Resistance is the first thing we learn in our physics classes after current and voltage. We know it is the property of a material due to which it resists the flow of current through it. In this post, I'll share with you some basics about it so I get to talk about the advances we have made in this field. In short, I'll be showing you the past, the present and the future of this property that every material has.
Just for a note, the scientists that studied electricity back in eighteenth century called themselves "The Electricians". This happened for almost a hundred years from around 1750 to 1870. The term electrician was later given to technicians from around 1870.
Our story begins at the birth of this concept. Two important things were noticed as soon as phenomenon of electric current was discovered.
The very first thing was that some materials like the metals and water etc. allowed current to flow through them while the nonmetals, wood, etc. didn't. This made us divide all materials in the two categories which we now know as conductors and insulators. Or we can simply say - those materials which conduct electricity, and those which don't.
The second thing we noticed was the metals, when conducting electricity, voltage and current was found to be different for:
- Different materials
- Same material with different sizes
- And also for same material of same size at different temperatures.
Studying these, we get the concept of resistance.
Ohm’s Law:
At a constant temperature, the value of current through a wire was found to be in direct proportion with the voltage applied across it. A fundamental law in electricity was hence discovered. This happened in 1827. You know this one... v α i, also written as v = ir. And as we all know, r, the proportionality constant was given the name resistance.
Fact no. 1 : The resistance of a conducting material remains constant unless it's size or temperature changes. For the same size, r rises as the material gets hotter and decreases as it gets cooler.
So as I said earlier, ohm's law means, only at a constant temperature, v = ir.
Using this law, the resistance values of all materials could now be calculated.
The advantage of electricity was that it could be transferred to any distance using wires. But one more problem was observed as the distance increased. Longer the material used, more the resistance it offered. And wider the material used, lesser the resistance it offered. And then this was studied, which gave birth to the concept of resistivity.
r α l/A
This means resistance is in direct proportion (increases) with the length of wire, and in inverse proportion (decreases) with the area of the same. The proportionality constant here, was given the name resistivity (ρ).
Another important concept is electric power, by which we simply mean the voltage and current multiplied.
p = vi
And if we put ohm's law in it, we get
p = (i.r) = i2r
And also, p = v (v/r) = v2/r
We also learned the series and parallel connection of wires at the same time. The increase in length of a conductor is just like two conductors in series, total resistance increases in both cases. Increasing area is just like putting them in parallel, total resistance decreases in both cases. I won’t go in detail to keep things simple, plus it’s basic, so most probably you already know these things.
Going further, what do we get from the concepts (and the calculations) of electric power and resistance?
The answer lies in the temperature rise that we observe when a material is conducting electricity. When we pass current through a wire, we are trying to transfer energy through it, in the form of electricity. And the wire, as it is heating up, implies that some energy is being lost in the form of heat. We can also say it is being consumed by the network. And we later found superconductors that do not consume any energy while current is being passed through them. A Dutch physicist, Heike Kamerlingh Onnes got super lucky while working on this. We’ll talk about him later.
Fact no. 2 : For some amount of power being supplied, every electric network consumes some amount of it and loses it in the form of heat.
For a constant voltage source, for example, a battery, the heat lost while passing current through the conductor is p = i2r. And hence we also use the term i2r loss in literature for heat loss. We don't call it v2/r loss for a good reason, let’s see why.
We can see, using i = v/r, we can also have
p = vi = v*(v/r) = v2/r
which seems just fine.
The problem here is, the resistance of materials can drop to zero. And that would complicate the equation p = v2/r a lot, while keeping it simple in case of p = i2r, which is correct since power dissipated really is zero when r becomes zero.
Now, all this time, we talked about the heat loss and all the stuff related to our fact no. 1 and 2. This is just enough knowledge for us to advance into the new things we have acquired to this date - the superconductors. Of course there is some quantum mechanics involved as well, which I will explain when we go into detail, but today, we will see where we have got with it, and what we are expecting from the future in this field.
