# magnetic field reducer?

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Suppose you have a current source with magnetic field and you want to reduce it by adding a metal near it to induce opposite magnetic field to cancel the one from the source (partially). What coil configuration must you use? Is this possible?

Don't reply by telling me to use particular wire shielding or bend the source wire. I'm just interested in the theoretical rationale of putting metal or coil configuration near the magnetic field source to create reverse magnetic field to cancel a part of it. How much magnetic field do you think is reduced (or cancelled)?? This is assuming the metal or coil put near it doesn't have any voltage or current source itself but just from the induced current from the magnetic field of the source.

qude

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All electrical circuits have a return path. The best you can do is to use a second wire for the return path and to twist it around the signal wire. You now have equal currents flowing in opposite directions in close proximity. The magnetic fields almost cancel out.

• posted

I'm asking whether putting a separate wire without voltage or current source with certain coil configuration can cause opposite magnetic field to be produced by the induced current, and what is the magnitude of it, Anyone knows?

qude

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yes it can. magnitude depends on configuration and incident field. i.e. a plate of aluminum can almost completely cancel a 60hz magnetic field due to induced currents.

• posted

Is it always cancellation.. or can you make the configuration such that the induced field would add up to the source increasing the magnetic field (or does the law of conservation of field applies here).

qude

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in antenna design you can configure conductors to increase the field in one direction but it always is decreased in some other direction because of conservation of energy. i haven't really thought of that for low frequency work, but would assume the same would apply, while reducing the field by cancellation is one direction it must increase in some other area, minus a bit for losses from resistive heating in the material used for the shielding.

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How would you calculate the losses?

In an earlier question I am asking about using the heating effect?

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very carefully. calculating induced currents and losses is not the easiest problem, especially in solid plates. it may be a bit easier in wire loops.

yes, if a current is induced by the magnetic field then there will be some loss due to heating in the material.

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But it's not always the case, isn't it. Suppose you turn a connected wire in parallel with the current of each in opposite direction. There would be cancellation of the magnetic field outside the parallel wires and nothing in between them. This means cancellation outside indeed occur. Where does the conservation of energy came into play here? There is no evidence it is increased elsewhere with the same magnitude as the ones outside that is cancelled.

qude

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i.e.

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There seems to be a great deal of confusion involved here. More so in your first message.

Note that inducing a voltage in a parallel wire (and this voltage producing a current), the best you will get is that the external field will be the same as with the original wire only as all you have is a crude transformer.

In the case of the wire being physically parallel with a return wire carrying the same current (e.g. a lampcord), partial cancellation (never full unless the wires can occupy the same point in space) will occur outside the wires. However, between the wires, the field will be not be "nothing" but will be enhanced as the two currents are then additive with regard to the magnetic field. Sketch the field around a current into the page and that due to a current out of the page. Bring the sketches together- note the relative directions of the fields in different locations. I can give you a set of equations for calculation of the field due to a number of parallel current carrying wires if you wish. As to conservation of energy- there is no problem. The total field energy is dependent on the current and the position of the wires. If the position changes, then the field changes but there is work input or output to change the position In fact, conservation of energy can be used to calculate forces in such a situation. (Conservation of energy leads to : change in electrical energy in =change in magnetic field energy + change in mechanical energy + losses). This can be and is expressed in a number of ways in Electromagnetic Energy Conversion texts.

```--
Don Kelly
dhky@peeshaw.ca```
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To avoid confusion.

This is what I'm describing. A plate of aluminium put over a wire carrying 60 hz AC or fluctuating dc. Using the right hand rule, the current is clockwise looking from the source at the left. Now what I wanna understand is how does the induced current in the plate of aluminum (not in contact with it but put on top of it at a distance) produce magnetic vector that is in opposite to that of the wire. Does this something to do with Lenz law? What's the proof that induced current and magnetic field in the aluminum plate is opposite to that of the wire... which part of the plate the current flows?

Thanks guys.

qude

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to aid in figuring this out in your mind replace the plate with a whole bunch of little wire loops. as the field changes the db/dt induces a current flow in each of the little loops that will oppose the incident field. this is a simple case of induced current from a changing magnetic field. it gets much more complicated when you realize there are an infinite number of these loops and they are all connected to each other when it becomes a solid plate, but the small loop model can still be used to give good estimates of fields.

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I'm inquiring because I saw a product at a shop which has this cooper plate. It is supposed to lower emissions of magnetic field from appliances by simply being put near them. I wonder how many percentage of magnetic field are suppressed when say put directly on top a computer monitor or toaster, any idea? It's for those who are very EM sensitive or electrosensitives.

p6

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no, its for those who have more money than brains. at the edges the fields wrap around so for a small shield like that it would only reduce the field slightly for a small distance on the other side of it. even when shielding transformer vaults it requires not only doing the whole wall between the transformer and the equipment being protected, but a large part of the floor, ceiling and other walls... preferably wrapping the whole room.

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Related to Lenz' Law- yes. Related to conservation of energy- also true. The aluminum plate is essentially the secondary of a transformer (not a good one by any means). Look up transformer operation as well as the background for the expression Erms =4.44FN(flux maximum) for sinusoidal AC.

