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## Topic 12 Electromagnetic Induction

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### Title: Electromagnetic Induction Author: sporter Last modified by: sporter Created Date: 3/6/2007 6:54:01 PM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Topic 12 Electromagnetic Induction

1
Topic 12 Electromagnetic Induction
2
Electromagnetic induction
• Make a coil using wire. The coil should be wide
enough to easily move a magnet inside

3
Electromagnetic induction
• Put your coil in this circuit. The multimeter
should be on the µA scale.

µA
4
Electromagnetic induction
• MOVE a magnet in and out of the coil. Watch the
meter!

µA
5
Electromagnetic induction
• If a magnet is moved inside a coil an electric
current is induced (produced)

6
Generator/dynamo
• A generator works in this way by rotating a coil
in a magnetic field (or rotating a magnet in a
coil)

7
Motor generator
• If electric energy enters a motor it is changed
into kinetic energy, but if kinetic energy is
inputted (the motor is turned) electric energy is
produced!

8
The Motor Effect
• When a current is placed in a magnetic field it
will experience a force (provided the current is
not parallel to the field). This is called the
motor effect.

9
The Motor Effect
• The direction of the force on a current in a
magnetic field is given by Flemmings left hand
rule.

Thumb Motion
First finger Field direction
Centre finger Conventional Current
10
The Motor Effect
Can you copy this please? WITH DIAGRAM!
• The direction of the force on a current in a
magnetic field is given by Flemmings left hand
rule.

Thumb Motion
First finger Field direction
Centre finger Conventional Current
11
Sample question
• In this example, which way will the wire be
pushed? (red is north on the magnets)

12
Sample question
• In this example, which way will the wire be
pushed? (red is north on the magnets)

Current
Field
13
IB Level!
14
Electromagnetic Induction
• Imagine a wire moving with velocity v in a
magnetic field B out of the page.

Wire moving with velocity v
L
v
Region of magnetic field B out of page
15
• The electrons in the wire feel a force (the
motor effect) which pushes the electrons to the
right. This creates a potential difference in the
wire.

Electrons pushed this way (left hand rule)
L
v
16
• The field in the wire that produces this
potential difference is given by E V/L

e.m.f. (voltage) across the wire in the magnetic
field
L

-
v
17
• The force produced by this field E V/L would
push the electrons back again, but this is
opposed by the force on the electrons due to the
magnetic filed F Bev

L

-
v
18
• There exists a balance between the force on the
electrons due to the field in the wire and the
force due to the field
• eE Bev

L
v
19
• eE Bev
• since E V/L, V vBL

L
v
20
• V vBL
• This means that a conducting wire of length L
moving with speed v normally to a magnetic field
B will have a e.m.f. of vBL across its ends. This
is called a motional e.m.f.

Wire moving with velocity v
L
v
Region of magnetic field B out of page
21
• My hero!

22
• Consider a magnet moving through a rectangular
plane coil of wire.

N
S
23
• A current is produced in the wire only when the
magnet is moving.

N
S
24
• The faster the magnet moves, the bigger the
current.

N
S
25
• The stronger the magnet, the bigger the current.

26
• The more turns on the coil (same area), the
bigger the current.

N
S
27
• The bigger the area of the coil, the bigger the
current.

N
S
28
• If the movement is not perpendicular, the
current is less.

29
Magnetic Flux (?)
• Imagine a loop of (plane) wire in a region where
the magnetic filed (B) is constant.

B
30
• The magnetic flux (?) is defined as ? BAcos?
where A is the area of the loop and ? is the
angle between the magnetic field direction and
the direction normal (perpendicular) to the plane
of the coil.

B
31
• If the loop has N turns, the flux is given by
• ? NBAcos? in which case we call this the flux

B
The unit of flux is the Weber (Wb) ( 1 Tm2)
32
• It can help to imagine the flux as the number of
lines of magnetic field going through the area of
the coil. We can increase the flux with a larger
area, larger field, and keeping the loop
perpendicular to the field.

B
33
I built the first electric motor and generator
too. I refused all prizes and awards because that
would detract from Gods glory.
• As we seen, an e.m.f. is only induced when the
field is changing. The induced e.m.f. is found
using Faradays law, which uses the idea of flux.

