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Electromagnetic Induction
Magnetic Flux
The flux density of a magnetic field is defined by the
equation:
F=BIl
where F is the force on a conducting wire
I is the current in the conducting wire
l is the length of the conducting wire
The magnetic flux density gives us an idea of how strong the
field is; it is a measurement of how close together the field
lines are.
The unit is the Tesla T
Consider a loop of wire in a uniform magnetic field:
B
Cross- sectional
area A
The magnetic flux linked by a particular loop is given by:
mag. flux density x area of loop
F = BA
( If the flux density is stronger, the lines are closer together and
so more lines, “flux”, are linked.)
If you have N loops of wire then the total flux linked is N times
greater:
B
Cross- sectional
area A
The total magnetic flux linked is B A N = N F
This makes sense when you think of a solenoid; having more
turns makes a stronger magnetic field.
The unit of magnetic flux is the Weber Wb
1 Wb m-2 = 1 T
Electromagnetic Induction
As the coil is moved, an emf is induced in it.
Since the circuit is complete, a current can flow.
Electromagnetic Induction
Reversing:
direction of movement
poles of magnet
connections to ammeter
reverses direction of
current
A straight conductor moving in a magnetic field
N
S
Consider a wire moving in a magnetic field
The electrons in the wire are moving perpendicular to the field,
so there is a force on them given by FLHR.
This force is given by F = B q v
Electrons are forced perpendicular to B and v, i.e. along the
length of the wire
This means an emf is induced in the wire.
N
S
If you think about FLHR, you can see the force pushing the electrons
along the wire gives rise to a conventional current flowing out of the paper.
If you then apply FLHR rule again, this produces a force opposite
to the original movement.
So, emf induced causes a current to flow which opposes
change which caused it.
This is known as Lenz’s Law