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Unformatted text preview: Class 12 Generators, Motional emf, and Review for Midterm Exam #1 Physics 106 Fall 2011 Press CTRLL to view as a slide show. Learning Outcomes Today we will discuss: I Magnetic dipole in a nonuniform field I Induced electric fields and Faraday’s Law I Lenz’s Law Learning Outcomes Today we will I learn about generators I look at the details of a motional emf problem I review for Exam #1 Generators Generators I Generators (usually) change θ by rotating a coil in a magnetic field. Generators I Alternating Current (AC) generator I Like an "inverted" AC motor rotate the coil and current comes out I Two separate commutators AC Generators I Basic operation of the generator AC Generators I = NBA ω sin ω t I = when the normal to the loop is parallel to the field I = max when the normal is perpendicular DC Generators I The contacts to the rotating loop are made by a split commutator DC Generators I The output voltage always has the same polarity I To produce a steady current, many loops and commutators around the axis of rotation are used Motional emf Motional emf I A rod of length ‘ moves with constant velocity through a uniform field I The electrons in the conductor experience a magnetic force I F = qvB I The electrons tend to move to the lower end of the conductor Motional emf I The top of the rod is positive and the bottom of the rod is negative I An electric field is produced in the conductor I Charges build up at the ends of the conductor until the downward magnetic force is balanced by the upward electric force I As with the velocity selector: qE = qvB , v = E / B Motional emf I There is a voltage difference across the rod. In terms of potential, the electric field is E=V/l I This gives: V = B ‘ v Motional emf in a Circuit I A moving bar has negligible resistance I As the bar is pulled to the right by an applied force, the free electrons in the bar experience a magnetic force I This causes an induced current because the charges are free to move in a closed path Motional emf in a Circuit What is the magnetic flux? Φ B = BA = B ‘ x Motional emf in a Circuit What is the magnitude of the EMF? = Δ BA Δ t = B ‘ Δ x Δ t   = B ‘ v Motional emf in a Circuit What is the current? I = R = B ‘ vR Motional emf in a Circuit What is the power dissipated by the resistor? P = I = ( B ‘ v ) 2 R Motional emf in a Circuit What is the magnetic force on the bar? F = I ‘ B = B ‘ v R ‘ B ( B ‘ v ) 2 R Motional emf in a Circuit What work is done in moving the bar a distance Δ x ? W = F Δ x = ( B ‘ v ) 2 R Δ x Motional emf in a Circuit What is the mechanical power of pulling the bar? P = W Δ t = B 2 ‘ 2 v R Δ x Δ t B 2 ‘ 2 v 2 R Test Information What is the test like?...
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 Fall '09
 Physics, Magnetic Force, Magnetic Field, Electric charge, motional emf

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