Consider a simple situation:

Of an infinite wire and a loop kept in front of it. We are to find the mutual inductance of the system.

$\phi = Mi$, where M denotes mutual inductance.

The method we adopt to solve such questions is:

  • Make current flow through the object you like and calculate the flux because of it through other object.

  • So we make current $i$ flow through the wire.

  • Calculate the flux through the square loop using proper integration techniques.

  • and end up with something like :

$\phi = \text{many constants}\times i $

Now we compare it with :

$\phi = Mi$

And say:

$M=\text{many constants}$.

I am able to understand the physical sense of these questions. Basically, if we make varying current flow through the wire we get varying flux which leads to induced emf in the square loop.

Now, what I don't understand is the physical significance of such questions:

Find the inductance of the unit length of double tape line: enter image description here

What is the purpose of these questions? To solve them, we assume current is flowing through both sheets and calculate the total flux through the region between them. But what use is it? There will be only eddy current fields between the two sheets if we make the current flowing through them vary.

  • $\begingroup$ The idea of inductance is useful for changing currents, but to calculate inductance you don't have to assume changing currents. $\endgroup$ Aug 21 '18 at 10:05
  • $\begingroup$ I think what you are asking is how the method of calculating (or the definition of) mutual inductance which is used in the 1st problem can be applied in the 2nd problem. In the 1st problem you can identify flux from the wire through the current loop, but in the 2nd problem it is not possible to identify a loop. $\endgroup$ Aug 24 '18 at 0:19

Two parallel tapes or plates make up a transmission line, in which case the currents would flow along the plates in opposite directions.

The inductance per unit length of such line (as well as the capacitance) could be used to determine its characteristic impedance: $Z_0=\sqrt \frac L C$.


The importance in my line of work is that stray inductance can cause all sorts of problems with fast signals, which change on $\mu$s and ns timescales. It can make sensitive measurements difficult and make it difficult to apply electrical pulses in experimental apparatus.


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