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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.

Can you copy this sentence into your books please.
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
Faradays Law
  • My hero!

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

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

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

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

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

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

N
S
28
Faradays Law
  • 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
    linkage.

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
Faradays law (at last!)
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
Faradays law

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!
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