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For more information, please see full course syllabus of AP Physics C/Electricity and Magnetism
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Torque on a Current Carrying Loop

  • If the magnetic field B is parallel to the plane of a loop carrying a current I, the torque exerted by B on the loop is IAB, where A is the area of the loop.
  • In general, the torque is given by I A x B, where A is a vector whose magnitude is the loop area and which is directed normal to the loop plane.
  • The magnetic moment of a loop of area A carrying a current I is the product m = IA and is directed normal to the plane of the loop: if the four fingers of the right hand circle around in the direction of the current, then the thumb points in the direction of m.
  • The torque exerted by a magnetic field B on a magnetic moment m is given by the cross product m x B. The potential energy of a magnetic dipole m in a magnetic field B is the dot product –m.B.

Torque on a Current Carrying Loop

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
  • B-Field Parallel to Plane of the Loop 0:27
    • Loop in the X-Y Plane
    • Net Force on Loop
  • B-Field Not Parallel to Plane of the Loop 15:16
    • Loop in the X-Y Plane, Free to Rotate in X- Direction
    • Force on Out of Page and Force in to the Page
    • Loop Turns Through 90 Degrees
  • Magnetic Moment 36:26
    • Any Current Loop Has Current 'I'
    • Electric Dipole in Electric Field
    • Potential Energy
    • Magnetic Potential Energy of Dipole
  • Example 43:33
    • Circular of Radius 'r' With Magnetic Field and Pass Current
    • Torque
  • Example 1: Loop in Magnetic Field
  • Example 2: Rotating Charge