Is there any reference about how long magnetic remanence lasts, or is it permanent?

Quote from: rukiddingme on April 12, 2010, 04:27:21 AM
Is there any reference about how long magnetic remanence lasts, or is it permanent?

Try to google the issue, and see what you find. Usually most magnets are lasting for very long time, but temperature, exposure of other stronger magnets, and how you store them is important factors.

Vidar

Quote from: Low-Q on April 12, 2010, 08:54:32 AM
Try to google the issue, and see what you find. Usually most magnets are lasting for very long time, but temperature, exposure of other stronger magnets, and how you store them is important factors.

Vidar

That's not exactly what I am trying to figure out.

There are actually two aspects to the question.

1.) If you have an electromagnet and you pulse the coil, the core becomes magnetized. A little of the magnitism is retained by the core, but most of it dissipates. How long does it take for most of the field to dissipate?

2.) If you pass a magnet past a piece of metal, the piece of metal becomes magnitized. Some of the magnitism is retained by the metal, but most of it dissipates. How long does it take for most of the field to dissipate?

Here's the wiki article about remanence: http://en.wikipedia.org/wiki/Remanence

Remanence is the magnetization left behind in a medium after an external magnetic field is removed. It is denoted in equations as Mr. In engineering applications it is often assumed that the magnetization  M is synonymous with the residual flux density B (they differ by a factor of µ0, BR = µ0M) hence the remanence is frequently denoted as BR  (see the image). Only substances that can be magnetized, ferromagnetic and ferrimagnetic materials such as iron, have remanence.

To the right is an image of the hysteresis loop. I can't figure out where the time is shown.

Is there any reference that can give an approximate time frame for this? Are we talking a millisecond, ten milliseconds, a hundred milliseconds, five hundred milliseconds, a second?

I realize that it depends on the material. If anyone could give a guess how long it takes for the field to dissipate for cold rolled steel, it would be greatly appreciated.

Thanks in advance.

Quote from: rukiddingme on April 12, 2010, 11:23:07 AM
That's not exactly what I am trying to figure out.

There are actually two aspects to the question.

1.) If you have an electromagnet and you pulse the coil, the core becomes magnetized. A little of the magnitism is retained by the core, but most of it dissipates. A does it take for most of the field to dissipate?

2.) If you pass a magnet past a piece of metal, the piece of metal becomes magnitized. Some of the magnitism is retained by the metal, but most of it dissipates. How long does it take for most of the field to dissipate?

Here's the wiki article about remanence: http://en.wikipedia.org/wiki/Remanence

Remanence is the magnetization left behind in a medium after an external magnetic field is removed. It is denoted in equations as Mr. In engineering applications it is often assumed that the magnetization  M is synonymous with the residual flux density B (they differ by a factor of µ0, BR = µ0M) hence the remanence is frequently denoted as BR  (see the image). Only substances that can be magnetized, ferromagnetic and ferrimagnetic materials such as iron, have remanence.

To the right is an image of the hysteresis loop. I can't figure out where the time is shown.

Is there any reference that can give an approximate time frame for this? Are we talking a millisecond, ten milliseconds, a hundred milliseconds, five hundred milliseconds, a second?

I realize that it depends on the material. If anyone could give a guess how long it takes for the field to dissipate for cold rolled steel, it would be greatly appreciated.

A in A.

Now I see Sorry! Well the answer is still how the metal is treated with temperature, other magnetic materials etc. To cancel the magnetism, you can use a coil with a high freqyency and gradually decrease the amplitude till zero. Then the metal should be neutral again. But at room temperature with no other treatments, I really don't know for how long it will last. I have however magnetized screwdrivers, and the magnetism is considerably weaken after a week or so.

Vidar

Quote from: Low-Q on April 12, 2010, 11:59:42 AM
Now I see Sorry! Well the answer is still how the metal is treated with temperature, other magnetic materials etc. To cancel the magnetism, you can use a coil with a high frequency and gradually decrease the amplitude till zero. Then the metal should be neutral again. But is room temperature with no other treatments, I really don't know for how long it will last. I have however magnetized screwdrivers, and the magnetism is considerably weaken after a week or so.

Vidar

I think you are thinking about retentivity:
http://en.wikipedia.org/wiki/Retentivity

The retentivity of a material  is its capacity to remain magnetized  after the external magnetizing field has ceased to exist.

I was under the impression that when you pulse an electromagnet, the core becomes magnetized. When the pulse ends, there is a period of time that it takes for the magnetic field to dissipate. This is the number I am looking for. How long a piece of steel takes to lose its magnetism after being pulsed.

Maybe I have it wrong, maybe a steel core loses it's magnetic field instantly, but that doesn't seem intuitive.

I'll keep researching this and if I can find the answer, I'll post it here.