Wire wrapped around copper

1. Jan 20, 2010

crockman1

will copper wire wrapped around insulated copper wire induce a current to some degree? and if electricity is flowing through the wire of witch the copper is wrapped around will it induce current in the wrapped wire?

2. Jan 20, 2010

jambaugh

The wrapping itself will not induce current, however the current in the wrapped wire will also have a component parallel to the core wire. You will get the same effect simply by running the outer wire parallel to the core wire (along the same length that it would have been wrapped) rather than wrapping around it.

3. Jan 20, 2010

crockman1

whoops i meant to say: if there is current running through the outside wire will it make make a current in the wire its wrapped around? i think what you said means that there will be a current induced in the core wire in the direction of the current in the wire that is wrapped around it, and running a single wire on the outside will do the same thing? will a current in the core wire produce current in the outer wind/ skin? what if i wrap foil around an insulated core and run electricity through it? the core?

Last edited: Jan 20, 2010
4. Jan 20, 2010

jambaugh

Right, you can think of it as a 1:1 transformer but just unwound. The changing B field around one wire will also pass around the other so it can induce an emf in the other wire.

Note the current itself doesn't induce a current in the second wire. A change in current casing a change in the B field around the second wire will induce a voltage difference along the wire which in turn, given a circuit will drive a current. That's why transformers must use AC.

5. Jul 6, 2011

crockman1

wow im dumb what i meant to say was: will *insulated* copper wire with an alternating current pumped through it wrapped around a copper wire induce a current in the copper wire core? And the other way around? U know, if the copper core is electrified w/ ac voltage?

Last edited: Jul 6, 2011
6. Jul 6, 2011

crockman1

heres an interesting thought, will there be an increase in voltage if the end of the core wire is connected to the beginning of the wrapped wire? What if this setup was coiled around to produce a flat spiral, and the core wire was electrified by a high voltage oscillator such as a tesla coil?

7. Jul 6, 2011

Staff: Mentor

As jambaugh already said, at best you can make a 1:1 transformer. Can you please explain back to us why that is true? We can save a lot of time here if you do that...

8. Jul 7, 2011

crockman1

really? How come there is a different effect when the wire is coiled around an iron core compared to running the wire along a piece of iron?

9. Jul 7, 2011

wbeaty

The issue is confused because you're using a spiral wire.

Try this question instead:

Suppose we have a hoop-shaped coil, or a pancake-shaped coil. Suppose we pass another wire through the center of the hoop, with this extra wire oriented perfectly axially. If we now apply AC to the hoop-coil, will we see any inductive coupling? Will a voltage appear across the axial wire which passes through the hoop?​

A cylindrical spiral coil has a component of electric current in both the axial and toroidal directions. Yes, the axial component does induce a current in a second wire (you just have two parallel wires in that case, and AC in one parallel wire will certainly produce strong magnetic induction in the second wire.)

But a hoop-coil or a pancake-coil has only the toroidal current, so things become much clearer.

10. Jul 7, 2011

jambaugh

With a linear wire pair, the B field of one wire only wraps around the other wire once. This doesn't provide much mutual inductance between two wires. They are weakly coupled.

Looping the wires in a coil increases the mutual coupling thus increasing the amount they couple to each other. Firstly the primary wire being around a circle N times creates N time the B field for a given current. Secondly that B field travels around the second wire more than just once since it circles many windings. The two wires are then strongly coupled.

This winding of the wires also couples each wire to itself in a negative fashion causing it to induces a reverse voltage when it changes current. This is how you make an inductor (as opposed to a transformer) which has the effect of resisting changes in its current.

This effect also is why the transformer is not a short circuit on one side (w.r.t. AC current). It will resist increases in current to the extent that very little current can flow before the voltage cycles. When the second half of the transformer is connected to a load then the induced voltage causes a secondary current which opposes this self inductance in the first coil in exactly the amount that the power output will be proportional to the input power, actually they'll be equal minus the losses due to non-ideal performance.

Make it a straight wire and this reactive self inductance is much less and you'll loose much more power in the wire acting as a resistive heater rather than as an electromagnet.

11. Jul 18, 2011

crockman1

thanks for the info!

12. Nov 19, 2011

Abbarue

I have a similar question:
If I wrap say #28 copper wire around #18 iron wire would this work as an inductor?
Then if I wrapped another layer of #28 copper around that would it work as a transformer?

Maybe the main question I should be asking is:
What is the main purpose of making transformer cores thicker.

13. Nov 19, 2011

crockman1

i think there might be some interesting effects! i have ran this one through my head a bunch of times. you may discover an extremely efficiant electromagnet if you use the right ratios. try a whole bunch of turns of small gauge wire around annealed guitar strings (heat the guitar strings to red-hot to soften them because [theoretically] that makes an awesome core for this) i called the idea the fractal transformer! i havnt built it yet though...