Hello again,
I think Eric's example of the incandescent bulb is a good one to look at.
The true relationship between E and I is exponential like:
r=A*v^B {note v is a variable here}
yes we still call it 'ohmic' sometimes because it dissipates power as heat.
But it doesnt follow Ohm's Law until we force it to by linearizing the curve at one particular voltage point for example:
Rv=A*B*I^(B-1) {note I is a constant here}
and now we assume it obeys Ohm's Law over a small operating range:
E=I*Rv
But in this case we forced it to obey Ohm's Law and realize that this will only hold over a short range of I.
However, the device dissipates power over the entire range so sometimes we call it ohmic anyway because resistance is an energy dissipator. Perhaps a better terminology would be to simply call it 'resistive'.
Wikipedia Ohmic Contact said:An ohmic contact is a non-rectifying junction: an electrical junction between two conductors that has a linear current–voltage (I-V) curve as with Ohm's law.
Put the worms back in the can please.
The incandescent lamp is a "resistive element'.
If you fix temperature and some other variables and R=V/I holds true for the specified region of interest, it's ohmic. Passing current will likely raise it's temperature and R=V/I will fail. At some point, there will be voltage breakdown.
Yea, so it's k= V(temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, C negligible, negligible etc. ) divided by I (temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, Capacitance negligible, Inductance negligible etc. ) it's "ohmic"
The incandescent lamp can be thought of as a "current dependent resistor".
The NTC and PTC thermister is a temperature dependent resistor.
If I fix temperature what good are they?
"Ohmic" is an adjective e.g. "Ohmic contact"
The key here is the "region of interest". I expect an "Ohmic contact" to be a linear relationship between V and I under the specified conditions. I would not expect it to look like a diode, because that is a "rectifying contact" and I don't expect it to act as a voltage source.
When we join dissimilar metals we call that a "junction" or even 1/2 junctions. We have terms like "contact resistance", incandescent lamps, NTC thermisters and PTC thermisters.
If you want to put your NTC or PTC thermister in a temperature controlled oven, I'll let you call it ohmic, otherwise forget it.
Voltage breakdown is irrelevant to the definition of ohmic. R=V/I is always true no matter whether it is ohmic or not. Ohmic is defined according to the physics text book I quoted as having a linear V-I curve or constant resistance over the range of interest.
Hi again,
Well we have to be careful with this R=V/I.
In the universe we dont always find resistance just because we find voltage and current. If something is non resistive it of course can still have a V-I curve. That makes it non ohmic, or at least that's how it is sometimes referred to (non resistive is probably the better terminology again). In this case it does not dissipate heat. I am sure you are aware of this i just wanted to point it out for clarification in the thread. So R=V/I isnt always true.
Hi again,
Yes, but you only need one contrary instance to show that R=V*I isnt always true, which was my main point, which you just proved so well
Yes, storage elements are not ohmic so they wont follow R=V*I in general because there is no R. I was pointing that out partly because it's not true for some elements in the universe and partly just to show something that was really non ohmic, or as we all seem to prefer, non resistive.
Hi,
Yes but this is a tangent to the main idea. The main idea does not require a discussion on parasitics.
All insulators are dielectric and all dielectrics have an effective series resistance and parallel leakage. Air in sparkplugs that ionizes a and detonate have a negative resistance in series with the positive ESR.
Reactive elements if they were ideal ( which does not exist ) would not have a series resistance, but they every one has a conductor with loss and the dielectric and a conductor such as film so they all have ESR.
Basic everything has resistance except perhaps a vacuum with no contamination, until sufficient voltage is applied to cause breakdown.
Basically everything that exists in nature and man-made has resistance such that with sufficient voltage or current, thus heat is generated.
Obviously ideal parts are a goal to reach that often comes at a compromise in cost, size, or some other attribute.
Many material have a strong temperature coefficent such as some Metal OXides used for PTC fuses or ICL (NTC) inrush current limiters, Tungsten when hot is about 10x the resistance of cold.
All diode and semi's when saturated have a bulk series resistance or ESR that may be called RdsOn or Rce in BJT's that is fairly constant within a wide tolerance.
Hi,
Ratch:
Perhaps you were talking about something else. It sounded like you were talking about the series resistance that almost always accompanies inductance or capacitance. I apologize if it misinterpreted your intent.
You said before something can be non ohmic, it has to have resistance. So it sounds like you are saying that everything has resistance, but resistance is considered a parasitic in cases of storage devices, where the main part of the behavior is the storage of energy not the dissipation of energy. Feel free to clarify your meaning though.
Tony:
Everything real has resistance in most cases, but when we talk theory we separate the various components so that we can understand the pieces better. Only after we study the pieces do we put it back together again and then start making observations about the whole. In the case here, the inductance or capacitance stores energy and does not dissipate energy, and this is even true in real life if we are just talking about the energy storage part and neglecting the parasitics. So we can say that the capacitive part is non ohmic.
Ratch which part did you not understand? Conductance is a property of every material, without exception. Leakage is a weak parallel conductance specified for dielectrics but all dielectrics which are insulators have a rated series leakage current and thus effective series resistance.How does the above eclectic series of facts pertain to the idea under discussion?
Ratch
Ratch which part did you not understand?
Conductance is a property of every material, without exception. Leakage is a weak parallel conductance specified for dielectrics but all dielectrics which are insulators have a rated series leakage current and thus effective series resistance.
Materials are either conductors, insulators or semiconductors. All have ohmic losses.
One non-Ohmic loss is associated with magnetic eddy currents which are not ohmic in nature but rather due to the resistance of rotating magnetic domains. THese are the only "non-ohmic" losses I can think of but these are magnetic losses not conduction losses.
Acadamia uses the term "non-ohmic" to mean OHm's Law does not apply. I reject this definition, because companies that make MOSFETS and BJT's specifiy an OHM's LAW equivalent series resistance so I can use OHM's LAW all the time for any saturated diode. Diodes Inc have more than 50 patents on their devices with all have Rce specified which satisfies Ohm's Law.
If you have followed any of my forum answers you would already know this as I used Ohm's Law all the time for Semiconductors including LEDs. Thermal characteristics affect all devices so Ohm's Law strictly applies for a fixed material temperature with a coefficent.... which is why all semi's are rated at 25'C
MrAl,
I am saying that before nonohmic or resistive nonlinearity can be declared, a resistance must exist.
Ratch
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