1Q30.40 - Fluctuating Magnetic Fields/Cons. of Angular Momentum

Code Number:
Demo Title:
Fluctuating Magnetic Fields/Cons. of Angular Momentum
Effects of Variable Magnetic Fields on a Star
Area of Study:
Rotating Air Table, Rotating Plate with Added Lead Strips, Compressed Air, Winch, Pulleys, Gloves, and Table Rod Stand for Pivot Pulley.

See also 1Q30.40 in Astronomy.

This demonstration requires a room with a compressed air line.

In most stars the inner core is rotating faster or slower than the outer layer so that there is a readily detectable shear force.  If a star has a variable magnetic field and this field increases some of the outer layer are tied to the inner core by the magnetic field and forced to rotate at the same speed.  Since total angular momentum is conserved there is a detectable change in the rate of rotation for both systems.

Our version of this is to drop a heavy small diameter rotating mass onto a lighter larger diameter rotating mass.  The small diameter plate with the lead strips is approximately 3 times heavier than the rotating air table (small diameter plate contains about 75% of the mass of the total system of the two plates).  By gently dropping the heavy mass onto the lighter mass you may demonstrate frictional slippage as the two masses achieve a common rotation rate.

NOTE: There are some sharp edges here so it is advisable to wear gloves when trying to establish your initial rotation rates.

  • Yau-Jong Twu, "From Rolling Without Slipping to Sweet Spots", TPT, Vol. 58, #3, Mar. 2020, p. 218.
  • Robert Drosd and Leonid Minkin, "Measuring the Coefficient of Kinetic Friction by Exploring Dynamics of Rotational Motion", TPT, Vol. 58, #3, Mar. 2020, p. 176.
  • Paul Gluck, "MBL Experiment in Angular Momentum", TPT, Vol. 40, #4, Apr. 2002, p. 230.
  • Robert Ehrlich, "F.14. Transfer of Angular Momentum", Turning the World Inside Out and 174 Other Simple Physics Demonstrations, p. 81 - 82.
  • Robert Ehrlich, "F.3. Dropping Objects onto a Rotating Casserole Cover", Turning the World Inside Out and 174 Other Simple Physics Demonstrations, p. 69 - 70.

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.