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Electromagnetic Induction

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Electromagnetic Induction Is the induced current cw, ccw or zero? The external magnetic field is increasing The external magnetic field is decreasing The right side ... – PowerPoint PPT presentation

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Title: Electromagnetic Induction


1
  • Electromagnetic Induction

2
21.7 Magnetic Fields Produced by Currents
  • In 1820, H.C. Oersted discovered that a
    current in a wire caused a deflection in a
    magnet.
  • This led to the conclusion that moving charges
    were magnets, producing their own magnetic field
    and exerting forces on other magnets.
  • It seemed natural for scientists to investigate
    the opposite Can a magnet generate a potential
    difference and thus cause current flow, in much
    the same way as a battery?

3
Induced Current
  • A current creates a magnetic field.
  • Is the opposite true?
  • Can a magnetic field create, or induce a current?
  • In 1831, Michael Faraday made an exciting
    discovery

4
Faradays Discovery
  • To test this hypothesis, Faraday used 2 separate
    circuits that were not connected and wrapped each
    around an iron ring that did not conduct
    electricity.
  • He thought by closing the switch on the left, the
    magnetic field of the left wire would magnetize
    the iron ring, which in turn might induce a
    current in the wire on the right.
  • But no luck, as the meter shows.

5
Faradays Discovery
  • Disappointed, he shut down for lunch.
  • But when he opened the switch to cut off the
    current in the left circuit, the meter suddenly
    moved, showing a momentary current in the wire on
    the right.
  • It quickly went back to zero.

6
Faradays Discovery
  • Baffled, he closed the switch again.
  • And noticed that the meter jumped again
    momentarily, and this time the meter needle went
    the other way.
  • I bet he actually did it a bunch of times before
    he noticed that it went the other way..

7
Faradays Law
  • Faraday found that he could induce a current in a
    closed coil of wire with an external magnetic
    field, but only if the magnetic field through the
    coil is changing.
  • This is an informal statement of Faradays Law.
  • A moving magnetic field causes charge to flow,
    just as moving charge causes a magnet to deflect.

8
22.1 Induced Emf and Induced Current
There are a number of ways a magnetic field can
be used to generate an electric current.
It is the changing magnetic field that produces
the current.
9
  • A square loop of copper wire is pulled through a
    region of constant magnetic field. In which
    case, if any, does current flow?
  • A. 2,3,4 B. None C. 2,4 D. All

3
2
1
4
10
22.1 Induced Emf and Induced Current
We know that a current is induced in the presence
of a changing magnetic field. How do we calculate
the magnitude and direction of the current? To do
so, we must discuss a quantity called the
magnetic flux, F.
11
22.3 Magnetic Flux
GRAPHICAL INTERPRETATION OF MAGNETIC FLUX
  • We can think as the magnetic flux as proportional
    to the number of magnetic field lines that pass
    through a surface. We can change the magnetic
    flux by
  • changing the magnetic field strength(B) as shown
  • changing the size of the loop
  • changing the angle between the normal to the
    loop and the direction of B

12
Magnetic Flux due to an external magnetic field
  • Fm AB cos ?
  • A cross-sectional area of the loop (m2 )
  • B strength of magnetic field (Tesla)
  • ? angle between B, and normal to A
  • Units 1 weber 1Wb 1Tm2

13
Rank the magnitude of magnetic flux that passes
through the coil, shown below (edge view) in
three different orientations relative to an
external magnetic field. Rank from greatest flux
to least.
  1. 1,2,3 b. 3,2,1 c. (1,3), 2 d.(3,1), 2

14
Magnetic Flux
  • A uniform magnetic field has a magnitude of 0.078
    T and is uniform over a circular surface of
    radius 0.10 m. The field is oriented at an angle
    of 25 with respect to the normal of the surface.
    What is the magnetic flux through the surface?

15
Magnetic Flux
  • A uniform magnetic field has a magnitude of 0.078
    T and is uniform over a circular surface of
    radius 0.10 m. The field is oriented at an angle
    of 25 with respect to the normal of the surface.
    What is the magnetic flux through the surface?

Answer 2.2 x 10-3 Wb
16
Change in Magnetic flux
  • A long, narrow rectangular loop of wire is moving
    toward the bottom of the page with a speed of
    0.019 m/s (see the drawing). The loop is leaving
    a region in which a 2.1 T magnetic field exists
    the magnetic field outside this region is zero.
    During a time of 1.0 s, what is the magnitude of
    the change in the magnetic flux?

