# Inductive acceleration

1. Oct 6, 2007

### cala

Hello.

I want to ask you about an idea to get a free coil movement when it's moving on a specific magnetic field, and at the same time it also gives electric energy... and without replacing the energy from outside...

OK, Don't be scared with these statements, and please read the idea. Then, if you consider it is wrong, please tell me why.

Magnetic induction says that the field generated on a coil that is inmersed into a variable magnetic field always opposes the change on that magnetic field. From this point of view, magnetic induction is an inertia or tendency to oppose magnetic changes.

Usually, the aproach of a magnet pole (either north or south pole) to a coil increases the field on the coil, and their sepparation decreases the field on the coil, so magnetic induction is always seen as an opposing force to movement.

But... ¿What about a way to increse the field when the magnet is leaving the coil, and decrease it when the magnet pole is approaching?

In this case, the usual mechanical opposition would turn into a mechanical force of acceleration for the coil!

I've attached a schema of such a system:

We have a track with alternated polarity magnets.

Then, imagine you move a coil perpendicular to the magnets, from the left to the right of the picture.

1 - We start at the middle of the north pole of a magnet. As the coil LEAVES the north pole magnet, the field INCREASES (instead of decreasing as usual). The coil tries to avoid the field increase, and then it creates a north pole... Directed to the north pole of the magnet that it is leaving!, so we have REPULSION, and then the coil MOVES AWAY the north pole magnet.

2 - When the coil arrives at the area between two magnets, the field is maximum, but there is no magnetic induction (so no force).

3 - Then, the coil APPROACHES to a south pole magnet. Usually, the field should increase, then the magnetic induction would create repulsion, restraining the coil... but in our case, the field DECREASES as the coil approaches, so the coil opposes this change generating a north pole directed to the south pole magnet, that is to say we have magnetic ATTRACTION, so the coil APPROACHES more to the south pole magnet.

4 - Now, when the coil gets to the middle of the south pole magnet, magnetic change and induction are at their maximum values... but there is no field crossing the coil.

5 - When the coil LEAVES south pole magnet, usually we should see a decreasing field, but in our case, the field INCREASES!, So the coil opposes to it, and generates a south pole directed to the south pole magnet. Then we have REPULSION between magnet and coil, so the coil MOVES AWAY from the south pole magnet.

6 - Finally, the coil APPROACHES a north pole magnet, the field crossing the coil should increse, but it DECREASES. The coil opposes this magnetic variation generating a south pole directed to the north pole magnet, so there is ATTRACTION between them. The coil APPROACHES to the north pole magnet, till it gets to the middle of the north pole magnet, where the magnetic change, and thus the induction are maximum, but there is no field crossing the coil.

Then the sequence repeates. I mean, as we have inverted the usual relation between coil movement and magnetic field variation in the whole track, we have the magnetic induction creating a mechanical force of acceleration instead of restraining the coil!... and we have also the electric energy of the induction itself!.

The coil gets accelerated when moving into the track, and also it extracts electric energy from the induction! (??)

You could think that at the same time you have the coil generating a magnetic pole to accelerate it, you have the other side of the coil generating the opposite pole, and then you've got acceleration into the other direction, restraining the coil... but the distance between the coil and the previous magnet is always greater than to the next magnet, so the accelerating force is always greater than the restraining force.

What do you think?

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• ###### inductive acceleration.JPG
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Last edited: Oct 7, 2007
2. Oct 6, 2007

### Staff: Mentor

Huh? Your diagram shows the field weakening as you move away from the north pole--the field lines spread out.

Regardless, the induced current always acts to oppose the change in flux through the coil. Per Lenz's law, the induced current creates a force that opposes the coil's motion.

3. Oct 6, 2007

### cala

Yes, your argument of the lines spreading is a good point. The lines spread as the coil leaves the magnet.

But also, it cuts more lines as it pass by.

If you have the coil vertically oriented (i mean, the field generated on the coil is perpendicular to the position of the magnets), you can see that there are almost no flux lines across the coil surface when the coil is in the middle of the magnet, and almost all of the flux lines are crossing the surface of the coil when it is between two magnets.

So as it leaves magnets, the number of crossing lines increase (and the angle of these lines becomes more paralel to the normal of the coil surface), and the lines crossing the coil reduce (and also the angle of these lines increase respect to the normal) when the coil approaches to a magnet.

The Lenz law, from Wikipedia, states:

"Lenz's law states that induced current opposes the change in flux producing it."

