Concepts of Physics

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

Electromagnetic Waves do not Travel in Conductors

Introduction

Electromagnetic waves are all around us. We can transmit or receive the EM waves at our wish by using the mobile phone. While these waves move freely in empty space and insulating materials, they quickly damp out in conducting media. A simple but fascinating way to show this is described in this demonstration.

Apparatus

A steel box, a mobile phone

Procedure

Put you mobile phone in the steel box and close the box. From a different mobile phone call the number of the first one. Are you able to get the connection? Often it says `the subscriber phone is out of range'. Bring the phone out of the box and again make a call. It rings as usual.

Discussion

The electromagnetic wave contains electric and magnetic fields. Conduction media contain large number of free electrons. When EM wave travels in a conducting medium, the E-field causes motion of these free electrons resulting in a current. Thus energy is lost as Joule heating and the EM wave dampens. There are other channels of energy dissipation too but this is the most dominant. As a result, EM waves, especially of high frequency, are not able to penetrate steel wall of the box.