17
Change in Magnetic flux
  • A long, narrow rectangular loop of wire is moving
    toward the bottom of the page with a speed of
    0.019 m/s (see the drawing). The loop is leaving
    a region in which a 2.1 T magnetic field exists
    the magnetic field outside this region is zero.
    During a time of 1.0 s, what is the magnitude of
    the change in the magnetic flux?
  • Ans .00319 Tm2 or Wb

18
22.4 Faradays Law of Electromagnetic Induction
  • Faraday found that he could induce a current in
    a closed coil of wire with an external magnetic
    field, but only if the magnetic field through the
    coil is changing. This is an informal statement
    of Faradays Law.
  • The formal statement relates the changing
    magnetic flux to an induced emf (e), which is the
    potential difference that causes a current to
    flow in a closed conductor. The magnitude of e,
    the emf, induced in a coil of N loops is

SI Unit of Induced Emf volt (V)
19
22.4 Faradays Law of Electromagnetic Induction
Example 5 The Emf Induced by a Changing Magnetic
Field A coil of wire consists of 20 turns each
of which has an area of 0.0015 m2. A magnetic
field is perpendicular to the surface.
Initially, the magnitude of the magnetic field
is 0.050 T and 0.10s later, it has increased to
0.060 T. Find the average emf induced in the coil
during this time.
20
(Worksheet) Flux Ranking Task
21
(Worksheet)Changing Flux
22
An MRI
  • Magnetic resonance imaging (MRI) is a medical
    technique for producing pictures of the interior
    of the body. The patient is placed within a
    strong magnetic field. One safety concern is what
    would happen to the positively and negatively
    charged particles in the body fluids if an
    equipment failure caused the magnetic field to be
    shut off suddenly. An induced emf could cause
    these particles to flow, producing an electric
    current within the body. Suppose the largest
    surface of the body through which flux passes has
    an area of 0.026 m2 and a normal that is parallel
    to a magnetic field of 3.5 T. Determine the
    smallest time period during which the field can
    be allowed to vanish if the magnitude of the
    average induced emf is to be kept less than 0.010
    V.

23
An MRI
  • Magnetic resonance imaging (MRI) is a medical
    technique for producing pictures of the interior
    of the body. The patient is placed within a
    strong magnetic field. One safety concern is what
    would happen to the positively and negatively
    charged particles in the body fluids if an
    equipment failure caused the magnetic field to be
    shut off suddenly. An induced emf could cause
    these particles to flow, producing an electric
    current within the body. Suppose the largest
    surface of the body through which flux passes has
    an area of 0.026 m2 and a normal that is parallel
    to a magnetic field of 3.5 T. Determine the
    smallest time period during which the field can
    be allowed to vanish if the magnitude of the
    average induced emf is to be kept less than 0.010
    V.
  • Ans 9.1 s

24
  • A current-carrying wire is pulled away from a
    copper wire loop as shown. As a result, an emf
    (and therefore a current) is induced in the loop.
    To maximize the emf, we should
  • pull the wire away from the loop more slowly
  • pull the wire away from the loop more quickly
  • push the wire towards the loop at the same speed.

25
Lenzs Law
  • There is an induced current in a closed
    conducting loop only if the magnetic flux is
    changing (either B, A or ?). The direction of
    the induced current is such that the induced
    magnetic field opposes the change in flux.

26
Using Lenz Law
  • Determine the direction of the external magnetic
    field.
  • Determine how the flux is changing. Is it
    increasing, decreasing, or staying the same?
  • Determine the direction of an induced magnetic
    field that will oppose the change in the flux.
  • Increasing induced magnetic field points
    opposite the external magnetic field.
  • Decreasing induced magnetic field points in the
    same direction as the external magnetic field.
  • Constant no induced magnetic field.
  • 4. Determine the direction of the induced
    current. Use the right-hand rule.

27
Lenzs Law
  • An induced current in a coil or loop can be
    created 2 ways
  • Change the size or orientation of the coil in a
    stationary magnetic field.
  • Change the strength of the magnetic field through
    a stationary circuit.
  • Both of these create a changing magnetic flux.

28
  • The current exists because the changing magnetic
    flux has induced an emf. In a closed circuit
    with a resistance, R
  • I e/R
  • The current is a consequence of the induced emf.
  • The emf is a consequence of changing flux (Fm )

29
http//phet.colorado.edu/en/simulation/faraday
30
Lenzs Law
  • An external magnetic field, B is directed into
    the page. A sliding rail completes the circuit.
    The rail is pushed to the right, increasing the
    area of the circuit.

31
Lenzs Law
  • In which direction is the induced magnetic field?
  • into the page
  • out of the page
  • zero, since field is stationary
  • in the direction of the moving rail

32
Lenzs Law
33
  • A current-carrying wire is pulled away from a
    copper wire loop as shown. As a result, an emf
    (and therefore a current) is induced in the loop.
    in what direction is the current?
  • clockwise
  • counterclockwise
  • No current

34
Is the induced current cw, ccw or zero?
  1. The external magnetic field is increasing
  2. The external magnetic field is decreasing
  3. The right side of the loop turns into the page
  4. The left side of the loop turns into the page

35
  • Positive () current comes out the top of the
    loop and enters the bottom.
  • Negative (-) current comes out the bottom of the
    loop and enters the top.
  • For A and B, and C is the current , -, or zero?
  • A. The magnet is pushed into the loop.
  • The magnet is pulled back to the left.
  • The loop turns so the normal is in the plane of
    the page.
  • If the magnet is pulled out more loop more
    rapidly than in A, does the current increase,
    decrease or stay the same?
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