# Determining Magnetic Polarity

Discussion in 'General Electronics Chat' started by BR-549, Sep 22, 2016.

1. ### BR-549 Thread Starter Well-Known Member

Sep 22, 2013
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Can the magnetic polarity of a coil be determined from a schematic? Let's say that we have a battery with a coil and resistor in series. Can you tell the north pole of coil?

Can anyone show me how to do this?

With conventional current......charge flows from + battery to coil.

With electron current....charge flows from - battery to coil.

Is the north magnetic pole on the battery side of coil or the resistor side of coil?

2. ### crutschow Expert

Mar 14, 2008
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For current flow (positive to negative) you use the right-hand-rule to determine the magnetic polarity (which requires you know the physical winding direction).

3. ### GopherT AAC Fanatic!

Nov 23, 2012
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Why would it matter which side is N or S for an inductor or transformer? Are you specifically talking about an electromagnet?

4. ### BR-549 Thread Starter Well-Known Member

Sep 22, 2013
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Alrighty crutschow.....let's try that. We'll make the coil a right handed coil. That means that if we are using conventional flow, causes us to use our right hand fingers the wrap in direction of flow around coil.........thumb will point to the north magnetic pole.

This would mean that the north pole is on the resistor side.

Using electron flow....and wrapping LEFT hand fingers to wrap in direction of flow.......thumb pointing to north pole.....north pole is resistor side also.

Interesting.........the Polarity of the charge carrier......did not matter.

Now let's use a left handed coil. For conventional flow, using right hand coil rule....fingers in direction of current around coil......north pole is on battery side.

Using electron flow and left hand coil rule......fingers wrapped in direction of current around coil....north pole at battery side also.

Again the polarity of the charge carrier did not matter.

Let's put another coil in parallel with first coil....... but far enough away for no interaction. One coil is left handed and one coil is right handed.

Now............this is interesting. Look at those poles.

Does the Direction of charge flow determine the north pole? We can invert the pole with direction.........but not determine it.

So the Polarity of charge and the Direction of charge does not determine or set the reference of the pole.

The handedness of the coil........determines the north pole.

It's the handedness (rotational direction) of the current (positive charge or negative charge), that determines the north pole.

Notice that handedness can direct the magnetic with the current flow or direct the magnetic against the current flow.

This, along with physics experiments shows the charge itself has handedness. And that the phase of the magnetic with the electric charge can be set or reversed with a handed helical path.

5. ### GopherT AAC Fanatic!

Nov 23, 2012
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So, you are saying that if I make a left turn with my car and I feel a force pushing me to my passengers seat (Right), and the opposite effect if I make a right hand turn, then the car has handedness?

6. ### MrAl Distinguished Member

Jun 17, 2014
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Hello,

As to the original question...

Another way to determine the polarity of a coil is to bring one end near a CRT oscilloscope. It has to be a CRT type though because the electron beam is used to detect the coil mag polarity. This technique uses the Lorentz force to detect the coil polarity.
The coil is energized with some current through it. With the beam set to provide a point in the middle of the screen, bring one end of the coil close to the left side of the scope cabinet. If the beam defects downward, then the end of the coil closest to the scope cabinet is the North pole. If the beam deflects upward then it is the South pole. That is the same thing that will happen if you bring a magnet close to the left side of the cabinet also.

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7. ### BR-549 Thread Starter Well-Known Member

Sep 22, 2013
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I just use a compass.

8. ### atferrari AAC Fanatic!

Jan 6, 2004
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Could you say exactly how and why you identify the North Pole in your device using a compass?

9. ### BR-549 Thread Starter Well-Known Member

Sep 22, 2013
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I identify the north pole of a coil using the left hand coil rule. If you use positive charge flow for current, you must use the right hand coil rule. You may verify that rule with a compass. The same way we find the earth's magnetic poles. Be aware that a compass points to a fountaining in (or return) pole. A south magnetic pole. Which is located at earth's north pole.

10. ### MrAl Distinguished Member

Jun 17, 2014
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Hi,

Yes a compass would work if the field is strong enough. You can make a compass yourself if you dont have one, by using a magnet and sewing thread. Suspend the magnet with the thread and let it unwind until it stops rotating, then knowing the north direction from your present location, you can make the magnet face with N and S and you can use that for a compass. There are other ways do make one, but this seems the simplest and takes the least amount of 'parts'.

11. ### BR-549 Thread Starter Well-Known Member

Sep 22, 2013
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If we wound half a coil left handed and the second half right handed........we would get a coil that has like poles at the ends, and like poles in the center.

A three pole coil.

Current in one direction gives you a two south pole coil. Reverse the current and we have a two north pole coil.

Would the inductance change in a three pole coil of 20 turns vs a 20 turn two pole coil?

12. ### GopherT AAC Fanatic!

Nov 23, 2012
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Change, yes. Magnitude, it depends on a lot of things.