So, the Lenz law is not related to the movement of the coil, it's related to the field variation, whatever the movement that caused it was.

As I explained before, usually approaching magnet means more field, and leaving magnet means less field, but this situation is reversed in this setup (so aproaching of the pole means decrease of the field for the coil surface, and leaving the magnet away means that the flux increases on the coil surface).

Last edited: Oct 6, 2007
4. Oct 6, 2007

### Staff: Mentor

This is true.

Let's say that parallel to the x-axis (the direction in which the coil moves, which is normal to the plane of the coil) that the field (and flux) through the coil increases as the coil moves away from a north pole. (That's equivalent to the coil's moving straight towards the south pole of a magnet aligned along the x-axis.) The induced current will oppose that increase, creating an induced magnetic field along the x-direction, which will create a force on the coil opposed to its motion. (Since the coil is aligned and constrained to move along the x-axis, it's the x-component of the field that counts.)

Sure it's related to the movement of the coil. What counts is the flux through the coil--any change in that flux is (of course) due to the movement of the coil. Any such change will be opposed by the induced current.

[Edit: Corrected typo; previously stated "north pole" corrected to read "south pole".]

Last edited: Oct 7, 2007
5. Oct 7, 2007

### cala

When you say "Let's say that parallel to the x-axis (the direction in which the coil moves, which is normal to the plane of the coil) that the field (and flux) through the coil increases as the coil moves away from a north pole. (That's equivalent to the coil's moving straight towards the north pole of a magnet aligned along the x-axis.)" you are completely right.

But... the equivalent x-aligned north pole magnet approaching is not on the front of the coil, it is behind, so i'll add to your sentence:

"Let's say that parallel to the x-axis (the direction in which the coil moves, which is normal to the plane of the coil) the field (and flux) through the BACK SURFACE OF THE coil increases as the coil moves away from a north pole. (That's equivalent to THAT SURFACE OF the coil's moving straight towards the north pole of a magnet aligned along the x-axis.)"

So you have a coil moving away from a magnet, but its movement is equivalent to a coil approaching to a magnet ON THAT SIDE (not on the front of the coil).

6. Oct 7, 2007

### cala

I've updated the picture, to show the coil positions and its crossing magnetic lines.

Maybe this picture will help to clarify the idea.

You can see it on the first post of this thread.

7. Oct 7, 2007

### Staff: Mentor

No, actually I had it flipped around. I should have said: "That's equivalent to the coil's moving straight towards the south pole of a magnet aligned along the x-axis." (If you move into a region where the field is increasing to the right, you're effectively moving towards the south pole of a magnet.)

Realize that the flux lines pass through both sides of a coil.

I'll restate things. When the coil moves away from the north pole (along the x-axis) the field is increasing and directed to the right. Thus the induced current and field will be directed to the left: the magnetic field of the coil will be aligned N-S (left to right) and moving (effectively) towards a south pole and thus pushed back.

8. Oct 7, 2007

### cala

Yes, you're right again, but as i said in the first post:

"You could think that at the same time you have the coil generating a magnetic pole to accelerate it (take N - N for example), you have the other side of the coil generating the opposite pole (S - S), and then you've got acceleration into the other direction, restraining the coil..."

But the distance of the interaction that helps the movement is always greater than the one that restrains it, so the resulting force is always helping the movement of the coil:

In the case of the N - N interaction, the distance (back of the coil) is less than the S - S interaction distance in front of the coil, so the N - N repulsion is bigger, helping the movement.

When the coil is leaving a south pole magnet, the interactions interchange: the S - S back interaction force is greater than the front N - N interaction force, helping the movement again.

When the interaction is S - N (coil approaching a north pole magnet), the back of the coil has N - S interaction, but in this case, the S - N front interaction force is greater than the N - S back interaction (because the distance is less to the north pole magnet in front of the coil than to the previous south pole), so helping the movement again.

When the coil approaches a south pole magnet, the N - S interaction is greater than the S - N attraction at the back of the coil, then the coil induction is helping the movement again.

So, yes, there is a restraining force, but in every situation, the helping force is greater than the restraining one.

Last edited: Oct 7, 2007
9. Oct 7, 2007

### cala

Ok, new diagram with the two forces of each one of the coil surfaces:

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10. Oct 7, 2007

### Staff: Mentor

All this talk about "restraining" and "helping" forces is obscuring the simplicity of the situation. All the coil "cares" about is the flux passing through it. As the coil moves to the right, the flux changes. No matter how the flux changes, the induced current always opposes that change.

