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How do electricity and superconductors work?

by h1010134
Tags: electricity, superconductors, work
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h1010134
#1
Dec23-12, 07:53 AM
P: 11
Electricity has been a mysterious entity plaguing my mind since last year when I learnt about it in class. I am unable to understand why electrons move the way they do when a circuit is formed, nor what is the driving force to make them do so (which is voltage difference, and I don't understand that either). Electron flow has been analogued as a water pump system in class, and that does work in allowing me to visualise resistance etc, as how water in the analogue comes into contact with the walls of the pipe. However, the analogy fails to show me what is voltage, as the water pump does not magically work on it's own - it requires electricity. So, I'd like to be clearer on how electricity works, preferably explained with a visual image.

As to superconductors, I already have an image of it, gained from some source on the web. The image is this - Normal conductors are shown as a messy train station, with electrons analogued as people, moving to the trains in a disorderly fashion, thereby bumping into other things (nuclei) and other people (hence the resistance). In superconductors, these people pair up and move to trains orderly, thereby not hitting anything. What I'd like to know is why electrons pair up in superconductors, especially since they should repel each other due to their like charges.

Any response would be appreciated. Thanks!
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mfb
#2
Dec23-12, 08:22 AM
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P: 11,917
Voltage difference => electric field => force on electrons

In the water pipe analogy, voltage corresponds to pressure or height of the water level.

as the water pump does not magically work on it's own - it requires electricity.
And electric power supplies do not work on their own either, they need other forms of energy (chemical in batteries, mechanical in generators, light in photovoltaics, ...).

What I'd like to know is why electrons pair up in superconductors, especially since they should repel each other due to their like charges.
Their interaction with the atomic lattice and its vibrations gives a (small) attractive force. For low-temperature superconductors, this can be described with the BCS theory. For high-temperature superconductors, it is an open question.
Drakkith
#3
Dec23-12, 11:01 PM
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Quote Quote by h1010134 View Post
Electricity has been a mysterious entity plaguing my mind since last year when I learnt about it in class. I am unable to understand why electrons move the way they do when a circuit is formed, nor what is the driving force to make them do so (which is voltage difference, and I don't understand that either).
Voltage is a difference in electric potential. Imagine you have a positive charge and a negative charge separated a short distance apart. Since they attract each other they will feel a force that accelerates them towards each other. This is electric potential. The POTENTIAL for work to be done. (Work being the movement of the charges)

Now, if you throw a circuit into an area where electric potential is the same everywhere, you won't get any net movement through the circuit. Your circuit as a whole will just have a slight imbalance in charge between two sides that aligns with the electric field. (AKA negative charge will build up slightly on one side and positive on the opposite) You need a DIFFERENCE in potential to cause current to flow. Imagine you are on a plateau. You have gravitational potential energy, as does your buddy nearby, but you can't use gravity to do any work between you. If however, your buddy stands off the side a little bit and is now lower, he has less potential energy than you, and you now have a DIFFERENCE in potential energy between the two of you. You can use this difference to do work, such as dropping a ball to him. The same applies to a circuit. If you have a DIFFERENCE in electric potential then charges at one point will not have the same force as charges at another point and they will move.

The full technical version can be found at the link below, as electric potential is a measure of the amount of electric potential energy of a particle. From the example above each charge has potential energy since they are attracted to each other.

http://en.wikipedia.org/wiki/Electric_potential_energy


Electron flow has been analogued as a water pump system in class, and that does work in allowing me to visualise resistance etc, as how water in the analogue comes into contact with the walls of the pipe. However, the analogy fails to show me what is voltage, as the water pump does not magically work on it's own - it requires electricity. So, I'd like to be clearer on how electricity works, preferably explained with a visual image.
It doesn't really matter how the analogy does it, just realize that voltage is pressure from the pump. You could replace the electric pump with a manual one. Either way it's just an analogy, so you won't be able to visualize it completely no matter what is said.

As to superconductors, I already have an image of it, gained from some source on the web. The image is this - Normal conductors are shown as a messy train station, with electrons analogued as people, moving to the trains in a disorderly fashion, thereby bumping into other things (nuclei) and other people (hence the resistance). In superconductors, these people pair up and move to trains orderly, thereby not hitting anything. What I'd like to know is why electrons pair up in superconductors, especially since they should repel each other due to their like charges.

Any response would be appreciated. Thanks!
This is a complicated topic. What happens is that as the electrons move through the lattice, they leave behind a "hole" where they used to be. This hole has a positive charge and attracts another electron to it. At low temperatures two electrons can "pair up" because of this effect. Since they are now paired up, in order for one electron to bump into anything, it must break this bond between itself, the hole, and its paired electron. This requires energy to break the bond. At low temperatures there isn't enough energy to break this bond so the electrons don't "bump" into anything and thus have zero resistance. This is just a basic explanation, and an understanding of Quantum Mechanics is necessary to really grasp this topic.

h1010134
#4
Dec25-12, 10:59 AM
P: 11
How do electricity and superconductors work?

Wow QQ that's going to be alot of reading material ^^ thank you!


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