Understanding self-inductance in a solenoid.

  • Thread starter Thread starter zenterix
  • Start date Start date
AI Thread Summary
The discussion focuses on understanding self-inductance in a solenoid, approximating it as an infinite solenoid with a defined magnetic field. The changing magnetic field induces an electromotive force (emf) due to electromagnetic induction, as described by Faraday's law. The self-inductance is expressed as a proportionality constant between magnetic flux and current, leading to the concept of self-induced emf that opposes changes in current, in accordance with Lenz's law. The participants explore the implications of the induced current and its relationship to the solenoid's loops, questioning how the back emf is calculated and its effect on the overall current. The discussion emphasizes the importance of understanding these concepts for deeper insights into electromagnetic phenomena.
zenterix
Messages
774
Reaction score
84
Homework Statement
I'd like to understand the concepts of mutual and self inductance.
Relevant Equations
Suppose we have the following setup
1712380117616.png

That is, a solenoid with ##N## turns, length ##l##, radius ##R##, and a current ##I## flowing.

Let's approximate the solenoid as an infinite solenoid (ie, ##l## is very large).

Then, the magnetic field inside the solenoid is

$$\vec{B}=\mu_0 nI\hat{k}=\frac{\mu_0NI}{l}\hat{k}\tag{1}$$

Suppose ##\frac{dI}{dt}> 0##. Then, in the solenoid the magnetic field is changing and so is the magnetic flux.

This variance of the magnetic field creates a phenomenon called electromagnetic induction.
Faraday conjectured that this phenomenon is due to a non-electrostatic electric field ##\vec{E}## which satisfies

$$\mathcal{E}=\oint_C\vec{E}\cdot d\vec{s}=-\frac{d\Phi_B}{dt}=\frac{d}{dt}\iint\vec{B}\cdot d\vec{A}\tag{2}$$

where ##\mathcal{E}## is called the electromotive force. This isn't an actual force. It has units of voltage. It is like a potential difference but isn't one either since this new electric field isn't conservative.

As I understand it, in the case of the solenoid, we have ##N## turns, not loops.

On the other hand, we can approximate the solenoid as being composed of ##N## loops.

The flux through the solenoid is then just the sum of the flux through the ##N## loops.

$$\Phi=N\iint_{\text{turn}}\vec{B}\cdot d\vec{A}=n^2l\mu_0n\pi R^2I$$

(I am not sure about the term "flux through the solenoid" above. Flux is defined relative to a surface, so this term doesn't seem correct).

There is a linear relationship between ##\Phi## and ##I##. The proportionality constant is the self-inductance ##L=n^2l\mu_0n\pi R^2##.

Why is this important?

It seems to be because we have the concept of self-induced emf ##\mathcal{E}_L##.

$$\mathcal{E}_L=-N\frac{d\Phi_B}{dt}\tag{3}$$

$$=-N\frac{d}{dt}\iint_{\text{turn}}\vec{B}\cdot d\vec{A}\tag{4}$$

$$=-N\frac{d}{dt}\left (\frac{\mu_0 NI}{l}\pi R^2\right)\tag{5}$$

$$=-\frac{N^2\mu_0\pi R^2}{l}\frac{dI}{dt}\tag{6}$$

$$=-n\mu_0l\pi R^2\frac{dI}{dt}\tag{7}$$

If this back emf were not present, the magnetic field would simply increase due to the presence of ##I## in (1).

At this point, as I understand it, we add the whole new field ##\vec{E}## that we mentioned previously. It doesn't come from previous theory, but rather from experiments.

This is the part I'd like to understand more deeply.

I have a few questions but let me tackle them in parts.

The negative sign in equations (3)-(7) are there because of Lenz's law, which seems to also be simply a law obtained from experimental evidence.

$$\mathcal{E}_L=-L\frac{dI}{dt}=-n^2l\mu_0 \pi R^2 \frac{dI}{dt}=\oint_C \vec{E}\cdot d\vec{s}\tag{8}$$

The rightmost integral has units of voltage of course, which is energy per charge.

