Ohmic and Non-Ohmic Resistances

[Thender21]

I reviewed some information in archives that is consistent with my thoughts.

A resistor or resistance is considered Ohmic if it has a linear IV plot. A proportional increase in amperage for a proportional increase in voltage.

Ohm's law doesnt break but if the resistance changes at a given applied voltage then it may not be ohmic or in other words it is nonlinear.

A light filament is a good example. So is a diode. Even a solenoid may not be ohmic while the core saturates. Inductors generally seem like they wouldnt be considered ohmic.

For me what is mainly relevant is whether bad connections caused by loose wires that should be bolted down, or by heavily corroded wiring (green green).

If those are Ohmic, a resistance check with an ohm meter can be semi useful.

Spark plug gaps that ionize and become conductive come to mind...

What types of things tend to be Ohmic and what types arent?

 

[Diver300]

As you mentioned, lamps are an example. Filament lights will take far more than half current if the supply voltage is halved. LED lamps get interesting. Ones with normal ballast resistors will take far less than half current if the supply voltage is halved. Current limited LED lamps can take a constant current over a range of supply voltages, while lamps with switch mode drivers will often take more current if the supply voltage is reduced.

 

[ericgibbs]

A resistor or resistance is considered Ohmic if it has a linear IV plot. A proportional increase in amperage for a proportional increase in voltage.
Ohm's law doesnt break but if the resistance changes at a given applied voltage then it may not be ohmic or in other words it is nonlinear.

hi,
You are incorrectly stating Ohms law.
Ohms law says the Current thru a conductor is proportional to the applied voltage and inversely proportional to the conductors resistance.
I = V/R.
Linearity does not come into Ohms law.

So as the tungsten filament heats up its resistance increases, so Ohms law still applies.

Semiconductors are usually considered non linear devices, but these also obey Ohms law for any given value of Current and Voltage at that instant.

E

 

[KeepItSimpleStupid]

Chemistry isn't my strong point, but with connections cleanliness and the avoidance of dissimilar metals is important.

Just connecting different materials together can create a corrosive environment: http://www.engineeringtoolbox.com/electrode-potential-d_482.html

We have electro-chemical corrosion too: http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_7:_Electrochemical_Corrosion

One of the most troublesome corrosions I have seen is where flux after soldering was not removed. In one case, I could not convince the manufacturer to fix their cleaning step. Under a scanning electron microscope, corrosion was easily seen. The fix turned out to be boiling the received items in baking soda and applying an electroless gold coating. Then these probes could survive high temperatures <=200 degrees C. Without the treatment, the probes would turn black.

Older screws were Cadmium plated. This was probably a good thing. Cadmium acts as lubricant.

I don't assemble anything without anti-seize.

You had mentioned Stabilant 22 on another forum. You should look at Conducto-Lube for higher current connections. Dielectric grease is also useful. It's great for lamps. I either use dielectric grease or petroleum jelly with all bulbs in the car or home.

In the industry where I came from, fingerprints were severe contamination. I still have the habit of using disposable PVC gloves and Nitrile and neoprene when appropriate.

In the automobile, remember drip loops. A good example of a drip loop is the service entrance wiring to a house. the wire dips first, so that water drops at the dip and doesn't run into the mast head. This needs to be followed in the automobile too. Water should not be allowed to follow a wire into a connector without dropping below that connector first.

FWIW: Wikipedia defines an Ohmic contact as one with a linear relationship. http://en.wikipedia.org/wiki/Ohmic_contact Ohms law states that the relationship between voltage and current is a constant.

never tried it, but it's possible that a little moisture around a corroded connection MIGHT result in a small battery.

 

[ericgibbs]

Ohms law states that the relationship between voltage and current is a constant.

Morning Kiss,

Ohms law as defined does state that relationship between voltage and current is a constant, but no where does it mention linearity over a range of measurements.

Its only when we use Ohms law to calculate a Current value at a known Voltage and Resistance and then try to extrapolate that value to determine another Current value using the initial values of V and R is where the linearity factor comes into play.

Using the extrapolated measured Current and Voltage values it will obey Ohms law.

E

 

[KeepItSimpleStupid]

Agreed, per Wikipedia "At any instant of time Ohm's law is valid for such circuits [Composed of Resistive elements and AC voltages] , but an "ohmic contact" is one with a linear relationship as per Wikipedia as well.

