# Coil electrodynamics

Started by October 27, 2011
```Can anyone please clarify the following for me?

Two part question:

If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
max 2V min) into one end and an inverted copy with a similar negative
offset (ie -2V max -6V min) into the opposite, what, in terms of
electrodynamics, occurs within the coil?

If I now split the coil in half and ground the center point, what
happens now?

Karl Roberts

```
```"Karl Roberts"

> Two part question:
>
> If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
> max 2V min) into one end and an inverted copy with a similar negative
> offset (ie -2V max -6V min) into the opposite, what, in terms of
> electrodynamics, occurs within the coil?
>

**  The voltage across the coil varies from 4 to 12 volts in a sine wave
fashion  -   IOW an 8 volt p-p sine wave with a DC component of 8 volts. So
a current will flow in the coil that depends on it's impedance at the
frequency of the sine wave AND it's DC resistance.

The mag field inside the solenoid will be in proportion to the instantaneous
value of the combined currents.

> If I now split the coil in half and ground the center point, what
> happens now?

**  This is ambiguous.

What does '"split the coil in half " mean ?

Grounding the center point has no effect,  on it's own.

Good enough for homework ??

...  Phil

```
```On Fri, 28 Oct 2011 12:10:28 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>
>"Karl Roberts"
>
>> Two part question:
>>
>> If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
>> max 2V min) into one end and an inverted copy with a similar negative
>> offset (ie -2V max -6V min) into the opposite, what, in terms of
>> electrodynamics, occurs within the coil?
>>
>
>**  The voltage across the coil varies from 4 to 12 volts in a sine wave
>fashion  -   IOW an 8 volt p-p sine wave with a DC component of 8 volts. So
>a current will flow in the coil that depends on it's impedance at the
>frequency of the sine wave AND it's DC resistance.
>

But don't two identical waves 180 degrees out of phase cancel to zero?
Or does the symetrical DC component change this?

"yes".

This is what I am trying to visulaize/understand.

Karl Roberts
```
```Karl Roberts wrote:
> Can anyone please clarify the following for me?
>
> Two part question:
>
> If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
> max 2V min) into one end and an inverted copy with a similar negative
> offset (ie -2V max -6V min) into the opposite, what, in terms of
> electrodynamics, occurs within the coil?

It melts.
>
> If I now split the coil in half and ground the center point, what
> happens now?
>
> Karl Roberts
>
>
>
>
```
```On Oct 28, 9:00=A0am, karlrobe...@infoline7.com (Karl Roberts) wrote:
> On Fri, 28 Oct 2011 12:10:28 +1100, "Phil Allison" <phi...@tpg.com.au>
> wrote:
>
>
>
>
>
> >"Karl Roberts"
>
> >> Two part question:
>
> >> If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
> >> max 2V min) into one end and an inverted copy with a similar negative
> >> offset (ie -2V max -6V min) into the opposite, what, in terms of
> >> electrodynamics, occurs within the coil?
>
> >** =A0The voltage across the coil varies from 4 to 12 volts in a sine wa=
ve
> >fashion =A0- =A0 IOW an 8 volt p-p sine wave with a DC component of 8 vo=
lts. So
> >a current will flow in the coil that depends on it's impedance at the
> >frequency of the sine wave AND it's DC resistance.
>
> But don't two identical waves 180 degrees out of phase cancel to zero?
> Or does the symetrical DC component change this?
>
> "yes".
>
> This is what I am trying to visulaize/understand.
>
> Karl Roberts

try sci.electronics.basics. All the coil sees is the pd between its
ends

NT
```
```"Karl Roberts"

> But don't two identical waves 180 degrees out of phase cancel to zero?

**  Only if they are summed.

In your example the coil sees the * difference *.

As any load connected between two terminals does.

Very basic stuff.

Try " sci.electronics.basics "   -  next time.

...   Phil

