# magnetic field reducer?

Discussion in 'Misc Electronics' started by qude, Jun 20, 2005.

1. ### qudeGuest

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

2. ### CWattersGuest

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.

3. ### qudeGuest

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

4. ### DaveGuest

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.

5. ### qudeGuest

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

6. ### DaveGuest

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.

7. ### Gerald RobinsonGuest

How would you calculate the losses?

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

8. ### DaveGuest

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.

9. ### qudeGuest

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

10. ### Don KellyGuest

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.

11. ### qudeGuest

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

12. ### DaveGuest

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.

13. ### qudeGuest

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

14. ### DaveGuest

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.

15. ### Don KellyGuest

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.

16. ### CWattersGuest

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.

17. ### qudeGuest

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
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

18. ### DaveGuest

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

19. ### qudeGuest

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

20. ### DaveGuest

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.