Weber (Wb)
define one weber
the magnetic flux when a field of magnetic flux density 1T passes at right angles through a coil with area of one metre squared
things to remember about ɸ=BA
ɸ unchanged if loop raised as field is uniform
A is the area only in contact with the field
magnetic flux linkage
when there are several coils (motors and generators)
flux linkage = Nɸ
number of turns x magnetic flux
UNITS - Wb turns
moving bar magnet through coil of wire (N pole to ground)
second peak greater and shorter time interval due to acceleration due to gravity
peaks opposite direction due to Lenz’s law
induced emf is 0 when in magnet due to field coil being parallel to flux density
number of coils are direc proportional to induced voltage
define magnetic flux linkage
the product of the number of turns in the coil and the magnetic flux
Faraday’s law
induced emf is directly proportional to the rate of change of magnetic flux linkage
ε∝ ∆(Nɸ) / ∆t
direction of induced emf when magnet moves through coil
direction of induced emf and hence the current changed direction when magnet is pulled away and pushed towards the coil due to lenz’s law
Lenz’s law
the direction of any induced emf or current is always in a direction that opposes the change producing it
conservation of ENERGY
combining Faraday’s and Lenz’s laws
ε = -∆(Nɸ) / ∆t
what is constant for AC generator
ε = -∆(Nɸ) / ∆t
ε = -∆(BANcosϑ) / ∆t
BAN is constant
how is AC current generated
there is magnetic field surrounding the magnet and when the magnet spins, the field lines are cut, causing a change in magnetic flux inducing an emf and current. because the field is rotating, AC is produced, emf induced causes electrons to flow one way then the other
emf max occurs when
Nɸ is 0
emf is 0 when
Nɸ is at maximum
emf relation to flux linkage and time graph
-ve gradient
how transformers work
an AC current causes the core to be magnetised and demagnetised. this rapid change in flux means equally fast change in flux in secondary coil as iron core links flux to secondary coil, inducing AC and emf in secondary coil opposite to direction of flux change causing it
the faster the change in flux…
higher the induced emf
step up and down transformers
up - Vs>Vp
down - Vs<Vp
transformers coil equation
Ns/Np = Vs/Vp = Ip/Is
why is core soft iron
easy to magnetise and demagnetise quickly, also improves efficiency
100% efficiency for a transformer =
output power from Vp = Vs
why is a transformer not 100% effiecient?
heating of P and S coil
eddy currents induced in core
- heating due to current
how to improve efficiency of transformers
using low resistance to make more efficient and reduce heating effect of current
laminate core to minimise eddy currents
voltage for transmission lines - national grid
high to minimise heat loss and reduce heating effect of larger current
