Transcript Electromagnetic induction

Electromagnetic induction
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.
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?
Flux - The rate of flow of energy through
a given surface
• flux density B
(The strength of your magnetic field)
• magnetic flux, F.
F = BA (A = Area)
• Flux Linkage, N F
– (N = number of
turns)
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 dF/dt
Fleming's Right hand rule
p133
Kinetic energy recovery
systems
Toyota
• http://www.youtube.com/watch?v=evZC8fVrP4
F1
• http://www.youtube.com/watch?v=09kn
BT2gqqU
Inducing an Emf (no current yet)
•
Connect the coil of wire to the microvoltmeter 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?
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.
v = velocity
V = Voltage
E = Electric field
B = 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 = BLv
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
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
=BxA/t
B x A can be called the magnetic
flux, F.
Thus induced emf = F / t
= rate of change of magnetic flux
And more generally
emf = d F/ dt
So how can you increase the induced
voltage?
L
Flux Linkage (N F)
• 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 F /dt
Sketching Flux Patterns
S
N
S
S
N
–
+
N