I am writing a lab report which aims to investigate which material is most suitable as a high voltage overhead conductor. We are suppose to produce a current-voltage graph to display whether or not the given materials follow ohm's law. How does determining whether the material follows ohm's law investigate the suitability of a material as a conductor?
What properties of a material can be determined if it does not follow ohm's law?
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That's a good question -- I have a hard time seeing that it does. It's possible that the purpose of the lab is just to learn how to determine whether a material is ohmic and that everything about choosing overhead conductors is a bunch of tripe to make the lab appear to have some relevance to some real-world problem. I see that kind of nonsense all the time in text books. Have they provided any information on what properties are valuable for high-voltage overhead conductors and why?
Hi,
Well it could be just a side issue they want you to look at, but on the other hand if it does not follow Ohm's Law but instead follows Jackobean's Law (which states that a conductor that passes 10 times the current has 10 times the resistance) then you could be in a lot of trouble if it is used as an overhead conductor.
We are also assuming probably incorrectly that if it does not follow Ohm's Law that is a bad thing, but that probably is not the right view either. For example a super conductor does not obey yet it would be pretty darn good. Note they never make a logical connection between Ohm's Law and either good or bad.
The other Law quoted above is a fictitious law just for illustration, but a real law would be something like the diode law, where even if you pass a very light current you get a 0.65v voltage drop which could be bad.
voltage vs current
No, we have been given nothing.
So it probably is just useless prattle to give some illusion of relevance. You are just trying to determine if the conductor is ohmic or not.
Which one on x axis?
Which one of y axis?
You don't seem to pay attention to detail. I am done here.
My understanding of resistance is: A current flows through a material and clashes with the materials ions which resist the currents flow and dissipates the ?energy? of the current into heat energy.
So, with that in mind, are their more free ions in non-ohmic materials? Or is there no definite answer as the electrical properties of non-ohmic materials don't have a common property other than they are non-ohmic?
voltage y axis and current x axis--> gradient equals y/x and Resistance=Voltage/Current
An ohmic material is merely one that close obeys Ohm's Law over some range of voltages and currents of interest. This, in turn, merely means that the voltage across it is directly proportional to the current through it -- the proportionality constant is the "resistance" of the material. No material (that I know of) is ohmic over all voltages and current and, even more so, over all other parameters, particularly temperature. Furthermore, even over small ranges view materials exactly obey Ohm's Law. But many materials are sufficiently close to being ohmic over a sufficiently broad range of voltages and currents that we call them ohmic even though they strictly aren't.
But all we can say about non-ohmic materials is that they do not obey Ohm's Law (over our region of interest). The relationship between voltage and current is not linear -- it could ANYTHING other than linear. It could be exponential, it could be a square law, it could even be non-monotonic. The voltage could grow faster or slower than linear with current, so I don't see how anyone could claim that ohmic materials are fundamentally better for overhead high voltage transmission lines.
When you say "a versus b" the universal understanding is that a is the dependent variable and b is the independent variable and, furthermore, that the independent variable is plotted on the horizontal axis. If you want to do something different, THEN you need to explicitly state which is which.
Hi,
I mentioned already that there was no implication that ohmic or non ohmic materials would be better in the original question. They just simply asked everyone to look at that.
In other words, if they say, "look at some trees", do they mean that trees that are smaller are worse than trees that are bigger in some way...or trees that are bigger are worse than smaller...i dont think so.
Sometimes questions are not worded right either so the interpretation is difficult or impossible. We had a question in a class a long time ago that i remember that no student knew how to interpret exactly so we had to ask for a better wording. I would also wonder if this could have been from a translation.
Okay, so how is determining whether a material is ohmic relevant to investigating whether it is suitable for use as a high-voltage conductor? This is like telling someone that you want them to write a report investigating which building material is suitable for skyscrapers by having them determine which paint color dries fastest.
Hi,
Well, i thought i said something about this already but i understand this is a unique problem.
One of the problems i see here is that we were not given a list of the materials yet. That would help narrow this down, but we dont have that so i was looking in the broadest sense that would include everything that could possibly make up a conductor, and i came up with an extreme example just to illustrate the concept.
Also, note that the question as posed exactly to start with:
"We are suppose to produce a current-voltage graph to display whether or not the given materials follow ohm's law."
and then the obvious question that would come from that:
"How does determining whether the material follows ohm's law investigate the suitability of a material as a conductor?"
This all sounds like a paraphrase because they are asking us to produce a current voltage graph, but then simply decide if the material is ohmic or not. A single vote of ohmic or non ohmic can not in itself produce a complete answer, but before we can vote we have to first draw the graphs, and thus we already have the information needed to draw a conclusion. So maybe the paraphrase is skewing the question a little, which happens a lot.