There are four possibilities:
(1) Flux to the right and increasing (like moving towards the south pole of a magnet). The induced field will point to the left, thus opposing the motion.
(2) Flux to the right and decreasing (like moving away from the north pole of a magnet). The induced field will point to the right, thus opposing the motion.
(3) Flux to the left and increasing (like moving towards the north pole of a magnet). The induced field will point to the right, thus opposing the motion.
(4) Flux to the left and decreasing (like moving away from the south pole of a magnet). The induced field will point to the left, thus opposing the motion.

11. Oct 8, 2007

### cala

The situation is not so simple.

There are two interactions at any position of the coil: One with the magnet behind the coil, and the other with the next magnet, in front of it. So there is not only one force, but two.

The resultant of these forces is what is of interest for us.

For exmaple, i said that the force in the direction of movement will be always greater than the opposing one, and that is true... in magnitude.

But also, the angle of the interaction is important.

So maybe, in the x-line of movement, the components of these two forces could be similar...

But anyway, we eliminate or compensate some (or all) the opposing force of induction in some way.

In normal generators, the two forces of torque are opposing the movement. In this setup, one of the forces is opposing the movement, and the other is helping the movement.

Don't you think so?

From this point of view, the coil seems to be capable of turning or moving freely at the same time it's generating electric energy, and that is something more than normal generators do.

Is there something else i'm missing?

If all I said till this moment is correct, then it'll be time to do a deeper analisys... or construct the device.

But anyway, don't you think there are two forces or interactions? Don't you think the opposing force will compensate with the other side of the coil force? Why not?

12. Oct 9, 2007

### Staff: Mentor

You are complicating things. All that influences the coil is the magnetic field it passes through. That magnetic field is created by contributions from all nearby magnets. So the interactions with magnets in front and behind the coil are already included in the magnetic field.

As the coil moves, one of the four situations I outlined previously occurs. In each case, the induced current in the coil flows in a direction that opposes the change in flux. As that current cuts through the lines of field, a force is created that is opposite to the direction of motion of the coil. To keep the coil moving at constant speed requires an input of energy.

I suggest you review Faraday's law, Lenz's law, and motional EMF. (And conservation of energy, while you're at it. That's the one you think you've gotten around. Nope!)

13. Oct 9, 2007

### orubi

Starting from situation I): a moving coil that approaches the magnetic field B, generated by a central magnet. Assume direction of B as shown in the drawing. Then, as the coil (right side of the picture) approaches the magnet the intensity of B increases. An induced voltage is, thus, generated, producing an electric current i that flows in the direction shown in the picture (see http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magloo.html#c2 for details). This current induces a magnetic effect Bind, just like a little magnet with its north pole pointed towards the central magnet's north pole would do. The coil, being represented as the little magnet, meets a north pole at its left side that makes it experience a force F which tends to move it apart from the central magnet. F opposes the direction of motion of the coil and is proportional to the coil's speed V1. Once we' ve reach this point, it is clear that if the coil approaches an area of space where the intensity of the magnetic field increases, the coil itself would experience a braking force. Where does the kinetic energy lost go? Well, part of it will be used to heat the coil, considering its electrical resistance to current flow. Besides, another part of it will be store in form of magnetic energy in the coil's self inductance.

If there's still enough kinetic energy available to the coil as for passing the central magnet, then we will reach situation II): as the coil moves away from the central magnet, the intensity of the field B decreases. An induced voltage is, thus, generated, producing an electric current i that flows in the direction shown in the picture (left coil) (see http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magloo.html#c1 for details).This current induces a magnetic effect Bind, just like a little magnet with its north pole pointed towards the central magnet's south pole would do. The coil, beeing represented as the little magnet, meets a south pole at its right side that makes it experience a force F which opposes the direction of motion of the coil and is proportional to the coil's speed V2. What we've got now is that when the coil quits an area of space where the intensity of the magnetic field decreases, the coil would AGAIN experience a braking force. And the lost in kinetic energy will be again distributed between the same actors above-mentioned.
If we think about a collection of static magnets, and assuming the coil could pass the first of them, as soon as it approaches the second magnet the coil will enter an area of space where the magnetic field increases. We now know what will happen: it will experience the braking force again.
Conclussion: As Lenz's law states, the coil will brake until it stops, unless we provide it with an input of energy, just like Doc Al said.

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Last edited: Oct 9, 2007