# Rendering a magnet useless?

1. Jan 6, 2006

### InFiNitY1

hi

I was just thinking at school we learnt that a current running through a coil of wires with a iron core produces an electromagnet. so if you place a bar magnet instead of an iron core would this strenghten the magnet or if placed in opposite direction render the magnet useless (given that the electromagnet is strong enough)?

thanks (nothing to do in holidays)

2. Jan 6, 2006

### Mattara

Probably no.

Consider this: If you use two magnets they are more powerfull than if you only used one.

Conserning magnet field distortion....That isn't my best subject unfortunately, so do anyone else want to give it a try?

Last edited: Jan 6, 2006
3. Jan 6, 2006

### Danger

What a cool question! I don't know the answer, but my reasoning about it would be as follows:

an electromagnet works by temporarily aligning the dipoles in the same direction; a permanent magnet already has its dipoles aligned

the strength of an electromagnet varies with the applied current; the strength of a permanent magnet is pretty much fixed -- as far as I know, that strength in both cases depends upon how many dipoles are aligned

if the polarity of your current is such that it would magnetize a bar in the same direction as the permanent magnet, it would have no effect until it becomes strong enough to increase the number of dipoles aligned, then start to strengthen the field

if it's opposite, it would demagnetize the permanent one until it reaches zero field strength, then begin to reverse the field

I really have no idea of whether or not that's correct, but it's my initial take on the situation.

4. Jan 6, 2006

### Staff: Mentor

The easiest way to understand what happens is to look at the coercivity curve for a ferrous metal (like is used for the core of electromagnets and for permanent magnets). I did a quick google, and here's a typical picture:

http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/HysteresisLoop.htm

The curve shows the relationship between B and H for the material. You can see that as you put a higher and higher H field through the material, you get a higher B field in the material until it starts to saturate. B=uH until you get near saturation. The curve has hysteresis, in that you are left with some residual permanent B even after you remove any external H field. The "harder" the ferrous material, the more B is retained. "Hard" ferrous materials are used for permanent magnets, and "soft" ferrous materials are not.

So the answer to your question should be more obvious now. You can certainly impress an H field with a coil onto a permanent magnet to remove any residual B field, and "ruin" the magnet. Impressing an H field in the direction of the residual B field will give a higher magnetization temporarily, but removing the field will allow the magnet to return to its original state.

Exercise for the reader(s) -- how does a degaussing coil work?

5. Jan 6, 2006

I believe the magnetic fields would be additive the first scenario. The electric current creates a magnetic field with or with out the iron bar (look at a MRI) and I believe Danger has the second scenario correct.

6. Jan 6, 2006

### zoobyshoe

They employ a decaying AC current. That is: the current strength is gradually reduced to zero. This is because in practise simply applying an opposing field won't demagnetize a magnet, you just end up reversing it's polarity. The decaying AC current creates a magnetic field that alternates in polarity and whose strength gradually decreases leaving the domains in the ferrite as randomly oriented as possible.

7. Jan 7, 2006

### InFiNitY1

Hey Thanks for the replies everyone although im not a physics genius i do believe that in the first scenario it would strenghten the bar magnet but in the second im not sure and like Wizardsblade said i think something dramatic would happen could some descirbe what would happen or is it possible someone could try it (not sure if its a good idea!!)

8. Jan 7, 2006

### zoobyshoe

If you wrap a bar magnet with wire and hook it to the battery such that the coil field opposes the bar magnet field the coil field will simply remagnetize the magnet in the opposite direction. There won't be any explosions or anything bad.

9. Jan 7, 2006

### Staff: Mentor

Ding ding ding. But you're supposed to let the newbies answer, zoobie. Talk about intellectual overkill !!

10. Jan 7, 2006

### Staff: Mentor

Dramatic? Um, you did understand the hysteresis curve thing and my explanation, right? It's all just straightforward physical stuff. If you want dramatic, you need to look at experiments with your microwave oven or something. Better yet, somebody else's microwave oven.:rofl:

11. Jan 8, 2006

### zoobyshoe

Zooby rhymes with newbie. I got confused.