NCU Physics Demonstration Lab 國立中央大學物理演示實驗

Pendulum Stop

How a non-friction brake functions.

What?

This experiment shows the formation and effect of induced eddy currents produced by metal’s moving in a stationary magnetic field.

How?

Materials:

  1. Three equal length pendulums with bobs made of intact, comb-shaped, and hole-dotted aluminum sheet respectively.
  2. Few magnets.

Experiment:

  1. Observe the motion of the intact and the comb-shaped aluminum sheets without and within a magnetic field.

    Experiments demonstration

    ×Experiments demonstration

    http://videocentral.phy.tw/v/4e5a3bef8b5267d6

  2. Observe the motion of the intact and the hole-dotted aluminum sheets without and within a magnetic field.

    Experiments demonstration

    ×Experiments demonstration

    http://videocentral.phy.tw/v/22256d2154c08a75

  3. Observe the motion of the comb-shaped and the hole-dotted aluminum sheets without and within a magnetic field.

    Experiments demonstration

    ×Experiments demonstration

    http://videocentral.phy.tw/v/34c4618890792fb7

Why?

  1. Why is there a resistance force to the swing of aluminum sheets in a magnetic field?
  2. Will the comb-shaped aluminum sheet experience the same resistance in a magnetic field?

Think...

×Think...

  1. 1. The aluminum sheet experiences a change of magnetic flux when it swings in and out of the external magnetic field. The changing flux induces eddy currents and the currents form a magnetic field with a force against the swing direction of the metal sheet.
  2. 2. The slots on the comb-shaped aluminum sheet confine the induced eddy currents to many small current loops. These small currents are inadequate to induce a sufficient magnetic force to repress the motion of the aluminum sheet.

Questions

  1. Will the hole-dotted aluminum sheet experience a resistance force when it swings in a magnetic field? Why? At which size will the holes be sufficient to incur the effect of resistance?
  2. Discuss the possible circuits for the induced eddy currents on the metal sheet.
  3. Friction brake converts kinetic energy to thermal energy in the braking process. Where does the kinetic energy go in an Eddy Current Brake?
  4. Discuss and estimate the resistance force magnetically induced by conductors like flying airplanes or rockets cutting the Earth’s magnetic field.
  5. Why Eddy Current Brakes are not widely applied in the design of vehicles?

About the Experiment

Compare this experiment to E04 Falling Magnet Ring (Traditional Chinese), E06 Floating Aluminum Foil (Traditional Chinese) and E11 Magnet Space Stroll .

Reference

Benson, H. (1995).Chapter 31 Electromagnetic Induction. University Physics. John Wiley & Sons Inc.

Producer

v.1 Mr. Tseng(曾前助理), Chia-Yen Tai (戴嘉言)

v.2 Hung Chen(陳鴻)

Advisor

Tai-Lee Chen(陳泰利)

Written by

Tai-Lee Chen(陳泰利)

Translator

Hsiao-Ching Su