# Pulses, cables and toroidal coils

Hello.

I just came to imagine a "thing" that i'm not able to understand correctly, so maybe you can help me.

Imagine a high voltage source, and a circuit made of cable capable of handle high currents (with low resistivity) and a load with medium to high resistance (say 10 KOhm)

Now you connect the high voltage source to the rest of the circuit for just a moment, creating a pulse. During that time, there is high current developing on the circuit, and going through the load doing whatever work this sudden burst of current does on the load. When you disconnect, the current dissapears again.

As Oersted experiment showed, when you have current on a cable, you have a magnetic field around it, and if current changes, magnetic field changes as well.

Now imagine that you put a toroidal coil (with core) encircling the cable that goes between one terminal of the the source and one of the load.

When you connect/disconnect the source to the load, the changing current through the cable will be generated just like before, but now we will collect the changing magnetic field generated by the changing current with the toroidal coil, so we will have electric energy on the toroidal coil...

So first question is:

If we put the toroidal coil, we pick energy that we were not picking before from the changing magnetic field surrounding the cable. In that case... Do the presence of the toroidal coil affect the current or the electric conditions on the load of the electric circuit? Or the circuit will behave the same as before, independently of opposing or not to the changing magnetic field generated by the current?

If the answer is that we can make whatever we want with the field generated outside the cable without affecting the electric conditions or current on the electric circuit, then another question arises:

Imagine you add 10 times more cable between the source and the load. This changes resistance just a bit (because load is far more resistive than the cable), but you could encircle the cable with a coil 10 times longer, so pick up almost the same magnetic field, than before... but using a coil surface 10 times bigger! (or you could use 10 coils like the old one and connect them in series or parallel).

I mean, if you can encircle cables carrying current with toroidal coils without affecting the current that is running through them... You can get almost any value you want from the electromagnetic induction caused by the variable magnetic field!

For sure there is something I'm missing, but could you explain it?

Thanks.

## Answers and Replies

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berkeman
Mentor
Hello.

I just came to imagine a "thing" that i'm not able to understand correctly, so maybe you can help me.

Imagine a high voltage source, and a circuit made of cable capable of handle high currents (with low resistivity) and a load with medium to high resistance (say 10 KOhm)

Now you connect the high voltage source to the rest of the circuit for just a moment, creating a pulse. During that time, there is high current developing on the circuit, and going through the load doing whatever work this sudden burst of current does on the load. When you disconnect, the current dissapears again.

As Oersted experiment showed, when you have current on a cable, you have a magnetic field around it, and if current changes, magnetic field changes as well.

Now imagine that you put a toroidal coil (with core) encircling the cable that goes between one terminal of the the source and one of the load.

When you connect/disconnect the source to the load, the changing current through the cable will be generated just like before, but now we will collect the changing magnetic field generated by the changing current with the toroidal coil, so we will have electric energy on the toroidal coil...

So first question is:

If we put the toroidal coil, we pick energy that we were not picking before from the changing magnetic field surrounding the cable. In that case... Do the presence of the toroidal coil affect the current or the electric conditions on the load of the electric circuit? Or the circuit will behave the same as before, independently of opposing or not to the changing magnetic field generated by the current?

If the answer is that we can make whatever we want with the field generated outside the cable without affecting the electric conditions or current on the electric circuit, then another question arises:

Imagine you add 10 times more cable between the source and the load. This changes resistance just a bit (because load is far more resistive than the cable), but you could encircle the cable with a coil 10 times longer, so pick up almost the same magnetic field, than before... but using a coil surface 10 times bigger! (or you could use 10 coils like the old one and connect them in series or parallel).

I mean, if you can encircle cables carrying current with toroidal coils without affecting the current that is running through them... You can get almost any value you want from the electromagnetic induction caused by the variable magnetic field!

For sure there is something I'm missing, but could you explain it?

Thanks.
The magnetic fields of the cable and the toroid will be orthogonal, so there is no coupling if the cable runs through the toroid.

For there to be magnetic coupling, the cable would need to be wrapped around the toroid, with the wires parallel. In that case, you are adding inductance to your connection to your load resistance. Your current stores some energy in the coil, and the inductance of the coil resists changes in the value of the current according to the familiar equation:

$$V(t) = L \frac{dI(t)}{dt}$$

Thanks for your answer Berkeman.

Let me just describe what I think it's happening.

When the current goes through the cable, circles of magnetic field develop around the cable, just like coaxial layers encircling the cable.

If you put a ferromagnetic material encircling the cable, I think the flux lines will prefer to go into that material than into air, so now you have the changing density of flux lines happening inside the ferromagnetic material as the current on the cable changes. You can think of this as replacing the surrounding air outside the cable for "slices" of ferromagnetic material.