Note: Edit.

 

[Thender21]

hi,
You are incorrectly stating Ohms law.
Ohms law says the Current thru a conductor is proportional to the applied voltage and inversely proportional to the conductors resistance.
I = V/R.
Linearity does not come into Ohms law.

So as the tungsten filament heats up its resistance increases, so Ohms law still applies.

Semiconductors are usually considered non linear devices, but these also obey Ohms law for any given value of Current and Voltage at that instant.

E

Maybe thats not what I seemed to say but it ks exactly what I meant.

I think you could describe the distinction as whether one current and voltage measurement will be consistent with others.

Ohms law should apply in all cases but due to a change in resistance it may be a non linear relationship.

I womder if the filamemt is non linear because heat produced by operation increases resistance.

I think they call an I^2 R - current sauared times resistance an ohmic voltage drop? I am asleep and do not remember. Sleep posting!

Wire is gauged appropriately for the current otherwise the hest from electrical rssistance would drop an unacceptable amount of voltage and potentially create a fire hazard.

Things like semk comductors and magnetic fields have more complex non linearities but isnt that the filament issue?

Small increases in applied voltage grestly reduce filamemt life, disproportionately.

Even a loose battery terminal discnnecting at times can cause a voltage spike high enough to burn a filamemt out. Maybe not on the first time.

GM has a recall where the bulbs used were high wattage for the application (continuous running) and melted the headlamp lenses and burned out.

Mercedes has one where a resistor is added to reduce voltage to brake lighfs so they last longer.

https://www.hella.com/hella-za/assets/media_global/HASA_Bulbs_Catalogue_2012_LRes.pdf

Rule of thumb: If the supply voltage of a bulb is increased by 5%, then the luminous flux increases by 20%, but at the same time service life is halved.

LIFE EXPECTANCY OF A BULB

Service life and light efficiency among other things strongly depend on the supply voltage used.

Therefore, protective resistors are used in some vehicle types to ensure that the supply voltage doesn’t exceed 13.2 V. On the other hand, undervoltage, e.g. due to a defective generator, means exactly the opposite. The light now has a considerably higher red fraction and thus light efficiency is lower.



Page 10 - cool info.

By the way I care about ohmic or non ohmic because I want to know whether a resistance measurement will be a reasonably accurate predictor of loaded circuit performancs. Or whether voltage drops on the loaded circuit must be used.

It could be either way, there are complications.

I measured resistance on a DC motor commutator , different segments and the resistances were very low so maybe the most resistance comes from "CEMF" if you call that resistance.

And what happens if the motor stalls and there is no CEMF. Lr during starting...

Sleep!!!!

 

[MrAl]

Hi,

There are really two different interpretations of what is "ohmic" and what is not. I will explain both and show a clear example of this.

Ohm's Law is the first, and can be stated as:
E=I*R

but in this strictest sense R must be a constant. That's right, R must be a constant or it does not hold. If R is allowed to vary, then there might as well be no law at all. In this way Ohm's Law is sometimes said to be "self fulfilling".

The other 'view' is that anything that dissipates heat can be called "ohmic" which comes from the expression:
P=I^2*R

and this separates it from those elements that store energy which do not dissipate heat, but here R does not really need to be constant because even if it is not there is still power dissipated.

Now how can we be sure there are two different interpretations? All we need is a single device that can be interpreted using both definitions so we can see each in it's own light. Luckily we have such a device and is quite common: the ordinary silicon diode. Looking at this device we can see the differences and similarities between the two definitions.

We can separate the physical diode into two separate parts:
1. The ohmic contact part, which follows Ohm's Law quite closely.
2. The 'diode' part which is non linear for voltage and current.

The ohmic part follows E=I*R, while the base diode part follows an exponential.

The ohmic part follows Ohm's Law, while the exponential does not. So we can see the difference because of the different curves.
However, both parts generate heat. For the ohmic part we have:
Po=I^2*R
and for the exponential part we have:
Pe=I^2*r(v)
or:
Pe=i^2*r(i)

and Po and Pe are both real powers there is no imaginary part here. The difference is that Po can be calculated knowing only the current and a fixed constant, R, while for Pe the resistance changes with either the current or voltage (or both). This makes the two functions very different.

