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).