Now, I don't see any difference between wrapping a coil around each ferromagnetic core in this situation, and the usual way a transformer works. As long as there is magnetic flux changing inside the torus, this will cause the coil to develop electricity trying to opose those field changes...

Can you explain why the ferromagnetic torus with changing magnetic field inside will behave differently on a transformer than on this situation?

Is not the same principle working here in both cases?

Thank you.

berkeman
Mentor
Thanks for your answer Berkeman.

Let me just describe what I think it's happening.

When the current goes through the cable, circles of magnetic field develop around the cable, just like coaxial layers encircling the cable.

If you put a ferromagnetic material encircling the cable, I think the flux lines will prefer to go into that material than into air, so now you have the changing density of flux lines happening inside the ferromagnetic material as the current on the cable changes. You can think of this as replacing the surrounding air outside the cable for "slices" of ferromagnetic material.
Correct so far...
Now, I don't see any difference between wrapping a coil around each ferromagnetic core in this situation, and the usual way a transformer works. As long as there is magnetic flux changing inside the torus, this will cause the coil to develop electricity trying to opose those field changes...

Can you explain why the ferromagnetic torus with changing magnetic field inside will behave differently on a transformer than on this situation?

Is not the same principle working here in both cases?

Thank you.
The difference is the *direction* of the B-fields. You correctly say that the B-field lines encircle the straight wire. But when you make a coil in the shape of a solenoid or toroid, the fields combine to make a field that goes straight down the "tube" of the coil. If the wire is inside this tube, the wire's B-field is circling around the wire, parallel to the lays of the coil. And the field from the coil is traveling straight down the tube, parallel to the wire.

So the two sets of B-fields are orthogonal and do not interact.

Wire's B-field: http://www.pa.msu.edu/courses/1997spring/phy232/lectures/ampereslaw/rhrthumb.gif

Solenoid's B-field: http://basharspacetimeantenna.files...magnetic-field-in-a-straight-coil-of-wire.gif

.

Sorry, I think I explained incorrectly the positions of the cable on the electric circuit, and the toroidal coil setup.

Imagine you have the toroidal coil in mid air, and you pass the cable that carries current through the torus hole... That's what I'm talking about.

I'll try to post a picture later.

berkeman
Mentor
Sorry, I think I explained incorrectly the positions of the cable on the electric circuit, and the toroidal coil setup.

Imagine you have the toroidal coil in mid air, and you pass the cable that carries current through the torus hole... That's what I'm talking about.

I'll try to post a picture later.
Ohhh, that's different.

Yes, that is an N:1 transformer, where the toroid is the "secondary winding" and has N turns, and the wire through the hole in the middle counts as a 1-turn primary. That type of configuration is used for "Current Transformers", for example:

http://www.google.com/search?tbm=is...rrent+transformer&gbv=2&aq=f&aqi=g10&aql=&oq=

.

I post the picture.

In the first row of the drawing I show:

1 - The cable with changing current, and the generated changing magnetic field around this element.
2 - The position of the ferromagnetic core around the cable, and the absorption of the field into this material.
3 - The wrapping of the toroidal coil around the core. The flux lines of the magnetic field are perpendicular to the wrap, so this is just the same situation we have on a normal transformer secondary coil (I think).

In the second row I show:

1 - A longer cable, that has the same changing flux density all around its length (take for example triple length).
2 - The same cable, surrounded with toroidal coils all around its length (triple surface exposed to field changes, so triple generation of electric energy from the same cable current!)

In the third row I show the electric circuit and toroidal coils surrounding the circuit cable.

I'm still trying to understand it.

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berkeman
Mentor
I post the picture.

In the first row of the drawing I show:

1 - The cable with changing current, and the generated changing magnetic field around this element.
2 - The position of the ferromagnetic core around the cable, and the absorption of the field into this material.
3 - The wrapping of the toroidal coil around the core. The flux lines of the magnetic field are perpendicular to the wrap, so this is just the same situation we have on a normal transformer secondary coil (I think).

In the second row I show:

1 - A longer cable, that has the same changing flux density all around its length (take for example triple length).
2 - The same cable, surrounded with toroidal coils all around its length (triple surface exposed to field changes, so triple generation of electric energy from the same cable current!)

In the third row I show the electric circuit and toroidal coils surrounding the circuit cable.

I'm still trying to understand it.
As I said in my post just above yours, each is a 1:N transformer. But since the main cable is only 1 turn per solenoid, you don't couple much field to the toroids, and the increase in inductance for the straight cable is not all that much.

I just saw that you got the idea before posting the picture.

As I show on the picture, you can put a cable with "almost" any length you want, and using the same current on that cable, you could put any number of toroids around, and extract an arbitrary amount of energy from the same field variation...

