1. Electromagnetic induction

2. Important factors in inducing
currents
• 1. An emf is induced if the coil or the magnet
(or both) move (change in flux).
• 2. The size of the induced emf depends on
the speed of movement.
• 3. The induced emf depends on the strength
of the B field.
• 4. Changing the area inside the magnetic
field
• 5. Increasing the number of turns also
changes the flux linkage, and so induces a
greater emf.

3. What you are going to learn today
• What is magnetic flux, and magnetic flux linkage?
• What must happen to a conductor (or to the magnetic
field in which it’s placed) for electricity to be
generated?
• What factors would cause the induced emf to be
greater?
• What is Lenz’s law and what are the applications of
this law?

4. Flux - The rate of flow of energy through
a given surface
• flux density B
(The strength of your magnetic field)
• magnetic flux, Φ.
Φ = ΒΑ (Α = Αrea)
• Flux Linkage, N Φ
– (N = number of
turns)

5. Lenz’s Law
• Lenz’s Law states that the direction of
the induced current is always such as to
oppose the change that causes the
current.
• To include this idea in our formula, a
minus sign has to be introduced, giving;
• Emf = – N x dΦ/dt

6. Fleming's Right hand rule

7. p133

8. Kinetic energy recovery
systems
Toyota
• http://www.youtube.com/watch?v=evZ-
C8fVrP4
F1
• http://www.youtube.com/watch?
v=09knBT2gqqU

9. Inducing an Emf (no current yet)
• Connect the coil of wire to the micro-
voltmeter and place it close to the
magnet.
• 1. Move the magnet next to the coil.
What happens?
How does it depend on speed and
direction of movement?
• 2 .Move the coil next to the magnet.
What happens?
How does it depend on speed and
direction of movement?
• 3. Gradually unwind the coil in the
magnetic field. What happens?
• 4. Take the coil and crumple it up,
keeping it in the field. What happens?

10. Conductor in a magnetic field
Metal rod, length L in a
magnetic field moving with a
velocity v down the page.
An electron in the rod will
experience a force (= Bev)
that will push it towards the
end Q
The electrons will be pushed
towards end Q leaving end p
more positive
an electric field E builds up until
the force on electrons in the
rod due to this electric field
(= Ee) balances the force due
to the magnetic field.
Ee = Bev so E =Bv
For a rod of length L,
E = V/L and so V/L = Bv
Hence the induced emf = BLvv = velocity E = Electric field
V = Voltage B = Magnetic field

11. Completing the circuit
• The emf will now cause a current to flow in
the external resistor R. This means that a
similar current flows through the rod itself
giving a magnetic force, BIL to the left
• L is now the separation of the two
conductors along which the rod PQ
moves.) An equal and opposite force (to
the right) is needed to keep PQ moving at
a steady speed.
• The work done in moving the rod will equal
the energy dissipated in the resistor.
• In a time t, the rod moves a distance d = v
t
• Work done (FxD) on the rod = BIL v t
• Energy dissipated in R = power x time
= ItV
• giving BIL v t = ItV
• Emf (V) = BvL

12. However! You are increasing the
area inside the magnetic field
Emf (V) = BvL
In one second the area has increased
by Lv (A =Lv)
induced emf =
B x area swept out per second
= B x A / t
B x A can be called the magnetic
flux, Φ.
Thus induced emf = Φ / t
= rate of change of magnetic flux
And more generally
emf = d Φ/ dt
So how can you increase the induced
voltage?
L

13. Flux Linkage (N Φ)
• Increasing the number of turns of wire N in our circuit
increases the emf produced
• induced emf = rate of change of flux linkage
• emf = N x d Φ /dt

14. Sketching Flux Patterns
NS
NS
SN
– +