Why is current density a vector

[chiddler]

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?

 

[milski]

Because current is a movement of charges - it has direction of its own. For example in a conductor, the current will be along the length of the conductor and so will be the current density.

 

[chiddler]

but it's not measuring direction. it's just measuring how much current is in a given cross section.

for your example, current must be along length. where else could it be? for this reason, making it a vector seems unusual.

 

[milski]

Just because the direction is obvious does not mean that it is not there. 🙂 You could have a huge metal piece with some parts of it inside magnetic field and current flowing in it in a manner different from what you're used to seeing.

In other words: current is movement of charges. Movement has direction, so whatever else you want to do describing that movement can include this direction as well. In certain cases it might be simpler just to talk about the magnitude and ignore the direction but that does not mean it is not there.

 

[chiddler]

eh. i like it as much as "cathode ray" being electrons.

but i'll take it.

thanks for your help.

 

[milski]

I cannot think of a case which would be on the MCAT and it would matter if it's a scalar or a vector.

An analog for fluids would be considering only pipes with laminar flow and ignoring currents in open sea.