But what is ##C## in the case of our solenoid?
 
Last edited:
Physics news on Phys.org
Let me move to my next doubts.

From (8), we have ##\mathcal{E}_L<0##. Given our choice of normal vector ##\hat{k}## in the calculation of magnetic flux, it seems that ##\vec{E}## has fieldlines going clockwise in the solenoid wires.

There is an induced current opposite to the initial current which "generates" a field that opposes the increase in magnetic field. This induced magnetic field thus has field lines pointing down inside the solenoid.

Where in the calculations do we see this induced current?

We assumed, after all, that ##\frac{dI}{dt}>0##. But if there is induced current, isn't this rate of change affected?
 
I'm going to try to guess my way through the questions I posed.

I'm guessing that since we approximated the solenoid as ##N## loops, then ##C## is just one of the loops.

$$\mathcal{E}_L=-N\frac{d\Phi}{dt}=\oint_C\vec{E}\cdot d\vec{s}\tag{9}$$

$$\frac{\mathcal{E}_L}{N}=-\frac{d\Phi}{dt}=\frac{1}{N}\oint_C\vec{E}\cdot d\vec{s}\tag{10}$$

Does this mean that the back emf on one loop is ##\frac{1}{N}## the back emf on the entire solenoid?

Now, this division by ##N## doesn't seem to matter because

$$\frac{\mathcal{E}_L}{N}=\frac{1}{N}\oint_C\vec{E}\cdot d\vec{s}\tag{11}$$

$$\implies \mathcal{E}_L=\oint_C\vec{E} \cdot d\vec{s}\tag{12}$$

By symmetry, ##\vec{E}## is constant on a loop.

$$\mathcal{E}_L=E2\pi R\tag{13}$$

$$E=\frac{\mathcal{E}_L}{2\pi R}\tag{14}$$

$$=-L\frac{dI}{dt}\frac{1}{2\pi R}\tag{15}$$

Now, to be honest, I am not at all happy with this post as it seems very fishy and incorrect.

Is ##C## one of the loops? At this point I really don't know.
 
Last edited:
Closed loop ##C## is anything you choose it to be and encloses area ##S## over which the flux is changing. See my post #2 in your earlier post. If you choose ##S## to be the area of one turn, then ##C## is the circumference of one loop and ##\mathcal E## is the emf you get across an infinitesimally small cut in the loop. If you choose ##S## to be the area of all the turns, then ##C## is the circumference of all the truns and ##\mathcal E## is the emf you get across the two ends of the solenoid.
 
Thread 'Minimum mass of a block'
Here we know that if block B is going to move up or just be at the verge of moving up ##Mg \sin \theta ## will act downwards and maximum static friction will act downwards ## \mu Mg \cos \theta ## Now what im confused by is how will we know " how quickly" block B reaches its maximum static friction value without any numbers, the suggested solution says that when block A is at its maximum extension, then block B will start to move up but with a certain set of values couldn't block A reach...
TL;DR Summary: Find Electric field due to charges between 2 parallel infinite planes using Gauss law at any point Here's the diagram. We have a uniform p (rho) density of charges between 2 infinite planes in the cartesian coordinates system. I used a cube of thickness a that spans from z=-a/2 to z=a/2 as a Gaussian surface, each side of the cube has area A. I know that the field depends only on z since there is translational invariance in x and y directions because the planes are...
Thread 'Calculation of Tensile Forces in Piston-Type Water-Lifting Devices at Elevated Locations'
Figure 1 Overall Structure Diagram Figure 2: Top view of the piston when it is cylindrical A circular opening is created at a height of 5 meters above the water surface. Inside this opening is a sleeve-type piston with a cross-sectional area of 1 square meter. The piston is pulled to the right at a constant speed. The pulling force is(Figure 2): F = ρshg = 1000 × 1 × 5 × 10 = 50,000 N. Figure 3: Modifying the structure to incorporate a fixed internal piston When I modify the piston...
Back
Top