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Transcript
Magnetism and magnetic forces
Current off
coil
S
N
Molecular magnets
aligned randomly
Magnetic field
lines
(Representing
magnetic flux Φ)
Current on
N
coil
Magnetic field
lines
(Representing
magnetic flux Φ)
S
Molecular magnets
aligned North to South
Flux density
Flux
lines
B=Φ÷A
Φ=BxA
Flux density (B) is the
amount of flux (Φ)
(represented by flux
lines) passing
perpendicularly
through a given area
(A)
Force on a conductor
• When a current flows through a conductor in
a magnetic field a force acts on the conductor
The direction of the force depends on the
direction of the magnetic field and the direction
of the current.
These directions can be found using Fleming’s
Left Hand Rule
This is called the motor effect
Direction of current
Cross sections
of conductor
(wire)
Current coming out
of page(like an arrow
coming towards you)
Current going into
page(like an arrow
going away from you)
Force on a conductor
Current coming
out to page
Field Direction
N
S
direction of Force
(movement) up
Force on a conductor
Field Direction
N
Current
going in to
page
S
Force (movement) Down
Electric motor effect
If the conductor is part of a coil with the
current going into the coil on the right and
out on the left, the coil will spin ( as per an
electric motor)
N
S
Force on a conductor
Force = Flux density (B) x current (I) x length of
conductor in magnetic field (L)
F=BxIxL
EMF induced in a conductor
• If a conductor is moved through a magnetic
field an EMF (electro-motive- force) is induced
in the conductor which causes a current to
flow in the direction of the force.
• The directions can be found using Fleming’s
right hand rule
• This is called the generator effect
EMF induced in a conductor
Field Direction
N
S
Current (from
induced EMF)
going into
page
Movement up
through the field
EMF induced in a conductor
Field Direction
N
EMF induced in a conductor
Current (from induced
EMF)
coming out to page
S
Movement down
though field
EMF induced in a conductor
The magnitude of the EMF (hence current)
induced depends on the rate at which the
conductor ‘cuts‘ through the flux lines or ‘the
rate of change in flux:
E = -dФ/dt
The minus sign means that the induced emf opposes
the change in flux (Lenz’s Law)
EMF induced in a conductor
E = -dФ/dt
dФ/dt = dBA/dt (Ф = BA)
= dBLxL/dt ( L x L = A)
E = BLv (v (velocity) = dL/dt
B = flux density L = length of conductor in field v =
velocity of conductor through field