Aamc 8 ps #34

[Tenebrus]

The question asks for a characteristic of a good insulator but poor conductor. Choices were:

A. material contains no electrons
B. magnitude of electric field is always zero inside material
C. atoms in material can easily move from one side to other
D. electrons in material cannot move from one side to the other

Correct answer is D.

I was able to eliminate A and C, and I realize that D clearly looks like the more correct answer, but I'm curious as to what would happen if B were true? Wouldn't that prevent the movement of charges also?

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[MangoPlant]

No, think of a charged metal sphere. The electric field inside is 0 but it is still conductive because it has freely moving electrons. An electric field of 0 does not imply that the electrons cannot move.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/potsph.html

 

[Trayshawn]

Not sure I get the charged sphere example, but heres another way to think of it.

The only way for the electric field to ALWAYS be zero at any point in a material would be if it had no electrons (or any other charged particles for that matter)!! That makes it essentially the same thing as choice A, which we know is not a defining characteristic of an insulator.

There CAN be regions of space where the E field is zero in the presence of charge if the forces at that point cancelled out. But a 0 E field couldn't be true EVERYWHERE in unless there were no charges period.

Hope that helps, and I hope someone checks out my question - its killin me!

 

[Tenebrus]

Thanks for the help, I think I get it now!

 

[brownbaglunch]

Answer A seems unlikely as all materials (afaik) have protons and electrons. I would be highly suspect of this answer unless everything else seemed very wrong.

The electric field would theoretically be zero inside a perfect conductor. In response to an external electric field, the electrons of the conductor would re-arrange themselves with the charges oriented on the perimeter so as to eliminate any charge inside the conductor. While in actual practice this won't be strictly true, it is true enough that it makes answer B opposite of what is asked.

Answers C and D are opposite, so you should suspect that one of them is correct unless you see something better. At this point I crossed off A.

Electrons in an insulator are held strongly to their nuclei such that they resist the force of an external field and are not pulled along. Electrons in a conductor flow freely. B and C describe the same situation (good conductor, poor insulator). The correct answer is definitely D.

 

[rjosh33]

Electrons in an insulator are held strongly to their nuclei such that they resist the force of an external field and are not pulled along. Electrons in a conductor flow freely. B and C describe the same situation (good insulator, poor conductor). The correct answer is definitely D.

I think you meant the other way around. 🙂

 

[brownbaglunch]

I think you meant the other way around. 🙂

Indeed I did =). Fixed