Chap29_Notes - Physics 184 Physics for Scientists &...

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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 1 Physics 184 Physics for Scientists & Engineers 2 Fall Semester 2011 Chapter 29: Electromagnetic Induction
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 2 Faraday’s Experiments Consider the situation in which we have a wire loop connected to an ammeter so that we can measure current flowing in the loop We hold a bar magnet some distance from the loop, pointing the north pole of the magnet toward the loop While the magnet is stationary, there is no current flowing in the loop What happens if we move the magnet? Demo
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 3 Faraday’s Experiments (2) When we move the magnet toward the loop, we induce a positive current in the loop Now we turn the magnet around so that the south pole points toward the loop When we move the magnet toward the loop, we induce a negative current in the loop
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 4 Faraday’s Experiments (3) Now let’s point the north pole toward the loop but move away from the loop We get a negative current We turn the magnet around so that the south pole points toward the loop and move away from the loop We get a positive current
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 5 Faraday’s Experiments (4) We can create similar effects by placing a second loop near the first loop but with a more quantitative result as shown below If a constant current is flowing through loop 1, no current will be induced in loop 2 If we increase the current in loop 1, we observe that a current is induced in loop 2 in the opposite direction Thus not only does the changing current in the first loop induce a current in the second loop, the induced current is in the opposite direction
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 6 Faraday’s Experiments (5) Now if we have the current flowing in loop 1 in the same direction as before, and decrease the current as shown below, we induce a current in loop 2, in the same direction as the current in loop 1
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 7 Law of Induction From these observations we see that a changing magnetic field induces a current in a loop We can visualize the change in magnetic field as a change in the number of magnetic field lines passing through the loop Faraday’s Law of Induction states that: An emf is induced in a loop when the number of magnetic field lines passing through the loop changes with time The rate of change of magnetic field lines determines the induced emf
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10/28/2011 Physics for Scientists & Engineers 2, Lecture 26 8 Magnetic Flux To quantify the amount of magnetic field lines we define the magnetic flux in analogy to the electric flux When we introduced Gauss’ Law for the electric field, we defined the electric flux as For the magnetic field, we can define magnetic flux in analogy as The unit of magnetic flux is the weber (Wb) E E dA   B B dA   2 1 Wb 1 Tm
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Chap29_Notes - Physics 184 Physics for Scientists &...

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