Why think about it as two different ideas, why not just one with a variable resistance?
This is like asking why have resistors and transistors, why not just have transistors since we can use them as resistors if we bias them to our liking.

I'll post the diode curve if anyone is interested.

 

[cowboybob]

By the way I care about ohmic or non ohmic because I want to know whether a resistance measurement will be a reasonably accurate predictor of loaded circuit performancs. Or whether voltage drops on the loaded circuit must be used. ...

Ohm's Law is a linear equation (https://www.google.com/?gws_rd=ssl#q=linear+equations). As such it assumes an "instantaneous" and constant set of values for any two of the three variables and, as a result, by definition gives a result that is also a constant.

One set of variables and one result. Period. Anything changes and you must run the equation again.

I measured resistance on a DC motor commutator , different segments and the resistances were very low so maybe the most resistance comes from "CEMF" if you call that resistance. ...

At stall, a DC motor can be considered a simple, fixed resistive circuit (at least after the initial "in-rush" current has stabilized).

But, of course, motors have coils and rotate and thus they no longer exhibit simple "resistance", as defined or determined by Ohm's Law. Coils have, instead, inductance (or impedance).

Perhaps this (https://www.physlink.com/Education/AskExperts/ae517.cfm) will provide some insight into the concepts you are considering.

 

[MikeMl]

...
Semiconductors are usually considered non linear devices, but these also obey Ohms law for any given value of Current and Voltage at that instant.

Eric, I take issue with this statement.

Just because you can divide instantaneous voltage by the instantaneous current and come up with a ratio whose units are Ohms says nothing about that device being Ohmic.

 

[ericgibbs]

Mike,
I am not saying that, I was careful not to use the 'Ohmic' word, my context was passive resistors, the OP, I believe was posting about tungsten lamp filaments.

The point I was trying to make to the OP, is that Ohms law, using measured Current and Voltage values to calculate Resistance will always obey Ohms law, at the point of measurement.
Also Ohms Law, as defined does not mention linearity.

E

 

[JimB]

From the University of Guelph (Canada) we have the following definition:

1. Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that:
The potential difference (voltage) across an ideal conductor is proportional to the current through it.
The constant of proportionality is called the "resistance", R.
Ohm's Law is given by:

• 
  V = I R

where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is:

• 
  I = g V

2. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current.

Linky thingy:


And from the University of Kiel (Germany) we have:

Ohms Law and Materials Properties

**broken link removed** In this subchapter we will give an outline of how to progress from the simple version of Ohms "Law", which is a kind of "electrical" definition for a black box, to a formulation of the same law from a materials point of view employing (almost) first principles.
**broken link removed** In other words: The electrical engineering point of view is: If a "black box" exhibits a linear relation between the (dc) current I flowing through it and the voltage U applied to it, it is an ohmic resistor.
**broken link removed** The (extreme) Materials Science point of view is: Tell me what kind of material is in the black box, and I tell you:

1. If it is an ohmic resistor, i.e. if the current relates linearly to the voltage for reasonable voltages and both polarities; and
2. What its (specific) resistance will be, including its temperature dependence.
3. And everything else of interest.

And another linky thingy: https://www.tf.uni-kiel.de/matwis/amat/semi_en/kap_2/basics/b2_1_3.html


The item from Kiel University has MUCH more information if any one wishes to follow it up.


From a plain old electrical engineering point of view

R = V/I where R is the Constant of Proportionality, ie it is LINEAR.

JimB

 

[cowboybob]

Also Ohms Law, as defined does not mention linearity.

True enough, eric, but the equation is linear and as such, in a single use, is incapable of rendering anything but a linear solution.

 

[Thender21]

Hi,

There are really two different interpretations of what is "ohmic" and what is not. I will explain both and show a clear example of this.

Ohm's Law is the first, and can be stated as:
E=I*R

but in this strictest sense R must be a constant. That's right, R must be a constant or it does not hold. If R is allowed to vary, then there might as well be no law at all. In this way Ohm's Law is sometimes said to be "self fulfilling".

The other 'view' is that anything that dissipates heat can be called "ohmic" which comes from the expression:
P=I^2*R

and this separates it from those elements that store energy which do not dissipate heat, but here R does not really need to be constant because even if it is not there is still power dissipated.

