Download EM Induction 2

Faraday’s Transformer
Experiment 1831
Faraday expected to see a steady current in
the galvanometer circuit.
When is a voltage or current induced?
What would make the induced voltage
or emf larger?
Magnetic flux density, (or magnetic field strength), B is a
measure of the strength of a magnetic field per unit area and
is measured in teslas, T.
Magnetic flux, φ can be thought of as the total number of
field lines, so φ = B x A (Data Sheet)
It is measured in webers, Wb
When you move a wire or coil at right angles through a
magnetic flux the emf depends on the size of the flux being
cut through and the number of turns. B x A x N, which is
called the Flux Linkage, (Ф)
= BAN = Nφ (Data Sheet)
Also measured in Wb
When the coil (or magnet) is turned through 180
the flux linkage = -BAN
When the movement is parallel to the field lines,
ie. Along them, rather than through them, the
flux linkage = 0
When the magnetic flux is at an angle θ to the
(normal of the) plane of the coil the flux
linkage = BAN Cosθ
The magnetic flux φ = BACosθ
Where is the to the plane of the coil.
Electromagnetic Induction only occurs
when there is a change of flux
This can be achieved by:
Changing the flux itself
Moving the conductor through the flux lines
The larger the change of flux and the faster the rate
of change, the bigger the emf – the emf is
proportional to the rate of cutting through flux
lines. ɛ = NΔΦ/Δt (Data Sheet)
This is Faraday’s Law
Combining Faraday’s and Lenz’s laws gives
ɛ = -NΔΦ/Δt
• On a graph of magnetic flux, (BA) against time,
the size of the emf is shown by the gradient.
• On a graph of emf against time, the area
under the line shows the flux change.
Faraday’s Law of EM induction states that an
emf is induced when flux changes and is equal
to the rate of change of flux.
Lenz’s law states that the direction of the
induced emf is such that it opposes the change
of flux. According to the laws of energy
conservation, the emf must try to oppose or
resist whatever flux change is causing it.
An emf is induced when there is relative motion
between a conductor and a magnet.
Either can move.
Flux cutting always induces an emf, but a
current is only induced in a complete circuit.
Flux Linking is when an emf is induced by
changing the magnitude or direction of the flux
itself, eg. By AC current.
A change of 1Wb per second will induce an emf
of 1volt in a loop of wire.