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Lecture 9: Electromagnetic Induction (Ch. 22)

Lecture 9: Electromagnetic Induction (Ch. 22). 04-13-07. Induced EMF and Induced Current. Relative motion between the magnet and the coil is needed to generate a current; it does not matter which one moves.

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Lecture 9: Electromagnetic Induction (Ch. 22)

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  1. Lecture 9: Electromagnetic Induction (Ch. 22) 04-13-07

  2. Induced EMF and Induced Current Relative motion between the magnet and the coil is needed to generate a current; it does not matter which one moves. The current in the coil is called an induced current because it is brought about (or “induced”) by a changing magnetic field. Since a source of emf (electromotive force) is always needed to produce a current, the coil itself behaves as if it were a source of emf. This emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current.

  3. Induced EMF and Induced Current Cruise Control System An emf can be induced by changing the area of a coil in a constant magnetic field. Here the shape of the coil is being distorted so as to reduce the area. As long as the area is changing, an induced emf and current exist; they vanish when the area is no longer changing. If the distorted coil is returned to its original shape, thereby increasing the area, an oppositely directed current is generated while the area is changing.

  4. Motional EMF: EMF induced in a moving conductor RHR 1 When a conducting rod moves at right angles to a constant magnetic field, the magnetic force causes opposite charges to appear at the ends of the rod, giving rise to an induced emf. The induced emf causes an induced current I to appear in the circuit. The separated charges on the ends of the moving conductor give rise to an induced emf, called a motional emf because it originates from the motion of charges through a magnetic field. The emf exists as long as the rod moves. If the rod is brought to a halt, the magnetic force vanishes, with the result that the attractive electric force reunites the positive and negative charges and the emf disappears. Electric Force Magnetic Force Example 1

  5. Motional EMF and Electrical Energy Motional emf arises because a magnetic force acts on the charges in a conductor that is moving through a magnetic field. Whenever this emf causes a current, a second magnetic force enters the picture – See RHR-1 Violates Conservation of energy

  6. Magnetic Flux: Motional EMF and Magnetic Flux Motional emf, as well as any other type of induced emf, can be described in terms of a concept called magnetic flux. Magnetic flux is analogous to electric flux, which deals with the electric field and the surface through which it passes. The quantity BA is given the name magnetic flux and is represented by the symbol (Greek capital letter phi); thus       . The magnitude of the induced emf is the change in flux             divided by the time interval           during which the change occurs: General Expression for Magnetic Flux: UNIT: Weber: 1 Wb = 1 T · m2

  7. Graphical interpretation of magnetic flux Example 4 The magnitude of the magnetic field in (a) is three times as great as that in (b), because the number of magnetic field lines crossing the surfaces is in the ratio of 3:1.

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