This is almost how resistivity varies with temperature, forgive the errorss if any, didn't use a plotting software, it's just powerpoint...lol.
The effect of decreasing temperatures is super cool, well that’s the definition, but I meant awesome. Plus it earned some Nobel prizes!!
The first Nobel prize was given to the guy I talked about earlier, Onnes for his invention of liquid helium in 1908, which helped him in studying superconductivity, which earned him a second Nobel prize just three years later!! Super lucky guy huh?
When we drop the temperature of “some” material to the point where its resistance falls to zero, we call it a superconductor. This means no heat loss problem to deal with (i2r = 0). This temperature, below which the material becomes a superconductor, is called its critical temperature, Tc, also called as transition temperature.
The critical temperature of pure metals is very low. The first time superconductivity was observed by Onnes in Mercury, was at about 4.2 ⁰K, or almost -269 ⁰C. These days we have superconductors at relatively higher temperatures, of at most 203 ⁰K or -70 ⁰C for hydrogen sulphide under 150 GPa pressure, which we call HTC (High Tc) superconductors which earned Nobel prizes too.
Although one might say, since the resistance has already proven to be decreasing as the temperature decreases, it only seems logical that the resistance has to become zero at some point. This is not true. Copper, one of our most beloved conductors at room temperature, does not have this quality. And neither does gold. Superconductivity, actually has to do with how electrons in the atoms behave at low temperatures. If the electrons can form pairs called cooper pairs, the materials become superconducting. For this reason, it can only be described by what happens at a subatomic level, and this makes it a quantum mechanical phenomenon. Of course I'll dedicate a post to describing it in great detail, hopefully very soon, but for now, let's see some interesting facts.
Almost all images of superconductors show a magnet over them or a wire joining two of them to show either the meissner effect or the josephson effect respectively. We will talk about these in the next post.
So, back to our discussion, when a material becomes a superconductor, and some small amount of current is applied to it, the current never decays. It just stays there in the conductor forever, without any power source. I mean this, not only in a theoretical sense, but also practically. To give an idea of "never" decaying current, we can compare it with the longest amount of time we know - the age of the universe. Let's see what we get.
You see, absolute zero is a mathematical concept. There is no such thing as a zero quantity in physics, unless it is a convention. If we say there is something with zero amount, we mean that the quantity is so small that it can't be detected by any of our measuring instruments. So when we detect superconductivity and measure the resistance, we find that it is too small to be measured by any instruments we currently have. The heat loss given by i2r would be too small to detect too. Hence the decay in current can't be measured either. But, if we use some math, then theoretically, we might have something. And this something is the time taken by current in a superconductor to decrease to zero on its own, only due to heat loss, and this was found to be larger than the age of the universe!! So I guess now you understand what I meant when I said the current in a superconductor "never" decays.
We also have some materials that show something we call superinsulator properties at low temperatures. Meaning their resistance increases to infinity, but this has very few applications.
And if anyone asks what advancements have we made with the concept of resistance in almost two centuries, the answer would be, high temperature superconductors, which have a lot of hope in the future. We do not have a superconductor at room temperature... yet, but hopefully very soon. Seventeen people have earned Nobel prizes in this field so far. I’ve given the wikipedia page in the reference. And the list will keep adding up, there is no doubt about that.
Plus we don't have a theory that could help us predict where to look for a better HTC superconductor yet. And there is so much to talk about this field, but the article has already got too long. I just hope I gave a basic idea of what resistance and superconductors are, and in the next post I'll simply move with superconductors and some electromagnetics and quantum mechanics involved in it along with some modern applications like quantum computers!!!
See y'all....Take care.
Feel free to check out the references.
https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance
https://en.wikipedia.org/wiki/Superconductivity
https://en.wikipedia.org/wiki/High-temperature_superconductivity
https://home.cern/about/engineering/superconductivity
The part about resistance and power is basic physics and can be found in any college level physics textbook.