Electromagnetism

1. 12.1 – Lenz’s Law and Applications

2. Remember this: (from 12.1 – Electromagnetic Induction)
As a conductor moves through a magnetic field......
The current that flows makes Fleming’s Left-Hand
it’s own force on the Rule force created by
the Current
conductor.
You can use Fleming’s Left
Hand Rule to predict it too.
This force is in the opposite
direction to the motion that Force creating
is creating the current! the Motion
If there wasn’t an opposing Force being made then once you
started pushing the Conductor through the field it would
carry on making current for ever with no more force needed.
This would contradict the Law of Conservation of Energy

3. Lenz’s Law
This just says that ‘the direction of the Induced Current
is such that it opposes the change that causes it’.
‘If you push a wire through a field the induced current makes a
force that pushes back’
‘If a field is pointing one way and a conductor moves through
it, then the induced current makes a field that points the
opposite way’

4. Faraday’s and Lenz’s Laws – Example 1
A) Faraday’s Law
As the magnet approaches the coil the
B field increases and the changing
flux enclosed by the coil Induces an
emf in the coil that causes current to
flow.
The size of the emf is equal to the
rate of change of the flux enclosed
by the coil.

5. Faraday’s and Lenz’s Laws – Example 1
B) Lenz’s Law
The direction of the Induced
current is such that it opposes the
change producing it (the magnet
moving towards the coil)
The current in the coil has to make
a magnetic field that pushes the
Magnet away....
The Right hand grip rule shows
how the current must flow.

6. Faraday’s and Lenz’s Laws – Example 2
A) Faraday’s Law
When the current in circuit A flows, a magnetic field is created which causes
the magnetic flux enclosed by the coil in Circuit B to increase.
This increasing flux induces a current in coil B. It only happens for a milli-
second when it is switched on.
After that the flux is constant so there is no induced current.

7. Faraday’s and Lenz’s Laws – Example 2
B) Lenz’s Law
The direction of the current in B must oppose the change producing it (the
increasing field from A).
To do this the current induced in B flows to create a Magnetic field in the
opposite direction to that from A
this is how a transformer works

8. Magnet in Copper Pipe
Faraday’s Law
As the magnet falls through
the copper pipe there is a
changing Flux on the
copper. This induces eddy
currents in the copper pipe.
http://www.youtube.com/watch?feature=player_
embedded&v=dnK6oxbPSM4
Lenz’s Law
The eddy currents flow in
such a way that they create
a magnetic field which
opposes the change that
produced them.
http://www.youtube.com/watch?v=otu-KV3iH_I

9. Induction Braking – Eddy current braking
Faraday’s Law
As the magnet is brought near to
the rotating disc there is a
changing Flux on any point in
the copper. This induces eddy
currents in the copper disc.
Lenz’s Law
The eddy currents flow in sucha http://www.youtube.com/watch?v=SK0EdikjC24&featur
way that they create a magnetic e=related
field which opposes the change
that produced them.
The Railway Gazette
http://www.railwaygazette.com/news/sing
le-view/view/eddy-current-braking-a-long-
road-to-success.html http://demonstrations.wolfram.com/MagneticBraking/
You might need the Wolfram plugin for this to work

10. Induction Braking – Eddy current braking
Foucault’s Disc – a simulation
http://www.magnet.fsu.edu/education/tutorials/java/foucaultdisk/index.html

11. Quantum Levitation & Flux Pinning
Superconducting Disc
The upper disc is a
superconductor in which an
Induced Current is created by
the changing flux as it is
brought near to the lower
magnet. This induced current
flows constantly because the
superconductor has zero
resistance
http://www.youtube.com/watch?v=Ws6AAhTw7RA
Lenz’s Law
The induced currents flow in Explanation
such a way that they create a
magnetic field which opposes
the change that produced
them. http://www.divshare.com/download/17914105-09c
Flux Pinning http://en.wikipedia.org/wiki/Flux_pinning