- Thermal Properties Of Matter
- Heat And The First Law
- Change Of State
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- Kinetic Theory
- Gas Law
- Entropy And The Second Law
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Phase Change in Iron
Purpose:
To demonstrate Solid-Solid Phase change.
Procedure:
A 2.5 meter length of music or "piano" wire gauge No. 12 (0.029" dia) iron wire is suspended at its ends by ceramic insulators. A small weight (typically 50g) is suspended from the middle of the wire. A vertical scale is placed behind the weight to make its motion more evident.
A Variac attached to the wire is turned up until the wire glows yellow-red, expands and sags. The wire should be heated above 910ºC (1180ºK). Turn up the Variac slowly from 0 to about 95 V, 15 A. Leave the wire at a high temperature for a few seconds to allow the entire length of the wire to heat up. Turn off the Variac.Iron atoms are arranged in a body-centered cubic pattern (BCC) up to 1180ºK. Above this temperature it makes a phase transition to a face-centered cubic lattice (FCC). The transition from BCC to FCC results in an 8 to 9% increase in density, causing the iron sample to shrink in size as it is heated above the transition temperature.
This transition is more visible on cooling. When the Variac is turned off the wire will rise a few centimeters, halt momentarily and drop one to two centimeters, and then continue rising back to its original height.
Hints:
The iron wire is live and hot! DO NOT TOUCH!!! The variac should be set to 0 V before turning on to prevent blowing a fuse. Turn off the power immediately,
if the wire breaks.
Comments:
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