Do Currents Exhibit Inertia Even Without Inductance?

[lark]

The case I treated, as mentioned in the post, was electrons in copper at room temperature. You are correct that those numbers don't apply to a plasma which, as you point at, has both ion and electron charge carriers. It also has low density and long mean free path compared to a metal, and a host of special effects.

It's not true that E&M works differently, however. E&M is E&M !

It works differently in the sense that things like Ohm's law have to be
changed - I was looking around at plasma physics stuff and finding things about how an extra term gets added to Ohm's law. And there is "magnetic reconnection" in a plasma that is somehow related to the mass-inertia of charge carriers. So what I mean is, if you were writing down differential equations for current in a plasma they would look different.

Laura

 

[lalbatros]

Lark,

Currents indeed have inertia.
However they are not usually observed because of the high collision frequencies. In between two collisions, electrons (in copper for example) are indeed accelerated and their inertia does determine this acceleration. But with high collision rate the effect is not observable.

In particles accelerators, the particle beam is a current. Its inertia is observed easily: just by observing how much material the beam must cross before stopping. (but this is usually not the objective of these expensive experiments).

I plasma physics, more precisely in tokamaks, a current is induced in the plasma ring by an external magnetic field. When the plasma density is high, the plasma (fully ionised gas) behaves roughly like an excellent conductor and no inertia can be observed. However, in some circumstances, specially at low density, the most energetic electrons in this current see their collision frequency decreasing as their speed increase. These electron accelerate until they get out of the machine. These are called "run-away" electrons and can damage measurement devices. They show clearly their inertia: nothing can stop them in the plasma until they get out of the machine (within the tokamak a strong magnetic field keep most particles moving along the magnetic field lines inside the tokamak)

Michel

 

[questionmark]

inductance of seawater

an interesting case may be when you have an electric circuit comprising of two widely-separted electrodes placed into seawater passing an electric current between them driven by a source. the ions in the seawater are the charge carries for the seawater-part of the circuit, whilst electrons are the charge carriers for the remainder - in the cables. the ions have a much larger mass than the electrons. will the circuit have more/less/the same inductance compared to the case where a circuit with the same loop-area is contructed only using cables - i.e. no ions in the circuit?

 

[nikolatesla20]

Hm. All of these posts are interesting.

I too have always compared electrical inductance to physical inertia. I will agree they are not necessarily the same thing.

However, if we simply look at the behavioral level:

1. Current is applied thru a coil. The coil builds up a magnetic field. Upon removal of the current, the magnetic field collapes, thereby generating a reverse current (back EMF). This causes a large voltage to be generated (because the current has not where to go), releasing the stored energy (stored magnetic field).

2. A Physical object starts from a rest position to 60MPH. After reaching 60MPH, it makes contact with a solid graphite wall (the side of a mountain). It receives a large force in the opposite direction, causing a large release of energy (release of stored momentum).

They may not be mathematically equivalent, but they sure are mathematically comparable, even comparable at the physical observational level. And if we graphed the energy in each situation, we would see a similar graph with a large spike on the end. (Graph voltage for the coil, and graph force in lbs for the physical object).

I think the thing to realize however, is yes, the "inertia" of the inductor is not from the moving electrons. It is from the generated magnetic field. In the same vein, physical inertia is not from the body, but from stored energy in its motion. In this case I tend to think of inertia as "gravitational inductance"...

-niko

 

[leright]

Inductance has nothing to do with electrons staying in motion once they are in motion, which is what inertia describes...It has something to so with a current in a conductor being induced due to a time varying B-field. I don't see how this relates inertia to inductance.