34

The induced e.m.f. is equal to the (negative)
rate of change of magnetic flux, E -??/?t
35
Example question
• The magnetic field through a single loop of area
0.2 m2 is changing at a rate of 4 t.s-1. What is
the induced e.m.f?
• Physics for the IB Diploma K.A.Tsokos
(Cambridge University Press)

36
Example question
• The magnetic field (perpendicular) through a
single loop of area 0.2 m2 is changing at a rate
of 4 t.s-1. What is the induced e.m.f?
• ? BAcos? BA
• E ?? ?BA 4 x 0.2 0.8 V
• ?t ?t

37
Another example question!
• There is a uniform magnetic filed B 0.40 T out
of the page. A rod of length L 0.20 m is placed
on a railing and pushed to the right at a
constant speed of v 0.60 m.s-1. What is the
e.m.f. induced in the loop?

v
L
38
• The area of the loop is decreasing, so the flux
(BAcos?) must be changing. In time ?t the rod
will move a distance v?t, so the area will
decrease by an area of Lv?t

v
L
Lv?t
39
• E ?? B?A BLv?t BLv
• ?t ?t ?t
• E 0.40 x 0.20 x 0.60 48 mV

An important result, you may be asked to do this!
v
L
Lv?t
40
Lenzs Law
• The induced current will be in such a direction
as to oppose the change in magnetic flux that
created the current
• (If you think about it, this has to be so.)

41
Alternating current
• A coil rotating in a magnetic field will produce
an e.m.f.

N
S
42
Alternating current
• The e.m.f. produced is sinusoidal (for constant
rotation)

e.m.f. V
43
Slip ring commutator
• To use this e.m.f. to produce a current the coil
must be connected to an external circuit using a
split-ring commutator.

Slip-rings
lamp
44
Increasing the generator frequency?
e.m.f. V
45
Root mean square voltage and current
• It is useful to define an average current and
voltage when talking about an a.c. supply.
Unfortunately the average voltage and current is
zero!
• To help us we use the idea of root mean square
voltage and current.

46
Root mean square voltage

e.m.f. V
47
Root mean square voltage
• First we square the voltage to get a quantity
that is positive during a whole cycle.

e.m.f. V
48
Root mean square voltage
• Then we find the average of this positive
quantity

e.m.f. V
49
Root mean square voltage
• We then find the square root of this quantity.

e.m.f. V
50
Root mean square voltage
• We then find the square root of this quantity.

e.m.f. V
This value is called the root mean square voltage
51
Root mean square voltage
• We then find the square root of this quantity.

e.m.f. V
Emax
Erms Emax/v2
52
Transformers
• What can you remember about transformers from
last year?

53
Transformers
Np turns
Vp
Vs
Ns turns
Primary coil
Secondary coil
Iron core
Laminated
54
Transformers
• How do they work?

Np turns
Vp
Vs
Ns turns
Primary coil
Secondary coil
Iron core
55
• An alternating current in the primary coil
produces a changing magnetic field in the iron
core.

Np turns
Vp
Vs
Ns turns
Primary coil
Secondary coil
Iron core
56
• The changing magnetic field in the iron core
induces a current in the secondary coil.

Np turns
Vp
Vs
Ns turns
Primary coil
Secondary coil
Iron core
57
• It can be shown using Faradays law that
• Vp/Vs Np/Ns and VpIp VsIs

Np turns
Vp
Vs
Ns turns
Primary coil
Secondary coil
Iron core
58
Power transmission
• When current passes through a wire, the power
dissipated (lost as heat) is equal to
• P VI across the wire
• Since V IR
• Power dissipated I2R

59
Power transmission
• Power dissipated I2R
• Since the loss of power depends on the square of
the current, when transmitting energy over large
distances it is important to keep the current as
low as possible.
• However, to transmit large quantities of energy
we therefore must have a very high voltage.

60
Power transmission
• Electricity is thus transmitted at very high
voltages using step up transformers and then step
down transformers.

220 V
250,000 V
15,000 V
15,000 V
61
Dangerous?
62
Dangerous?
• Low-frequency electromagnetic fields can induce
currents in the human body!

63
Dangerous?
• Current evidence suggests that low-frequency
fields do not harm genetic material. This is not
fully proven or understood.

64
Whew! Thats it!