# Ohms Law & Digital Controllers

#### fxsts1

Joined Feb 15, 2004
3
I am trying to figure out what Ohm's Law has to do with Digital controllers. I understand the effect it has on analog systems

#### Dave

Joined Nov 17, 2003
6,969
What exactly are you confused about? :unsure:

#### fxsts1

Joined Feb 15, 2004
3
Sorry my keyboard died yesterday before I could finsh. What I am looking for is how does the law effect digital operations, I was thinking the same way as analog but I'm not sure. I know the resistant is lower with the advent of super/semiconductors or digital could be programed to account for the resistance to give adjusted readings.
The whole assignment was quite confusing, a paper on Ohn's Law and how it effects modern digital controllers. I got the ohm's law part but lost on the last. Thanks

#### Dave

Joined Nov 17, 2003
6,969
There is a version of Ohm's Law that appiles to semi-conductors. It is similar to the form you recognise and can be simply derived from Ohm's Law itself.

Current Density = Conductivity x Electric Field

Written J = σε

Where J and ε are vector quantities

There relation to Ohm's Law is similar to looking at it by saying:

J is equivalent to I
ε is equivalent to V
σ is equivalent to R

You may look at it and say, "doesn't that mean I = RV" but remember conductivity σ is equivalent to conductance, which is 1/R.

I could explain in more detail where this all comes from if you like, I found when I learnt it, that learning the derivation from one form to the other allowed me to understand the whole concept better.

Its also difficult to think of how to apply this version of Ohm's Law without an example to work through.

#### Battousai

Joined Nov 14, 2003
141
Originally posted by Dave@Feb 16 2004, 10:54 AM
J = σε
Look at that stylish epsilon. :lol: Go Dave

"How Ohm's law affects digital microcontrollers?" It sounds like kind of a funny topic to me. Ohm's law is such a general definition.

I'm not sure how this relates to digital microcontrollers but since you brought it up...

The superconductivity challenge is to fabricate a room temperature superconductor(Certain materials are superconducting below their critical temperature aka Tc). Currently superconductors with Tc's as high as 200K (?) have been realized. The resistance of a superconductor is 0 ohms (I=V/R ==> infinite current?). The current just runs in a superconductor forever. In fact in one experiment a while ago some scientists observed constant current flow in a superconductor for over a year (yeah, they cooled some material to below it's Tc for over a year).

#### Dave

Joined Nov 17, 2003
6,969
Originally posted by Battousai+Feb 17 2004, 07:08 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Battousai @ Feb 17 2004, 07:08 AM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-Dave@Feb 16 2004, 10:54 AM
J = σε
Look at that stylish epsilon. :lol: Go Dave

"How Ohm's law affects digital microcontrollers?" It sounds like kind of a funny topic to me. Ohm's law is such a general definition.

I'm not sure how this relates to digital microcontrollers but since you brought it up...

The superconductivity challenge is to fabricate a room temperature superconductor(Certain materials are superconducting below their critical temperature aka Tc). Currently superconductors with Tc's as high as 200K (?) have been realized. The resistance of a superconductor is 0 ohms (I=V/R ==> infinite current?). The current just runs in a superconductor forever. In fact in one experiment a while ago some scientists observed constant current flow in a superconductor for over a year (yeah, they cooled some material to below it's Tc for over a year). [/b][/quote]
Cheers Battousai, cut and paste job from Word!!

I've seen a little about those superconductors, someone got a Nobel Prize for Physics a couple of years ago for there work on them IIRC. The zero resistance is something to do with the behaviour of the electromagnetic field at that temperature, I can't fully remember.

#### fxsts1

Joined Feb 15, 2004
3
Thanks for the replies it helped
Doug

#### Dave

Joined Nov 17, 2003
6,969
Just to be a little more specific for you Doug, the current density J as the name suggests is the current density per unit area, i.e. A/m^2.

The electric field ε is actually a potential gradient defined as voltage per unit length, i.e. V/m. Analysis of this shows that when dV/dx = 0 the gradient is zero and the there is no electric field, any real value of dV/dx will imply there is an electric field present. Positive and negative values of dV/dx show the direction of the field, hence this is where the vector idea I mentioned before comes in.

Think of the conductor resistance in terms of R = ρL/A, where ρ = 1/σ.