031 – Drive by eddy currents

Reversal of eddy current brake  

Introduction to:  

Eddy currents, Lorentz force  

 

Material:  

  • Bar magnet        
  • Aluminum foil (approx. 30 cm x 30 cm)       
  • Plate (deep, soup plate), filled with water  

Observation:  

The aluminum foil rotates due to the circular motion of the bar magnet.  

Instructions:  

  • Fold the side edges of the aluminum foil 3 times on top of each other, alternating lengthwise and crosswise 
  • fold the resulting rectangle to a square by eye 
  • bend the 4 corners to the backside, so that a hexagon is formed   
  • place the finished hexagon on the water surface with the smooth side up 
  • move one pole of the magnet over the edge of the foil in a circular motion

Tips:  

  • move the magnet evenly and not too fast  
  • a bowl filled to the brim is optimal  

Explanation:  

  • Moving charges in a magnetic field experience a force perpendicular to the field lines, the so-called Lorentz force.
  • In the metal, there are free electrons moving with respect to the magnetic field.
  • The Lorentz force acting on them leads to disordered currents in the metal, the so-called eddy currents (induction).
  • According to Lenz’s rule, the induction effect is opposite to its cause (the rotation with respect to the magnet).
  • Therefore, a torque is created on the aluminum disk so that its motion converges towards the same rotation as the magnet.