Day 96: What Happens When a Copper Wire Stretches?

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In chemistry, I showed students a video of a long copper wire that stretches as weights are hung from the end. Then I asked students to draw what they thought was going on with the particles that make up the wire as the wire stretched.

We came up with several competing models:

  • the particles themselves became stretched out (on the left)
  • the space between the particles increases (also on the left)
  • the particles rearrange into a longer and narrower arrangement (on the right)

Here’s where I’m stuck: What can we do to test the different models to see what is actually happening?

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About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

3 responses to “Day 96: What Happens When a Copper Wire Stretches?”

  1. MrTschwall says :

    You could smash ionic substances with a hammer, as well as metals. The fact that the metals deform, but remain in tact would relate to what they saw. Inic compounds would break along clean lines, like salt, showing a different bonding type holding the structure together. Not sure how you could through molecular compounds in there with that method though. Still that would’ve relating intermolecular forces instead of the covalent bond. Really the metallic bonding of metals explains the ductile and malleable properties that other substances don’t have? Smashing them would certainly show that. Hope that helps.

  2. cookp says :

    You’ll have trouble validating any models from this experiment without serious imaging equipment. Atomic force microscopes will only provide a surface profile, and can give atomic scale resolution. Tunneling electron microscopes will allow you to "slice" through the wires and see the crystalline structure (which will most closely resemble the model shown in blue in your image). You’d have to contact a local university to gain access to either of these types of microscopes.

  3. Hillby258 says :

    If the molecules are being spread out, wouldn’t the wire become less dense? I’m not sure if you can be accurate enough, but it seems like a density measurement would suffice to differentiate between the two models.Also, if the molecules are being spread out, you would expect the width to remain the same, but it’s wishy washy between the two models because they both imply that the wire gets thinner.

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