kinetic inductance explained

Uploaded by karlberggren on 17.01.2011

Kinetic Inductance Explained
I'm gonna talk today about Kinetic Inductance. So kinetic inductance is different from what
you normally think of when you think of inductance because it's due to the kinetic energy of
the charge carriers rather than due to magnetic fields or magnetic inductance. So, if you
for instance think about any inductor, we can actually draw a kinetic inductor the same
way we would draw a magnetic inductor. It has a current, and the voltage across the
inductor varies with the rate of change of the current and the constant of the proportionality
as the inductance. If we think about the energy stored in the inductor, we can write that
as one-half L-I squared. And I should mention in this videos that I'm gonna assume that
you've got some circuits and physics background at least up to the sophomore level, so if
you need a refresher on some of these things you can leave some comments at the bottom
and we'll point out some resources that may help you.
So if we think about the magnetic field that's generated by current flowing through a wire,
what you'd realize is that that magnetic field stores energy. But there's another type of
energy that is stored from the current through the wire which we normally don't have to worry
about because in normal metals and at low frequencies, it's dissipated quickly into
the metal just through collisions and so it doesn't play a role in the circuits. But that's
kinetic energy, and the kinetic energy you can calculate from your classical mechanics,
it's just one-half times the total mass times the velocity squared. Now, we're kind of making
a simplification here to assume that everything, all these charge carriers in here, are moving
with the same velocity. But let's assume that, let's assume they have some charge E and they
have some number density which we'll call N, okay, so there's a simple relation that
relates the current to the average velocity and that's just the current as a cross-sectional
area times the number density times the charge times the average velocity, the charge carriers,
and you can re-arrange that if you like to do Algebra, and that's V equals I over A-N-E
and then you can substitute this into our kinetic energy relation which I'll do for
you. One-half times the mass which is just the mass of an electron times the number density
times the volume which is A times the length of the wire. L is the length of the wire,
so that's the total mass times the velocity which is I over A-N-E, this is squared. And
if you do the algebra carefully, make sure you don't lose any factors of, or do anything
like that, or you'd end up with one-half L-N over A-N-E squared, I squared. And then, if
you notice the analogy, this is a close analogy to the expression we used at the start for
magnetic field inductance, one-half L-I squared, and that's indeed the point because this term
here is effectively an inductance. So we call it L of K, and that's the kinetic inductance.
So, just a couple of things to note about it, so the kinetic inductance is a lot like
resistance in that it can be written as a geometric factor, the length of the wire divided
by the area, times a material definite factor which is the mass charged carrier divided
by their density times E squared, and that, so we can write that as L over A times something that we'll
call the inductivity which we'll write as a cursive L, and that's the inductivity.
So, that's all for today. This kinetic inductance plays an important role in thin superconducting
films, it dominates over the magnetic inductance in many cases. It also plays an important
role in high frequency fields interacting with metals, so it's the dominant form of
inductance in a plasmon for example, and so this area is something worth knowing about
that's not normally discussed.