Glass Conductivity Craig Johnson, Ph.D., P.E. Central Washington University Ellensburg, WA 98926-7584 www.cwu.edu/~cjohnson
Abstract
Glass is a common material used for windows, pots and pans. We generally think of it as clear, though ‘glass-ceramics’ like Corning Ware are typically opaque and often colored. And it’s common for glass to be thought of as a thermal insulator. We have ‘double-pane’ windows to protect us from the elements. We use ‘fiberglass’ to insulate our houses. Some of us also use glass devices to insulate against electrical power, such as fuse housings. But all properties are not as they seem. It turns out that glass can be used any which way we want! The same glass that we use for our protective windows can be used on our stove as a pot to heat water. The same ‘fiberglass’ that is used to insulate our houses is used as conductive pipes in heat exchangers (primarily because they don’t rust!). This an amazing display of ironic applications is presented in this module, along with PowerPoint slides for use to present this module.
Introduction
The thermal properties associated with ‘glass’ are typically connected with a ‘system’ that includes air. Fiberglass ‘insulation’ properties are mostly due trapped air: the ‘dead-air’ pockets insulate the house, not the glass fibers themselves. And those heat exchanger pipes made of fiberglass? Well, those are thin tubes of FRP (fiber reinforced plastic) that really don’t conduct heat that well, but are good enough to do the job if they are thin enough. This is especially true when considering the corrosion loss in conventional metal tubes. So, maybe thermal conduction isn’t the best example of ‘variable properties’ in ceramics.
But consider electrical conductivity (1) instead of thermal conductivity (2). In this module, we will explore wild variation in electrical properties that some ceramics allow. As with many properties, the electrical conductivity of glass changes with temperature. In this case we can focus our attention on simple soda-lime glass (3). In general, engineers are concerned with room-temperature applications of any material, unless they operate at extremes. The nominal electrical resistivity of glass (rho, in Ohm*m) is pretty high at 1013 (1). But this can change dramatically!
Module Content: This module is accompanied by a PowerPoint presentation that may be used in presenting the unit.
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Module Objectives: Students will be able to Use a torch, a voltmeter, and make the necessary electrical circuit connections. Quantitatively compare the electrical properties (e.g. resistance, conductivity) of glass at differing temperatures. Predict trends in properties of materials with changes in temperature.
MatEd Core Competencies Addressed: 6B1 Apply concepts of heat, including temperature, thermal conductivity, specific heat, etc. 8A Demonstrate the Planning and Execution of Materials Experiments
Type of Module / Mode of Presentation: This activity describes an in-class demo of electrical conductivity changes in glass with temperature. The accompanying PowerPoint may be used to help student comprehension.
Key Words / Key Phrases: Glass, Electrical Conductivity, Electrical Resistivity
Time required: This demo requires preparation time, either a previous set-up for the 110AC demo, or appropriate items to construct the voltmeter demo. Both scenarios can be completed within an hour. If the system is already constructed and activated, then the device/system can be used as a demo within a class period.
Pre-requisite knowledge: None
Target grade levels: Grade 9 to beginning college
Equipment and supplies needed (per participant or team of 2+): Environment: Each student or team will need: Table-top space (about four square feet) Glass rod (about three inches long, a quarter inch diameter) Voltmeter Torch (propane or butane is fine) Temperature probe (optional) Fused power strip, copper wire (~14 gauge), plank, light bulb w/base.
Curriculum Overview and Instructor Notes: If the curriculum is set up for a short demo, then the light bulb system is recommended. Since you can construct it prior to class, it offers a quick way to visibly show the effect of temperature on glass electrical conductivity.
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For a more general approach, voltmeters are common and relatively cheap. You can use the ‘alligator clips’ directly on the glass rod ends. Holding the glass rod so it is easy and safe to heat up requires a vertical stand or ceramic plate. Safety: Please use normal PPE (Personal Protective Equipment) during this lab. This includes using gloves for handling hot items. Eye protection is appropriate. Other items may be considered such as aprons for protecting clothing. Activity Description: In preparation, the instructor should run through this activity before using it. The demo scenario is self-evident. But the lab approach requires preparing the space and running an initial test. The lab itself involves an introduction, the activity, and then a debrief. Preparation: Preparation for the voltmeter activity: Determine the workspace. Set out the voltmeters, torches and required attachment tools. The glass rods can be set out, along with any other materials of interest.
Introduction: includes a discussion of electrical conductivity and how it relates to material bonding. There are major differences in bond energy between ionic/covalent vs. metallic. This is also shown in electronegativity values. A periodic chart can be handy to explore various elements and how they may behave. Though the glass rods show a dramatic change in electrical conductivity over temperature, many other materials do not. It may be of interest to test some rods of other materials. Activity (Testing the Effect): Students first complete the circuit, and place the rod for heating. An initial reading of resistance is appropriate, both direct (clip to clip) and then attached to the glass rod. When applying the torch, decisions can be made if you go to the hottest level and read the resistance, or if you apply the heat in steps (with corresponding temperature and resistance measurements). In the end, data will be recorded for post analyses. Debriefing: Debriefing is an important aspect of any educational activity. There are questions concerning both the concept of ‘conductivity’ and its application to materials.
Did the resistance readings make sense? Did the change in conductivity with temperature surprise you? Why? Do other materials behave similarly? If you don’t know: how would you find out? Variations of the activity: Other materials may be of interest. For example, the author has not found any metals that change electrical conductivity like glass. But there are a number of ceramics that may exhibit this trait.
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Comments this activity: It can be interesting to think of all the types of material properties and how they can change with altered environments. Whether it’s dozens of types of ice, or some alloy- dependent corrosion interaction: there are many scenarios that are simply alluring to an interested party. The fact that glass can exhibit such a wide range of conductivity is amazing. That other oxides can become superconductive stretches our predictive capabilities.
Evaluation of students: (questions, discussion or quiz items): 1) 6B1 (physical science concepts: Apply concepts of heat, including temperature, thermal conductivity, specific heat, etc.). How would you guess which materials might have changes in properties with changes in temperature? 2) 8A (experiments): Describe how you tested your material for properties (e.g. using a bulb or voltmeter to indicate/measure electrical conductivity). How could you improve the testing procedure?
References: 1. Wikihow, ‘Electrical Resistivity and Conductivity’, https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity 2. Wikihow, ‘Thermal Conductivity’, https://en.wikipedia.org/wiki/Thermal_conductivity
3. Wikipedia, search ‘Glass’, https://en.wikipedia.org/wiki/Glass
This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation DUE #1400619
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