Q & A: Controlling the Poles of a Magnet

Q:
I read your question and answer page that talked about making a magnet with a nail, wire, and a battery. if you do make a magnet like this which end will be north pole and which will be south? Is there a way to control which end becomes what?
- Jake
West Lafayette, IN, USA
A:
With that type of magnet, it is easy to control the poles. To control the which pole is which, you need to connect the battery in a certain direction. You'll also need to know which way the wire is wrapped around the nail.

The battery will be labled + and -. Look to which wire the + connects to. Now follow that wire from its + end to its - end and see which way the wire wraps around the wire. All right, now you want to wrap the fingers of your right hand around the nail in the same direction as the wire. Now give a thumb's up sign. This is called the right hand rule. Your thumb now points towards the North end of the electromagnet.

That last part might sound odd so I'll explain it in a little more detail. We say that current flows from + to - in a wire. When current flows in a circular loop, it produces a small magnetic field. The direction of the field is out of the loop. To determine which way out of the loop, you curl the fingers of your right hand in the same direction as the wire and stick your thumb out. Your thumb points in the direction of the magnetic field. If you add up a lot of these loops, you get a stronger magnetic field. The last part to do is determine which pole is which. Well, some guy in the past decided that the magnetic field leaves the magnet at the North pole and we've used that ever since. So the end of the magnet that your thumb points when you do the right hand rule is the North pole.

To change the poles, just change which way the battery is hooked up. This changes the direction the current flows and reverses the field.

(published on 10/22/2007)

Follow-Up #1: changing polarity of an electromagnet

Q:
If you change the poles of the battery to change the poles of the electromagner. ? Is the magnetism on the pole you want to reverse removed instantly or does one have to give it second shock. to clear the field? I am a little lost as far as the removal or just simply it drops in power momentarily as it does in electric motors
- gilbert (age 77)
Hallandale Fl.
A:
This can be a little tricky. First off, the electromagnet will have a large inductance. So the current will try to keep flowing. Just disconnecting the battery can cause a big spark.

OK. let's say you've decided to put up with that. The current decays very quickly since the resistance of the air between the dangling connections is so high. The electromagnet part very quickly loses its field. However, usually a powerful electromagnet has an iron core that gets magnetized to enhance the field. These cores typically don't relax back to being unmagnetized but keep a field of maybe 30 G (depends on the material) for a long time. When you turn on the electromagnet in the opposite way, it will nearly wipe out the memory of the old field, so you should fairly rapidly approach just the reverse of what you started with.

If you try to reverse field quickly, say with an ac current in the magnet, the cores have trouble keeping up. Their field lags behind a bit, and they warm up as they're magnetization is driven back and forth.

Mike W.

(published on 10/03/2012)