Self-inductance of a rectangular loop

In summary, the self-inductance of a loop of wire with current flowing through it is related to the magnetic field within the loop by the equation L = \frac{N \cdot \Phi_B}{I} where N is the number of turns of the wire, Phi_B is the self-inductance of the wire, I is the current, and r is the distance from the axis of the loop.
  • #1
De_Lille_D
3
0

Homework Statement


Given a rectangular loop of length C by length D (C >> D) through which a current I flows, calculate the self-inductance L of the loop (consider only the long sides of the rectangle).

Homework Equations



Self-inductance: [itex]L = \frac{N \cdot \Phi_B}{I}[/itex]

magnetic flux: [itex]\Phi_B = \int{B \cdot dA}[/itex]

magnetic field for a long conductor: [itex]B = \frac{\mu_0 \cdot I}{2\pi r}[/itex]

The Attempt at a Solution



sketch

A single loop: N = 1

C >> D, so we approximate the magnetic field in the loop by one made by 2 long conductors, meaning B is only dependent on the distance from the long sides.

Each of the 2 long sides contributes to the magnetic field constructively:

[itex]\Phi_B = 2 \int{\frac{\mu_0*I}{2\pi y} dx dy} = \frac{2*C*\mu_0*I}{2\pi} \int_0^D{\frac{1}{y}dy}[/itex]

This is where I'm stuck; the integral isn't convergent.
 
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  • #2
You're correct; if the wire is approximated as being infinitesimally thin, then the integral to calculate the flux diverges. My best guess is that either your professor wanted you to figure that out for yourself, or you have to do the calculation for a nonzero wire diameter -- which I don't think is an easy calculation.

p.s. Welcome to Physics Forums.
 
  • #3
Thank you so much! Yes, in the original question there are 2 long copper cylinders, but since it was a 2-part question, I thought that detail was irrelevant for the fist part (second part is on how much magnetic energy is left 20µs after turning off the power source). My course is in Dutch; that's why I didn't just copy the question.

Anyway, the only thing that would change are the boundaries of the integral: y = r..(D-r). This gives:

[itex]L = \frac {C \cdot \mu_0 \cdot \ln{\frac{D-r}{r}}}{\pi}[/itex]

Or should I account for the magnetic field within the cylinders as well? That integral would be divergent.
 
  • #4
Well, if the current is spread out over the cross-section of the wire, then B does not go to infinity inside the wire -- it actually becomes zero at the wire's central axis -- and the integral would not diverge. I have thought about this before, but I confess I have never come to a satisfactory answer for myself: if the current is distributed over the wire's cross-section, then where exactly is the "boundary" for the area over which flux is to be calculated?

I guess go ahead with your current approach, and use the wire's outer surface to do the integral. I'd be interested in hearing what your professor has to say about it! Meanwhile, I'm going to move this to the Advanced Physics subforum, in hopes that somebody else may chime in.
 
  • #5
Right, I meant converge...

magnetic field inside a long cylindrical conductor with radius R (r: distance to axis): [itex]B = \frac{\mu_0 I \cdot r}{2 \pi R^2}[/itex]
 
  • #6
De_Lille_D said:
Right, I meant converge...
:smile:
magnetic field inside a long cylindrical conductor with radius R (r: distance to axis): [itex]B = \frac{\mu_0 I \cdot r}{2 \pi R^2}[/itex]
Yes. The way I see it, there are two ways one might include the current inside the wire in calculating the flux:

1. Do the flux integral from 0 to r (the radius of the wire), add the result to what you have, and decide if it's a significant or a negligible contribution to the total flux;

or

2. Do the flux integral from -r to +r, so that the entire wire is included inside the area over which flux is to be calculated. Again, decide if it's a significant or a negligible contribution :wink:.

Hope that helps. Sorry I can't be more definitive in my suggestions.
 

What is self-inductance?

Self-inductance is a property of an electrical circuit that describes the ability of the circuit to generate an electromotive force (EMF) in itself when the current flowing through it changes.

What is a rectangular loop?

A rectangular loop is a type of electrical circuit that consists of a rectangular-shaped conductor, such as a wire, with its ends connected to form a closed loop.

How does self-inductance affect a rectangular loop?

Self-inductance affects a rectangular loop by creating a magnetic field around the loop, which opposes any changes in the current flowing through it. This causes the loop to resist changes in current and can lead to energy being stored in the form of electromagnetic energy.

What factors affect the self-inductance of a rectangular loop?

The self-inductance of a rectangular loop is affected by the number of turns in the loop, the size and shape of the loop, and the material the loop is made of. It is also affected by the presence of any nearby magnetic fields or other conductors.

How is the self-inductance of a rectangular loop calculated?

The self-inductance of a rectangular loop can be calculated using the formula L = μ0N²A/l, where L is the self-inductance in henries, μ0 is the permeability of free space, N is the number of turns in the loop, A is the area of the loop, and l is the length of the loop.

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