Table of Contents

Elasticity of Glass

Description

This demonstration consists of a glass bottle filled with water that's been dyed dark blue. The top of the bottle is stopped with a rubber stopper, and a glass capillary tube (formerly part of a thermometer) is stuck into the stopper.

Purpose

This demonstration shows the elasticity of a material usually thought of as brittle and inflexible. Demonstrates the measurable strain produced in glass as a result of force exerted by squeezing the bottle.

Apparatus

Setup

Notes

This demonstration can be finicky at times, you may notice that the water level will increase even as you hold it.

Only a very small change in the volume of the liquid is necessary to produce a substantial change in the height of the liquid in the capillary tube, so it's very sensitive to the environment. This effectively means that, in addition to providing an effective reading of the small changes in volume of the bottle, it's also an effective thermometer and barometer.

The bottle has an internal volume of 375ml, and the visible part of the capillary tube has a length of 7.5cm and an internal diameter of 0.3mm. (somewhere between 0.25mm and 0.5mm, this is a guess.) This means that to go between the capillary being empty and the capillary being full, there must be a reduction of the bottles volume of 5.3 microliters, or a change of 0.00141%.

If you do some math, you'll find that to achieve a similar change via expansion due to heating an increase in temperature of only 0.068ºC is required.3)

Detail showing the top of the bottle and the capillary tube sticking out.

Demo room information

Location —-
Maker Unknown
Current State Working
1)
If possible, try to add the droplet on top when the level of water inside the capillary reaches the top. This'll prevent the formation of bubbles, which are a bit annoying to get rid of.
2)
After adding water you might find that the newly added water is lightly colored because it hasn't mixed much with the rest of the strongly dyed water. This can be remedied by repeatedly squeezing the bottle so that the water in the capillary mixes with the rest of the water in the bottle.
3)
This is neglecting the thermal expansion of the glass, which should act against this change. That value is about ten times less than the value for water though, so I chose to ignore it. The actual change in temperature required should be ~10% higher.