Any clues on what could be wrong with extracting a lot of energy without interfering with the original circuit?

berkeman
Mentor
I just saw that you got the idea before posting the picture.

As I show on the picture, you can put a cable with "almost" any length you want, and using the same current on that cable, you could put any number of toroids around, and extract an arbitrary amount of energy from the same field variation...

Any clues on what could be wrong with extracting a lot of energy without interfering with the original circuit?
No, you cannot extract more energy than is supplied by the power source. Each toroid adds a small amount of inductance to the cable, so the energy that is coupled into each toroid comes at the expense of energy reaching the load. There is no difference between this and the situation where you add real inductors in series with the cable.

I know the current on the main cable has to be very high, but that's not the point.
The point is if the opposition of the toroids to the magnetic field developed by this current has some effect on that current or in the main electric circuit.

If not, then you can extract extra energy from a thing working this way (the main electric circuit doing some stuff, and also getting another source of electric energy from the "secondary" coils and cores around the main cable).

berkeman
Mentor
I know the current on the main cable has to be very high, but that's not the point.
The point is if the opposition of the toroids to the magnetic field developed by this current has some effect on that current or in the main electric circuit.
Correct, the inductance presented by the toroids affects the main cable's electric current. It opposes changes in current in the main cable.

If not, then you can extract extra energy from a thing working this way (the main electric circuit doing some stuff, and also getting another source of electric energy from the "secondary" coils and cores around the main cable).
Nope. There is no free energy. I'll allow you to ask a follow-up question if needed, but this thread is going to get closed pretty soon if you are trying to pursue a source of free energy. Free energy and perpetual motion machines are on the Banned Topics list for the PF for a reason. Mainly to avoid wasting time on pointless discussions, if that is what this thread has become...

Ok, so in fact there is an effect on the main current when you put a toroid around...

But what kind of effect is that?

So if you distort the magnetic field in some way, you get voltage drop or less current on the main circuit?

Ok, I just wanted to know what exactly is the mechanism that slows or alters or change the energy of the current on the main cable.

berkeman
Mentor
Ok, so in fact there is an effect on the main current when you put a toroid around...

But what kind of effect is that?

So if you distort the magnetic field in some way, you get voltage drop or less current on the main circuit?
See the inductor equation in my Post #2.

The inductance presents an impedance to changes in current. It's kind of like a resistance, but instead of the heat energy that a resistor produces from a current flowing through it, an inductance's impedance temporarily stores energy in the form of its internal B-field. In AC circuits, we treat the impedance of reactive elements (inductors and capacitors) much in the same was as we treat resistances. All of the energy is accounted for.

berkeman
Mentor
Ok, I just wanted to know what exactly is the mechanism that slows or alters or change the energy of the current on the main cable.
This article should be of help:

http://en.wikipedia.org/wiki/Inductor

.

Another question about this.

Each toroid is opposing the magnetic field generated by the main current... Is it not just like the magnetic lines being created on a non-ferromagnetic material?

I mean, with the coils... Is it not just like if the material surrounding the main circuit is not ferromagnetic? Are these two cases different?

I mean, the main electric circuit should work the same on air, under water or surrounded by iron, wood or any other material... How the toroidal coils achieve effects different than that just by being difficult for the magnetic field to develop?

berkeman
Mentor
Another question about this.

Each toroid is opposing the magnetic field generated by the main current... Is it not just like the magnetic lines being created on a non-ferromagnetic material?

I mean, with the coils... Is it not just like if the material surrounding the main circuit is not ferromagnetic? Are these two cases different?

I mean, the main electric circuit should work the same on air, under water or surrounded by iron, wood or any other material... How the toroidal coils achieve effects different than that just by being difficult for the magnetic field to develop?
No. The part I bolded is incorrect. Surrounding the wire with iron would increase the inductance of the wire. The external B-field of the wire now can store more energy in the iron than it could in air/water/wood, and hence the inductance of the wire goes up.

It's the same as the situation you have drawn. By running the cable through many N:1 current transformers, you are increasing the inductance of the wire.

Ok, thanks, two of my assumptions were wrong: The toroidal coils around the cable have effects on the current, and main circuit behaves differently surrounded by different materials.

Now, going further:
If the effect created in the main circuit is a increase on inductance... You mean that there will be a reactive power, and then the load will not dissipate the same real power as without the coils, isn't it?

berkeman
Mentor
Now, going further:
If the effect created in the main circuit is a increase on inductance... You mean that there will be a reactive power, and then the load will not dissipate the same real power as without the coils, isn't it?
Yes, series inductance can reduce the power delivered to the load, but no real power is being dissipated in the inductances (if their internal resistances are negligible). The issue has to do with the "Power Factor" for AC power distribution:

http://en.wikipedia.org/wiki/Power_factor

(boy, I'm really wearing out wikipedia today! )