```
```On Oct 28, 6:38=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:
> "Karl Roberts"
>
> > But don't two identical waves 180 degrees out of phase cancel to zero?
>
> ** =A0Only if they are summed.
>
> =A0In your example the coil sees the * difference *.
>
> =A0As any load connected between two terminals does.
>
> =A0Very basic stuff.
>
> =A0Try " sci.electronics.basics " =A0 - =A0next time.
>
> ... =A0 Phil

Karl, it may be easier to convince yourself that Phil is right by
considering what would happen if *identical* signals were applied to
each end of the coil. Now that's cancellation.

--
Joe
```
```
"Karl Roberts"  wrote in message news:4ea9fb6c.174734@news.tpg.com.au...

Can anyone please clarify the following for me?

Two part question:

If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
max 2V min) into one end and an inverted copy with a similar negative
offset (ie -2V max -6V min) into the opposite, what, in terms of
electrodynamics, occurs within the coil?

If I now split the coil in half and ground the center point, what
happens now?

-----------

Do you realize that voltage is relative? You can add any voltage to one
point as long as you add it to any other and the "physics" will not change.

So, your first case is identical to setting one side to 0 volts and the
other side to (2sin(wt) + 4) + (-4 - 2sin(wt)) = 0. So both ends of the coil
are at the same voltage. I could have set one end to Zeta(wt)^w +
cos(wsinw(wt))^t and wouldn't have changed anything.

In the second case when you ground the center you end up with two coils. You
can separate them mentally or even physically since you know(or forced) the
center(or even some other point) to be a certain value. It is exactly the
same when we know(force or decide) that something is "ground".

Note that this is distinctly different from the first case. In the first
case we have one coil and the second case we have two... but each one
similar to the others.

Anyways, if you think about it a bit you shouldn't have any issues. Use
resistors instead of coils.  The only difference with coils is that the
voltage drop across the coil may not be linear due to self and mutual
electromagnetic effects and fringe effects.

```
```On Fri, 28 Oct 2011 00:55:32 GMT, karlroberts@infoline7.com (Karl
Roberts) wrote:

>Can anyone please clarify the following for me?
>
>Two part question:
>
>If I take a solenoid and feed a positive DC offset sinewave (ie. 6V
>max 2V min) into one end and an inverted copy with a similar negative
>offset (ie -2V max -6V min) into the opposite, what, in terms of
>electrodynamics, occurs within the coil?

---
The voltage across the coil will cause a current through the coil
which will only be limited by the impedance of the coil at the
frequency in question.
---

>If I now split the coil in half and ground the center point, what
>happens now?

---
The center tap will always be at zero volts, as will ground, so
nothing will happen:

Version 4
SHEET 1 1228 784
WIRE 64 -160 -80 -160
WIRE 64 -112 64 -160
WIRE 64 0 64 -32
WIRE 176 0 64 0
WIRE 64 32 64 0
WIRE 176 32 176 0
WIRE 288 48 224 48
WIRE -80 192 -80 -160
WIRE 64 192 64 112
WIRE 288 192 288 48
WIRE -80 368 -80 272
WIRE 64 368 64 272
WIRE 64 368 -80 368
WIRE 176 368 176 112
WIRE 176 368 64 368
WIRE 224 368 224 96
WIRE 224 368 176 368
WIRE 288 368 288 272
WIRE 288 368 224 368
WIRE -80 480 -80 368
FLAG -80 480 0
SYMBOL ind 80 128 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L2
SYMATTR Value 1
SYMATTR SpiceLine Rser=.1
SYMBOL ind 80 -16 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L1
SYMATTR Value 1
SYMATTR SpiceLine Rser=.1
SYMBOL voltage -80 176 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value SINE(4 2 10)
SYMBOL voltage 288 176 R0
WINDOW 0 -53 5 Left 2
WINDOW 3 -242 110 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value PULSE(0 1 1 1E-6 1E-6)
SYMBOL sw 176 128 R180
WINDOW 0 32 15 Left 2
WINDOW 3 32 44 Left 2
SYMATTR InstName S1
SYMBOL voltage 64 288 R180
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value SINE(4 2 10)
TEXT -56 400 Left 2 !.model SW SW(Ron=1u Roff=1G Vt=0.5 Vh=0)
TEXT -56 432 Left 2 !.tran 0 2 0

--
JF
```
```On Fri, 28 Oct 2011 21:38:06 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

> Try " sci.electronics.basics "   -  next time.
>

What? ... and get answers from more people like me.

Karl
```