IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# How to slow a Rotating Conducting Disk?

## Introduction

There are many experiments to demonstrate Faraday's law of electromagnetic induction. Whenever a conductor is placed in a varying magnetic field or it moves under a magnetic field, emf is induced. If there are conducting paths available, currents start in the conductor which we call Eddy current. This experiment is one nice way to demonstrate eddy currents.

## Apparatus

A strong magnet, a disc made up of aluminium or copper, motor to rotate the disc, battery etc.

## Procedure

Mount the disc on the spindle of the motor. Connect the motor to the power source. Switch on the power so that the motor along with the connected aluminium disc starts rotating. Soon it will pick up a good speed. Now bring a magnet very close to the rotating disc. A pole should face the disc surface. The disc gradually slows down to almost a halt. Take the magnet a bit away. The disc again picks up speed.

The free electrons of the disc also move with the disc. When the magnet is kept near the rotating aluminium disc, the free electrons of the aluminium disc below the magnet experience magnetic force causing a motional emf in the conductor. This produces eddy currents in the disc. Energy is consumed in these currents putting more load on the motor. So the disc slows down.