Concepts of Physics

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

Electrical Conductivity Test

Objective

To get familiarize with good and bad conductors of electricity.

Introduction

The early topics in current electricity include the ides of electric conductors and insulators. Among conductors, some are good conductors and some are poor conductors.

Apparatus

A battery, LED, connecting wires

Procedure

Connect positive end of a LED (longer leg) to the positive end of the battery. Connect a wire to the negative end of the LED (shorter leg). The LED does not glow. This is because there is air gap between the two free ends A and B of wires connected to the negative ends of the battery and the LED. The current is not able to go through the air. Air in normal condition is a poor conductor.

Now, bring a metal rod and press the ends A and B at the ends of the rod. The LED glows showing that metals are good conductor of electricity.

Bring a plastic scale and connect A and B at the ends. The LED does not glow. Plastic is a poor conductor of electricity. Take a pencil and sharpen it at both the ends. Touch A and B at the ends of this pencil. Does the LED glow?

Bring some water in a beaker. Dip the ends A and B of the wires in water. Does LED glow? Dissolve some salt in water and dip the ends A and B in it. Does LED glow? Bring distilled water and put A and B in it. Does LED glow?

Discussion

Materials either allow electricity to flow freely through them or hold the charges on their surface. Those materials that allow electricity to flow freely, are conductors. Conductor do not store charge. Insulators hold electric charges on their surface and do not allow electricity to flow through them.

A good conductor has free electrons. The free electrons move within the conductor at random. The electrons do not `flow' in one direction. However, if there is difference in potential between the two ends of a conductor, the electrons are attracted to the positive end of the conductor and current flows.