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For more information, please see full course syllabus of AP Physics C/Electricity and Magnetism
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Motional EMF

  • When a conducting rod of length L moves with speed v in a magnetic field B that is perpendicular to both the rod and the direction of motion, an emf is induced across the rod. This emf, known as motional emf, is given by BLv.
  • If a rod, of length L, rotates with angular velocity w about a pivot at one end of the rod, and if a magnetic field exists in space, perpendicular to plane of rotation, an emf will be induced in the rod; it is given by (1/2)wBL^2.
  • Examples are provided in the lecture that show how to calculate the force that an external agent must exert in order to pull a rod on rails at constant speed in the presence of a magnetic field, and in the absence of this force, how the speed of the rod decreases with time.

Motional EMF

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Moving a Conducting Rod in Magnetic Field 0:24
    • Rod Moving in a Plane with Velocity 'v'
    • Charges Piles Up and Down Until Electric Force Balance 'B'
    • Equilibrium
    • Potential Difference, Distance to Length of Wire
  • Rod Pulled By External Agent 11:30
    • Resistance to Wire
    • Introducing Uniform Magnetic Field into The page
    • Finding Flux
    • Power Delivered to Resistance
    • Force Exerted by 'B' on Rod
    • Power By Agent
  • Sliding Rod 23:08
    • Resistance with a Sliding Rod and Magnetic Field 'B'
    • Push With Initial Velocity 'V0'
    • Finding Current = I
  • Rotating Rod 36:10
    • Magnetic Field into The Page
    • Rod fixed in Plane and Rotating
    • Induced EMF in Segment
  • Example 1: Bar in Magnetic Field
  • Example 2: Rod in Magnetic Field