# Coil electrodynamics

Discussion in 'Electronic Design' started by Karl Roberts, Oct 28, 2011.

1. ### Karl RobertsGuest

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

2. ### Phil AllisonGuest

"Karl Roberts"
** 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.

** 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

3. ### Karl RobertsGuest

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

It melts.

5. ### Phil AllisonGuest

"Karl Roberts"

** 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

6. ### DonMackGuest

"Karl Roberts" wrote in message
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.

7. ### Karl RobertsGuest

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

Karl

8. ### Phil AllisonGuest

"Don Mack"

" 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? "

** That is completely absurd.

The two voltage sources are summed in series, so the total is 8 + 4sin(wt).

** Drivel.

** More absolute drivel.

.... Phil

9. ### Karl RobertsGuest

In that case, given my original example of a + and - 2VDC offset for
each idendtcal sinewave, there would be a residual DC current flow at
4V from one end of the coil to the other.

Going back to having one sinewave inverted from the other, I
understand (now) the voltages add to 12 and the composite signal is
riding on the same 4V current.

Is that all correct?

OK, that's my last follow-up.

Karl Roberts

10. ### Phil AllisonGuest

"Karl Roberts"
" J.A. Legris"

** FFS pal - you just contradicted yourself.

The new case is VERY DIFFERENT from yuor example !!.

** Thick as a plank.

The offset voltage is 8 volts - cos that is the AVERAGE value of 8 +
4sin(wt)

The sine wave both adds to and subtracts from the DC offset - giving 4 and
12 as the min and max values.

.... Phil

11. ### Phil AllisonGuest

"Karl Roberts"
"Phil Allison"

Have a look and see who is supplying most of the answers.

.... Phil

12. ### MrTallymanGuest

Where do see the application here being that of a solenoid? He did not
mention one.
Since you seem to think he is referring to a solenoid maybe?

13. ### MrTallymanGuest

IOW, you react with: "When slapped, I dance like a little girl..."

Now, you tell us that you cannot even grasp a 100% *BASIC* netlist.

Logic 2
John G 0

14. ### John SGuest

Always wrong as usual. Look in the first sentence following the "Two
part question:" above, AlwaysWrong. Damn, you're stupid.

Since you couldn't find your ass with both hands, AlwaysWrong.