Why can current flow from power line to earth ground?

  • Thread starter DoobleD
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  • #26
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you can replace R1 and the wire it is connected to with a single resistor of LOWER resistance
Assuming the wires are perfect conductors, to replace R1 and the wire below it while still having the same situation, we would have to replace them by a single perfect conductor (or equivalently, simply remove R1)?
 
  • #27
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if you wanted to replace R1 and have the same situation, you would replace the wire with a wire of a bigger x-section

Two wires of 1mm^2 each in parallel carries the same current as one wire of 2mm^2

Putting wires/conductors in parallel allows more current to flow.
 
  • #28
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Hm, not sure I understand that last point. Since the wires are perfect conductors, why do their surface area matters?
 
  • #29
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its not the surface area (well IT IS in some instances, by forget about that) it is the X-section.

There is no such thing as a perfect conductor (except superconductors).

So a conductor is typically rated by how much current it can carry (before it starts to melt!)


Lets make up some numbers (to make things simple, rather than using real data)

Say you have a wire of x-section 1mm^2 and it can carry 1 amp
If you have two wires of x-section 1mm^2 then each wire can carry 1amp, so the total carried by the two wires is 2 amps.

Two wires of 1mm^2 is the same as one wire of 2mm^2

So you could replace the two 1mm^2 wires with one 2mm^2 wire and be able to carry 2 amps




Likewise, if you had a ten x 10mm^2 cables; that can carry the same as ONE 100mm^2 cable





(now this is not strictly true, because you have factors such as heat dissapation, so lots of cables with air around them allowing them to cool will be better than one big cable, but dont worry about that for the time being)
 
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  • #30
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Got it, I thought we were still in the idealized case. Thank you very much for your help William, much appreciated. Especially since my mistakes were huge basics!
 
  • #31
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I think you always had it; just over-thinking.

As an engineer, you will soon learn when to make the simplifications necessary -
ie when to think of the wire as "ideal" with no resistance and when to think of it as a very long, but low value resistor.
 
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  • #32
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I probably won't because I'm not a "real" student, and not trying to become an engineer! :) I am an online learner, using some free resources (mainly MIT OCW) to learn physics. I graduated in computer science. It'd be nice to switch field someday though!
 
  • #33
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good for you, those MIT courses are excellent.

If you are interested in circuit theory, then try and pick up this book
https://www.amazon.com/dp/0415662869/?tag=pfamazon01-20&tag=pfamazon01-20

Its aimed at HNC/HND Engineering level students (Higher Nationals are vocational programmes which are equivalent to the first and second years of an engineering degree); and is the bible for circuit theory. It really starts at the basics, and goes through more complex problems and covers everything you will ever need to know unless you become a specialist.
 
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  • #34
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Ok, I've looked a at it a little and it seems indeed fairly awesome, so I got the e-book right away. :D And there is a chapter about transmission lines! Thanks again.
 

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