All we have to do is change one word:
"We are suppose to produce a current-voltage graph [and] display whether or not the given materials follow ohm's law."
and suddenly it makes sense
With that, let's look at some of the simplest cases of just two such 'materials' and their ohmic properties and see what we can find out. After all, that's what this question is asking. Again, this is assuming the broadest interpretation of the question. Also, in reality we would probably be looking at the material resistance curves not using formulas as used here just for some quick examples.
Example 1:
Material #1: Aluminum wire, mostly ohmic, R=V/I
Material #2: Superconducter, mostly non ohmic
And the main question: Which one makes the better conductor?
Answer: It appears that knowing the ohmic property in this case helps figure out which one is better.
Example 2:
Material #1: Aluminum wire, mostly ohmic, R=V/I
Material #2: R=V/(A*(e^(V*K)-1)), with A,K such that R<V/I and V>=+0.7
Which one is better here? Material #2 again because R is always less than any ohmic conductor (assuming at least 0.7v of the right polarity).
Example 3:
Material #1: Aluminum wire, mostly ohmic, R=V/I
Material #2: R=V*10/I
Here Material #2 is worse because V*10/I is always greater than V/I.
Example 4:
Material #1: Copper wire, mostly ohmic, R=V/I
Material #2: R=V^2/I, V such that {-infinity < V < +infinity}
Here Material #2 is worse for |V|>1 and better for |V|<1 and the same for |V|=1.
The answer here would depend highly on the applications expected voltage drop.
Example 5, more real:
Material #1: Copper wire, mostly ohmic, mostly straight diagonal line curves up slightly
Material #2: Less pure copper wire: mostly ohmic, mostly straight diagonal line curves up more than material #1
Here Material #2 has more resistance as the current increases, thus #1 wins out.
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But your examples underscore the point that the ohmic vs non-ohmic nature of the material does NOT tell you anything about whether the materials is suitable or not.
Just knowing that material A is ohmic and material B is non-ohmic gives you no information at all about which is better suited. Draw the I/V curve for a material that is ohmic and I can draw a non-ohmic curve that is "better". Draw the I/V curve for a material that is non-ohmic and I can draw an ohmic curve that is "better".
So what is the point of the lab? To gain an understanding of ohmic vs. non-ohmic material and how to determine the characteristics. I doubt it is anything more than that. All of the stuff about high-voltage overhead power lines looks like it is just eye candy. I used a text (that I inherited from the previous instructor) when I was first teaching C that was supposedly centered around teaching engineering problem solving in the context of the twelve greatest engineering problems of the time. One such problem was the development of advanced materials for aircraft design and construction that would help save the planet by reducing the energy consumption associated with air travel and cargo operations. Leaving aside the issue of whether this problem even qualified as one of the twelve greatest engineering challenges, it talked about how developing such materials involves a lot of data collection and analysis and then the assignment that students had to do related to this engineering challenge was to write a program that asked the user to enter ten numbers and then print them out in sorted order.
Oh, and whether a superconductor is ohmic or not is a matter of scale. At most practical scales it is ohmic with a resistance of 0 Ω. At the fine level it is a generally treated as a power-law device with where V=Vo(I/Io)ⁿ, but k is so small that even at thousands of amperes the voltage is in the microvolt range or less until it approaches the critical current density.
Just knowing that material A is ohmic and material B is non-ohmic gives you no information at all about which is better suited. Draw the I/V curve for a material that is ohmic and I can draw a non-ohmic curve that is "better". Draw the I/V curve for a material that is non-ohmic and I can draw an ohmic curve that is "better".
So what is the point of the lab? To gain an understanding of ohmic vs. non-ohmic material and how to determine the characteristics. I doubt it is anything more than that. All of the stuff about high-voltage overhead power lines looks like it is just eye candy. I used a text (that I inherited from the previous instructor) when I was first teaching C that was supposedly centered around teaching engineering problem solving in the context of the twelve greatest engineering problems of the time. One such problem was the development of advanced materials for aircraft design and construction that would help save the planet by reducing the energy consumption associated with air travel and cargo operations. Leaving aside the issue of whether this problem even qualified as one of the twelve greatest engineering challenges, it talked about how developing such materials involves a lot of data collection and analysis and then the assignment that students had to do related to this engineering challenge was to write a program that asked the user to enter ten numbers and then print them out in sorted order.
Oh, and whether a superconductor is ohmic or not is a matter of scale. At most practical scales it is ohmic with a resistance of 0 Ω. At the fine level it is a generally treated as a power-law device with where V=Vo(I/Io)ⁿ, but k is so small that even at thousands of amperes the voltage is in the microvolt range or less until it approaches the critical current density.
Hi,
I pointed out that knowing JUST ohmic or non ohmic was not enough, but in the quest for that information we end up being able to solve the problem anyway.
For the supposed semiconductor version, i specified the voltage and also that the resistance was always less than V/I which tells you enough to know what region it is operating in and so how it should behave. That means that the undefined constants must meet these specs, so it's guaranteed to be in the low ohms range (plenty of bias current). I was going to set the voltage at 1v but didnt want to get carried away
