# Voltage vs Resistance. (Temperature analogy for metals)

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1. Oct 18, 2016

### DoubtfulOxymoron

Hello everyone,

From what i know electrical resistance for metals is a constant, but in fact it varies with temperature but not voltage. Explanation for metals and temperature is something like "heat, which is in fact average speed of electrons, does obstruct tranmission of other electrons called also electrical current, thus higher temperature = higher electrical resistance"
But I am not aware that voltage would affect resistance of metals (are there any extreme limits?).

Now the hard part. How can you explain the fact that high voltage does actualy decrease volume resistance of insulation materials? I am not saying there is any suitable analogy to previous example, but I am realy begging for solid explanation of this phenomenon.

I would prefer explanation on microscopic scale.
But any enlightement will be appreciated!

PS: I believe this could be better explained by material engineers, than electrical engineers.

2. Oct 18, 2016

### anorlunda

Are you familiar with the Drude Model? . Mean free path between collisions is a dominant material-dependent parameter.
https://en.m.wikipedia.org/wiki/Drude_model

Regarding insulators, I'm not sure what you are asking about. But there is a mechanism of high voltage damage that carves tracks in the medium that look like lightning bolt branching paths. Molecular structure is modified along the tracks.

3. Oct 19, 2016

### DoubtfulOxymoron

Yes thats what i thought too, it's called also electrical treeing. Some paths are being ionized more than others in insulation, this could explain decreased resistance or higher currents through the insulation. Thank you anorlunda.

Is there any detailed description why is this happening, or what material property does indicate severity of this phenomenon?

4. Oct 19, 2016

### anorlunda

A Google search on +treeing +insulation
turned up several papers on the subject.

Include the + signs in the search.

5. Oct 19, 2016

### DoubtfulOxymoron

Thanks again anorlunda, I have found useful link http://www.posterus.sk/?p=16307 . It is more clear now.

Although the phenomenon is not fully explained in science yet. Especialy I can't see very good indicator desriribing this material property. Only usefull information for me is that presence of water on ppm level is factor triggering treeing process.