Now how can we be sure there are two different interpretations? All we need is a single device that can be interpreted using both definitions so we can see each in it's own light. Luckily we have such a device and is quite common: the ordinary silicon diode. Looking at this device we can see the differences and similarities between the two definitions.

We can separate the physical diode into two separate parts:
1. The ohmic contact part, which follows Ohm's Law quite closely.
2. The 'diode' part which is non linear for voltage and current.

The ohmic part follows E=I*R, while the base diode part follows an exponential.

The ohmic part follows Ohm's Law, while the exponential does not. So we can see the difference because of the different curves.
However, both parts generate heat. For the ohmic part we have:
Po=I^2*R
and for the exponential part we have:
Pe=I^2*r(v)
or:
Pe=i^2*r(i)

and Po and Pe are both real powers there is no imaginary part here. The difference is that Po can be calculated knowing only the current and a fixed constant, R, while for Pe the resistance changes with either the current or voltage (or both). This makes the two functions very different.

Why think about it as two different ideas, why not just one with a variable resistance?
This is like asking why have resistors and transistors, why not just have transistors since we can use them as resistors if we bias them to our liking.

I'll post the diode curve if anyone is interested.

I believe I understand. A diode's conductivity changes depending on the magnitude and polarity of the applied voltage. My meter has a diode check function, I have heard .7V may be needed to "forward bias" it into a conducting state.

Some diodes, Zener? or Avalanche, will break down into a conducting state for a short time.

What is P? Power, in what terms exactly / how is it related to electrical units?

It seems to be a description of a measurement of work? I am tired :-(

Watts would be a measurement of the rate at which work is being done, by comparison?

Volts, of potential difference in electrical Charge,

Charge measured in coulombs.

And amps = movement of charge per second.

 

[MrAl]

Hello again,

Yes, P is power in watts, and can be calculated knowing other things like:
P=E*I (voltage times current)
P=I^2*R
P=E^2/R

Power gets a little more complicated when we have time varying signals, like sine waves and exponential waves, pulsed waves, etc. Then we have to describe the power as the "instantaneous power" or take the average over time and then we call that the "average power". Usually we use lower case for instantaneous and upper case for average:
p(t) would be instantaneous power,
P would be average power.

Average power is the sum of instantaneous powers over time divided by the period:
P=(1/Tp)*Integral(p(t)) dt, integrated from 0 to Tp.

and this can be easily approximated by just summing the powers over short time periods and dividing by the period:
P=(1/Tp)*(p(t1)*dt+p(t2)*dt+p(t3)*dt+...+p(tn)*dt)
where dt is just some small time increment.

 

[ericgibbs]

True enough, eric, but the equation is linear and as such, in a single use, is incapable of rendering anything but a linear solution.

hi Bob,
I am talking about the Ohms Law definition, not the equations derived from the law.

Obviously a single point measurement of I=V/R must be linear, but I still see online statements saying that for a tungsten lamp, that Ohms Law is not linear, which is just nonsense, when they really mean the Resistance plot of the lamp filament is non linear.
Some even show graphs of the 'Ohms law' how it is non linear.

E

EDIT:
https://www.bbc.co.uk/schools/gcseb...tyintheory/voltagecurrentresistancerev5.shtml

EXTRACT:
The filament lamp
current on the y axis and voltage on the x axis. A slightly curved line goes through the graph at 45 degrees.

Relationship between current and voltage for a filament lamp

The filament lamp does not follow Ohm's Law. The resistance of a filament lamp increases as the temperature of its filament increases. As a result, the current flowing through a filament lamp is not directly proportional to the voltage across it.
This is the graph of current against voltage for a filament lamp.

 

[KeepItSimpleStupid]

Hey. Ohms law doesn't exist in real life, but it's a really good aproximation when constrained.

There are parasitic inductances and capacitances in every real device. There is a circuit that won't work if three resistors are replaced by the series commbination. There are circuits that behave differently when SMT resistors are mounted on the edge. Voltages get generated in a wire in the earth's magnetic field. Voltage breakdown occurs at high voltage. Every "resistor" is a noise source reducing in magnitude depwnding on temperature. Absolute zero doesn't exist.

When I know when I can apply ohms law R=V/I, it works well. Two metals joined at different temperatures creates a voltage source.

