030 – Eddy current brake

Inductively braked screw-nut

Introduction to:

Lenz’s rule, eddy current, Lorentz force 

Material:

• Horseshoe magnet
• Metal screw-nut

Instructions:

• Set the nut into rotation by snapping your fingers.
• Hold magnet over it so that the nut is centered between the poles.

Tips:

• Hold the magnet at an angle and move it along with the nut to prevent it from sticking to the magnet
• Coins are unsuitable

Observation:

The rotation of the nut is strongly decelerated in the magnetic field.

Explanation:

• Moving charges in a magnetic field experience a force perpendicular to the field lines, the Lorentz force.
• There are free electrons in the metal, which spin with it, the Lorentz force on them leads to disordered currents in the metal, the so-called eddy currents (induction).
• According to Lenz’s rule, the induction effect is directed against its cause (the rotation in the magnetic field). So the rotation is decelerated.
• From an energetic point of view, rotational energy is converted into electrical energy and finally into thermal energy.
• The angular momentum is transferred to the magnet (not noticeable).