5D20.60 Conductivity in Glass

PURPOSE: To show that the resistance of glass is lower at higher temperature.

DESCRIPTION:  A light bulb is smashed with a mallet and the filament is cut off from the glass stem.  The broken base of the bulb is screwed into a lamp socket.  The base of the broken bulb contains a glass stem with metal contacts.  A torch is used to heat the glass stem until it begins to melt.  When the glass has melted the resistance drops and it begins to draw a current.  The current will cause the glass to continue to heat and glow.   Another 135 Watt lamp in series with the broken bulb is used as a ballast to control the current and also acts as a visible ammeter.

In the past this demo has been used as an example of a semiconductor in the Physics 79 course.  (The impurities in the glass are made mobile by heat.  In a silicon semiconductor the impurities in the silicon are made mobile by voltage.  Silicon itself is an insulator, but by doping the silicon with impurities it is made into a semiconductor that lies between an insulator and a conductor.  The resistance to current through the p-n junction of conjoined p-doped and n-doped silicon is be controlled by a voltage.  Glass is also made of silicon.  Glass, like silicon, at room temperature is an insulator that has impurities.  The impurities in glass can be made mobile by heat.  Heat causes the resistance of glass to pass through the semiconductor region quickly, as it travels from insulating to conducting.)

EQUIPMENT: 135 Watt bulb, a second bulb to break, series connected lamp base circuit, propane blow torch, box, mallet, gloves, goggles.

Set Up Notes:

Edited by Jerry Zani on 2/14/01