Temperature will really mess it up.

Cosmic rays messed up RAM chips.

I learned that electrons have circular orbits. They don't. It's the probabilty of finding an electron in a given space.

I learned that current flowed from + to negative. It doesn'tflow at all and electrons go the other way.

Lets keep things simple.

I've witnessed first hand that wigglih a wire generates a current and so does pushing on some types of insulators. I could meassure the conductance of paper and worked on detectors that needed to be at liquid nitrogen temperatures. A week at room temperature ruined the sensor.

 

[Ratchit]

I believe I understand. A diode's conductivity changes depending on the magnitude and polarity of the applied voltage. My meter has a diode check function, I have heard .7V may be needed to "forward bias" it into a conducting state.

Some diodes, Zener? or Avalanche, will break down into a conducting state for a short time.

What is P? Power, in what terms exactly / how is it related to electrical units?

It seems to be a description of a measurement of work? I am tired :-(

Watts would be a measurement of the rate at which work is being done, by comparison?

Volts, of potential difference in electrical Charge,

Charge measured in coulombs.

And amps = movement of charge per second.

I have published this on other threads previously.

I would like to call your attention to a misnomer in electrical science of the formula V = I*R, which is wrongly called Ohm's law.

OK, let's look next at a couple of good physics texts.

I will first quote from a college textbook called Physics, by Halliday & Resnick, 1967, page 780. It was written by David Halliday, Professor of Physics, University of Pittsburgh and Robert Resnick, Professor of Physics, Rensselaer Polytechnic Institute

-----------------------------------------------------------------------
"We stress the relationship V=I*R is NOT a statement of Ohm's law. A
conductor obeys Ohm's law only if its V vs. I curve is linear, that is, if R
is independent of V and I. The relationship R=V/I remains as the general
definition of the resistance of a conductor whether or not the conductor
obeys Ohm's law. . . . . . . . . Ohm's law is a specific property of
certain materials and is NOT a general law of electromagnetism, for example,
like Gauss's law."
-----------------------------------------------------------------------

Next a quote from another college textbook called Physics for Scientists & Engineers, by
Raymond Serway, Third Edition, 1990, page 745. It was written by Raymond A. Serway of James Madison University

-----------------------------------------------------------------------
"A current density J and an electric field E are established in a
conductor when a potential difference is maintained across the conductor.
If the potential difference is constant, the current will also be constant.
Very often, the current density in a conductor is proportional to the
electric field in the conductor. that is J=sigma*E where sigma is called the
conductivity of the conductor. Materials that obey the above equation are
said to follow Ohm's law, named after Georg Simon Ohm (1787-1854). More
specifically,

Ohm's law states that for many materials (including most metals) the
ratio of the current density and electric field is a constant, sigma, which is
independent of the electric field producing the current.

Materials that obey Ohm's law, and hence demonstrate this linear
behavior between E and J, are said to be ohmic. The electrical behavior of
most materials is quite linear for very small changes in the current.
Experimentally, one finds that not all materials have this property.
Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is
not a fundamental law of nature, but an empirical relationship valid only
for certain materials."
-----------------------------------------------------------------------

OK, what could be clearer? Ohm's law refers to the linearity between voltage and current, not relationship between voltage, current, and resistance. Yet the formula, V = I*R has been misnamed across countless classrooms, books and discussions. What does your textbook or school teach?

To summarize, certain materials like conductive metals follow Ohm's law, in that their V vs.I curve is linear. Ohm's law is a property of a material, not a general law of nature. Other conductive entities like diode junctions or gas discharge bulbs do not have the Ohm's law property, because their conductivity changes depending on what voltage or current is applied, causing their V vs. I curve to be nonlinear. In all cases, V = I*R is always correct, but it should not be called Ohm's law. It should be called the resistance or impedance formula.

No matter what you call V = I*R, circuits will still get designed and analyzed, and science will still progress.

Ratch

 

[MikeMl]

...No matter what you call V = I*R, circuits will still get designed and analyzed, and science will still progress.

What do you call V/I?
What do you call dv/di?

 

[Ratchit]

What do you call V/I?

I call it voltage divided by current. What do you call distance/speed?

What do you call dv/di?

I call it the derivative of voltage with respect to current. What do you call d (distance)/dt ?

Ratch