# Determining Electromagnet Polarity

Discussion in 'Electronic Basics' started by phaeton, Jan 6, 2006.

1. ### phaetonGuest

This would be easy to figure out if you had some sort of 'marked'
magnet lying around that indicated N and S poles.

However, let's assume you *don't*.

If we had an electromagnet set on a table, and we assumed that the
'core' of the windings were facing you, and the negative terminal was
on the left, the wire was wound CLOCKWISE on the core, and the positive
was on the right, how would we determine what pole was facing upwards
at us?

Is this a job for Lef Hand Rule?

TIA

2. ### phaetonGuest

I guess I should mention that i'm talking about DC current, not AC.

sorry.

3. ### John LarkinGuest

I have a supermagnet hanging from a string in my office, off the edge
of a bookshelf. It very strongly detents in the earth's field, and I
know which direction is North.

John

4. ### Ian MacmillanGuest

Look at one end of the coil, taking that current flows from positive to
negative.

If the current flows clockwise the pole at the coil end is south. Note that
the ends of the letter "S" point clockwise.

If the current flows anticlockwise the pole at the coil end is north. Note
that the ends of the letter "N" point anticlockwise.

In your example, if the positive wire came from the right and wound on
clockwise, the top face would be South.

Its a job for the right fist rule. Right hand, fingers curled, thumb up.
Fingers in direction of current (+ to -), thumb points North.

All the best
Ian Macmillan

5. ### ehsjrGuest

Right hand rule.

Ed

6. ### Jasen BettsGuest

The problem is insufficiently specifed.

is the electromagnet wound clockwise from negative to positive or
clockwise from positive to negative.

position of the terminals is irrelavent.
left hand rule for electron flow, right hand rule for conventional current.

assuming electron current flowing clockwise (leaving my right hand free to
type)

left hand grasps the left edge of the coil thumb pointing up to represent
current fingers curl round and point to me makinf the end facing me a North
pole.

Bye.
Jasen

7. ### JamieGuest

but that is magnetic north?, how about true north?

8. ### Guest

aslong as you love me baby

9. ### Rich GriseGuest

Yes. Electron Flow uses the Left-Hand Rule, and Conventional Flow uses
the Right-Hand Rule.

So your thumb should point north in either case. (although, 90% of
the time there's a fifty-fifty chance of guessing which is which, I
guess wrong - go to the dollar store and get a compass. ;-) )

Cheers!
Rich

10. ### Rich GriseGuest

If it had been AC, you wouldn't have said "negative" and "positive". ;-)

Cheers!
Rich

11. ### Rich GriseGuest

AH HAH!!!!!!!!!!

Finally, a logical explanation for that asinine "right-hand rule",
"conventional current" stuff! I guess the academician types need this sort
of little mnemonic, since they don't believe in electrons. ;-)

Cheers!
Rich

13. ### phaetonGuest

Excellent!

Thanks folks.... I'll have to take Rich Grise's advice and just buy a
cheap, marked magnet somewhere...

Although, in my experience with computer components, sometimes "cheap"
means "mismarked".

Maybe I have a brain tumor, but I always seem to consider electricity
from the "electron flow" angle, not "current flow" or "hole flow".

Is that good, bad, or indifferent?

thx again

14. ### Rich GriseGuest

Probably a fairly reliably-marked magnet would be an ordinary compass.

And "conventional current" vs. "electron flow" is almost irrelevant, as
long as it's clear which you're using in any given case - they're
practically indistinguishable - you just swap all of the + and - signs.

I was raised on electrons, but just to go easy on the ivy leaguers, I'm
happy to use conventional current to get around the issue. ;-)

Cheers!
Rich