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AP Physics Chapter 20 Electromagnetic Induction

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Examples: pacemakers stopping, migrating birds get lost, GPS won t work, etc. For N loops of wire, Faraday s Law of Induction can be written: = ... – PowerPoint PPT presentation

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Title: AP Physics Chapter 20 Electromagnetic Induction


1
AP Physics Chapter 20 Electromagnetic
Induction
2
Chapter 20 Electromagnetic Induction
  • 20.1 Induced Emfs Faradays Law and Lenzs Law
  • 20.2-4 Omitted

Homework for Chapter 20
  • Read Chapter 20
  • HW 20 p.658-660 3,5,6,8,9,10,11,13,14,18,21-24.

3
20.1 Induced Emfs Faradays Law and Lenzs Law
4
It is more like an electromagnet, because the
movements of the metal liquids create electrical
currents which change in strength and direction.
5
  • As we saw in Chapter 19, electric current
    produces a magnetic field. On the flip side,
    changing magnetic fields can be used to produce
    electric current.

electromagnetic induction - The
process of generating a current through a circuit
due to the relative motion between a wire and a
magnetic field when the wire is moved through the
magnetic field or the magnetic field moves past
the wire.
  • When there is no relative motion between the
    magnet and the loop, the number of field lines
    (in the diagram, 7) through the loop is constant.
  • The galvanometer (measuring current) shows no
    deflection.

6
  • Moving the magnet toward the loop increases the
    number of field lines now passing through the
    loop (now 12).
  • An induced current is detected by the
    galvanometer.
  • Moving the magnet away from the loop decreases
    the number of field lines passing through the
    loop (now 5).
  • The induced current is in the opposite direction
    as indicated by the opposite needle deflection.

7
  • The induced current can also occur if the loop
    is moved toward or away from a stationary magnet.
  • The magnitude of the induced current depends on
    the speed of that relative motion.

Exception When a loop is moved parallel to a
uniform magnetic field, there is no change in the
number of field lines passing through the loop
and no induced current.
8
  • Another way to induce a current in a stationary
    loop of wire is to vary the current in another
    loop close to it.
  • When the switch is closing in the right-loop
    circuit, the current buildup (typically over a
    few milliseconds) produces a changing magnetic
    field that passes through the other loop,
    inducing a current in it.
  • When the switch is opened the magnetic field
    lines through the left-hand loop decreases.
  • The induced current in this loop is then in the
    opposite direction.

9
  • The current induced in a loop is caused by an
    induced electromotive force (emf) due to
    electromagnetic induction.
  • An emf represents energy capable of moving
    charges around a circuit. We previously studied
    batteries as a chemical source of emf.
  • A moving magnet can create an induced emf
    in a stationary loop, which causes
    current.
  • In the case of two stationary loops, where a
    changing current in one circuit
  • induces an emf in the other, we call it mutual
    induction . .

10
Micheal Faraday (1791-1867)
  • Micheal Faraday and Joseph Henry conducted
    independent experiments on electromagnetic
    induction around 1830.
  • Faraday drew the conclusion
  • An induced emf is produced in a loop by
    changing the number of magnetic field lines
    passing through the plane of the loop.

11
magnetic flux (?) a measure of
the number of field lines passing through and
area (A). The variable we use to represent
magnetic flux is capital phi.
  • The area can be represented by a vector A
    perpendicular to the plane of the area.
  • b) When the plane of a rotating loop is
    perpendicular to the field and ? 0, then ?
    ?max BA.
  • c) When ? 180, the magnetic flux has the same
    magnitude but is opposite in direction ? -
    ?max - BA.
  • d) When and ? 90, then ? 0.

e) As the loop is rotated from an orientation
perpendicular to the field to one more nearly
parallel to the field, less area is open to the
field lines and the flux decreases. In general, ?
BA cos ?.
Side View
12
  • ?m BA cos ? magnetic flux
  • where B is the magnetic field
  • A is the area of the loop
  • ? is the angle between B and A
  • The unit of magnetic flux is the weber (Wb). 1
    Wb 1 Tm2
  • If the coil has N number of turns, then the
    total flux through the coil is the sum of the
    flux through each turn. Hence,
  • ?m NBA cos ? magnetic flux through a solenoid

On Gold Sheet
13
Example 20.1 A circular loop of radius 0.20 m is
rotating in a uniform magnetic field of 0.20 T.
Find the magnetic flux through the loop when the
plane of the loop and the magnetic field vector
are (a) parallel, (b) perpendicular, (c) at 60.
14
Examples pacemakers stopping, migrating birds
get lost, GPS wont work, etc.
15
(average for the time interval ?t)
For N loops of wire, Faradays Law of Induction
can be written ? - N ? ?m
? t
16
  • In other words, Lenzs Law states that the
    direction of the induced current opposes the
    increase in flux.

17
Example A
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20
Example B
21
Example C
22
On Gold Sheet

The polarity of induced emf is given by the right
hand force rule. Therefore, the force on the
electrons would be down.
-
  • The magnitude of the induced emf is called
    motional emf. .

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24
Example 20.2 A coil is wrapped with 100 turns of
wire on a square frame with sides 18 cm. A
magnetic field is applied perpendicular to the
plane of the coil. If the field changes uniformly
from 0 to 0.50 T in 8.0 s, find the average value
of the magnitude of the induced emf.
25
Example 20.3 A square coil of wire with 15 turns
and an area of 0.40 m2 is placed parallel to a
magnetic field of 0.75 T. The coil is flipped so
its plane is perpendicular to the magnetic field
in 0.050 s. What is the magnitude of the average
induced emf?
26
Example 20.4 An airplane with a wing span of 50
m flies horizontally with a speed of 200 m/s
above the Earth at a location where the downward
component of the Earths magnetic field is 6.0 x
10-5 T. Find the magnitude of the induced emf
between the tips of the wing.
27
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28
right to left through
left to right through the
29
left to right through the resistor.
right to left through the resistor.
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31
  • Check for Understanding
  • The unit of magnetic flux is
  • a) Wb
  • b) Tm2
  • c) Tm / A
  • d) both a and b
  • Answer d
  • 2. Magnetic flux through a loop can change due to
    a change in
  • a) the area of the coil
  • b) the magnetic field strength
  • c) the orientation of the loop
  • d) all of the above
  • Answer d

32
  • Check for Understanding
  • For an induced current to appear in a loop of
    wire,
  • a) there must be a large magnetic flux through
    the loop
  • b) the loop must be parallel to the magnetic
    field
  • c) the loop must be perpendicular to the
    magnetic field
  • d) the magnetic flux through the loop must vary
    with time
  • Answer d

33
Homework for Chapter 20
  • Read Chapter 20
  • HW 20 p.658-660 3,5,6,8,9,10,11,13,14,18,21-24.


34
Chapter 20 Formulas
?m NBA cos ? magnetic flux N is the number
of loops B is the magnetic field A is the
area of the loop ? is the angle between B and
A
  • - N ? ?m average induced emf
  • ? t N is the number of loops
  • ? ?m is the final minus initial magnetic flux
  • ? t is the final minus initial time
  • BLv induced motional emf
  • B is the magnetic field
  • L is the length of the conductor
  • v is the speed of the conductor
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