# On Inductance

Discussion in 'General Electronics Chat' started by kdillinger, Oct 7, 2014.

1. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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I have a quick question regarding inductance. If I were to ask you the question, "What is the inductance of this piece of wire?" what would your answer be?

It has been an awful long time since college, but is there not an assumption in this question? How can a wire have inductance when no loop is defined? No loop --> no surface area to integrate over. Said, Sir Maxwell.

I would presume once you have the return path (loop) defined, then you can start to calculate partial inductance of wires and PCB traces in circuit. THAT I am sure is another topic.

2. ### alfacliff Well-Known Member

Dec 13, 2013
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strieght wire has inductance too. it dosnt have to be coiled. ever hear of "stray inductance"? why does wire have no surface area?

3. ### wayneh Expert

Sep 9, 2010
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At DC or low frequency, it would be negligible. But consider a wire used as an RF antenna.

4. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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For inductance to exist, a current must be flowing. If I current is flowing, a loop must exist.

Right?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Inductance exists because when a current flows through a wire, a magnetic field will develop around that wire. And change in the magnetic field will induce a voltage along the wire. And this is why wire also has a inductance.

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8. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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I found a few PowerPoints by Dr. Archambeault. It is my understanding that a loop (or return path) must be defined first and then from that partial inductance can be calculated.

My question arises from discussions PCB traces where a person wants to know the inductance of a PCB track based solely on a given length. It's not that simple, and all the 'inductance calculators' one may find online are assuming *something*; I presume that *something* is a defined loop, path, closed surface.

9. ### alfacliff Well-Known Member

Dec 13, 2013
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what you are calling a loop is a completed crcuit. inductance is related to length of inductor, and the shape of the path. a streight path has inductance. a single turn coil has close to the same inductance as a piece of wire the same length as the coil. a multi turn coil has more inductance due to coupling of the individual turns than the length of wire. coils are made on printed circuits too, usually in the shape of a spiral or zigzag path. any ferous or non ferous material in the field of the coil also affects the inductance. ferous, iron and such, increase the inductance, and non ferous, brass and aluminum decrease the inductance.

10. ### The Electrician AAC Fanatic!

Oct 9, 2007
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http://www.ee.scu.edu/eefac/healy/indwire.html

11. ### crutschow Expert

Mar 14, 2008
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Inductance has no practical meaning if there is no current in the inductor. So, yes, there has to be a path for the current. But that's also true for the calculation of resistance and capacitance of a line. Do you somehow feel that inductance is different? It's not.

12. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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http://www.comsol.com/blogs/computing-the-inductance-of-a-wire/

So, let's expound a little bit. The inductance for a piece of of wire is, from what I see, calculated based on an ideal case: the wire exists in free space with no other current carrying conductors close by and the surface is bounded.

But this becomes far more complex in a real application like a PCB where one will have ground planes, VCC planes, and other current carrying conductors. To simply say "I have a PCB track that is 20 mils long, so the inductance of this trace is X" would be incorrect.

13. ### Lestraveled Well-Known Member

May 19, 2014
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When you start talking about the traces on a PCB, you are moving away from simple inductance and into transmission lines. A PCB trace has length, width, a dialectic and most often a ground plane. Now you have to consider wavelength and frequency content. Are PCB traces where you are headed?

Oct 9, 2007
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15. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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Yes. This is an exercise of 'thinking out loud'. The conversation starts out with a piece of wire as if that is equivalent to a PCB trace.

16. ### BR-549 Well-Known Member

Sep 22, 2013
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Inductance always has its' twin with it. If the circuit is steady, they will be sleeping. If the potential changes, capacitance will wake up. If the current changes, inductance will wake up. When the current changes, that changes the potential, and wakes more capacitance, which changes the current, which then wakes up more inductance. weeeeuuuuu. One loop?

17. ### nsaspook AAC Fanatic!

Aug 27, 2009
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One loop, one dual entity like in
http://en.wikipedia.org/wiki/Jefimenko's_equations#Discussion

It's not new physics, just a different way of looking at things.

Last edited: Oct 9, 2014
18. ### Lestraveled Well-Known Member

May 19, 2014
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OK. Are you ready to transition from the idealistic world of a single wire in space, to a trace on a PCB??

19. ### alfacliff Well-Known Member

Dec 13, 2013
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why all the fuss? wire has inductance., pc traces have inductance. pictures of either dont. you can make up thought games all day long and all you will do is confuse people, is that the purpose of this?

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20. ### kdillinger Thread Starter Active Member

Jul 26, 2009
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As long as you don't tell me that the only thing you need to know is the length of the track.