```--
Don Kelly
dhky@peeshaw.ca```
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fields

shielding

yeah there are lots of suspect products on the market that claim to "protect you from harmful radiation" - but if they worked whole industries would be out of business. It can be a bitch preventing EMC escaping from electronic devices like computers. I've done my time down a saltmine trying to design screening. Extreemly frustrating it can be.

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I checked further. Is it possible the plate of cooper or aluminum can be designed so that (by induction) it can produced a third waveform, by some kind of circuit. ES or Electrosensitives have so called neutralizing frequency and these swedish designers created some kind of copper plate in which when you put it near any source of magnetic field, it can produce a third waveform oscilating at a certain frequency. For example. You are tasked to design such circuit that is powered by induction (with power requirement even 0.01 mA or less) and oscilating at 20 Hz. How would you do that?

Also is it only magnetic field that can be used for contactless induction. How about electric field. Can electric field be used for contactless induction too? The former is Faraday law of induction, what's the second called (if at all)?

Thanks.

qude

via induction, works in conjunction with the electric field of the battery and the magnetic field of the quartz timing coil. Using these two fields, the Teslar chip is designed to produce a third waveform that pulses at 7-9 times

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rfid tags pick up one rf frequency and then use that to power their circuitry to respond on a different frequency. anti-theft labels in retail stores are even simpler and do a similar job but without the smart data of an rfid tag. adding a diode or two to a simple loop of wire you can double the frequency. you can also generate subharmonics of the exciting current with a properly tuned resonant circuit and a diode or two.

yes, an electric field can be used by having two plates instead of a coil. as the electric field changes it will produce a voltage difference between the plates that can be used to power devices. i think some power utilities use this for 'unpowered' devices on high voltage lines. many years ago one of the 'popular' magazines published a free power motor that ran on the clear air field gradient.

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Supposed you are to design it with pure copper foil or plate with no diode. Just wire perhaps imbedded in the copper surface. How would you do that. Supposed the input frequency is 2 Hz (by induction). And you want the output magnetic field to be 15 Hz.

For example you have a battery, let's say a small calculator battery. It should produce some electric field near it. If you put the copper tag nearby. What possible induction transfer can occur when it's clear you didn't put it in between the plates?

qude

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i would say its not possible without active components. harmonics and subharmonics require a non-linear component of some kind like a diode. non integer frequency multiplication is much harder and would typically require that the incident field be rectified and used to power some kind of active signal device like on an rfid tag.

the electric field from a small battery is very small and static except for when you physically move the battery or move the plate around it. but even then the change in the field is so small it would even be hard to measure without extremely sensitive instruments. also note that for a static field source like a battery a current would only be created when it was moving.

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