Author Topic: Electromagnet Staying Magnetized Without Electricity?  (Read 1688 times)

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Offline JdogTopic starter

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Electromagnet Staying Magnetized Without Electricity?
« on: February 02, 2010, 09:28:13 PM »
For part of a school project I have to have an electromagnet holding up something and drop it when the circuit breaks. For some reason even after the magnet does not have electricity, it is still staying magnetized enough to hold up the weight. Does anyone know why this happens, and a fix for it? The magnet is wound around either a steel or iron nail (most likely steel), I flattened out the tip which is holding the weight. The weight is a small steel nail wrapped in aluminum foil. The flattened tip of the electromagnet is touching the head of the weight. Unfortunately I can't just increase the weight because then the electromagnet wouldn't be powerful enough to hold up the weight if the platform it is mounted on shakes. Any ideas?

Offline madsci1016

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #1 on: February 02, 2010, 09:42:08 PM »
The core (the iron or steel)of the electromagnet will 'absorb' some of the mag field.

wikipedia:

"When the current in the coil is turned off, most of the domains lose alignment and return to a random state and the field disappears. However some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving the core a weak permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is called remanent magnetism. The residual magnetization of the core can be removed by degaussing."

Offline little-c

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #2 on: February 03, 2010, 03:29:57 AM »
some serrious science explanation as to why it wont work properly will give you shed loads of marks ;)


maybe reverse the electromagnet for a very short time to destabalise the iron core? or look up degaussing?

Offline dunk

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #3 on: February 03, 2010, 08:38:58 AM »
that's just the way physics works i'm afraid.
you'll need to modify your design.

have you considered attaching the iron core to the object you want to drop?
make your windings out of a non magnetic metal (eg, copper wire) and do not fix the iron core in place.
when power is applied the iron core will stay in the windings.
when you remove power the iron core will drop out.


dunk.

Offline JdogTopic starter

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #4 on: February 03, 2010, 03:35:41 PM »
I was told by my ap phiysics teacher that it's because I'm using steel. He said that the carbon locks the electrons in place and instead I should use pure iron. Any thoughts?

Offline Soeren

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #5 on: February 03, 2010, 04:48:07 PM »
Hi,

Yes, you need very soft iron to keep residual magnetism at bay (a stack of mains transformer core plates stacked will do - "glue" them together with lacquer and let dry under pressure).

To use your steel core, glue some rubber or plastic sheet (an inner tube should be fine, or a couple of layers of duct tape) on the ends that touches the thing you wanna lift, to weaken the holding force considerably when power's not applied.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline JdogTopic starter

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Re: Electromagnet Staying Magnetized Without Electricity?
« Reply #6 on: February 03, 2010, 05:20:28 PM »
Thanks for all the advice. I found an easy solution. Rather than having the electromagnet touch the nail at it's tip, I tilted the magnet on it's side and have the nail touch the magnet on the side of it. This works perfectly. Thank you all very much.

 


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