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Author Topic: what is the difference between a temporary magnet and an electromagnet?  (Read 4710 times)

Offline taregg

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are both are same ....becuse i read some in websites..... that say temporary magnet its wire current and coil..... and also they say electric magnet its wire current and coil...????
« Last Edit: 25/09/2013 19:08:35 by chris »


 

Offline chris

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My understanding is that a permanent magnet refers to a magnetic body in which the magnetic field is constantly present without the supply of energy or some other such influence. A temporary magnet is a body that can develop a magnetic field when induced to do so, such as by supplying an electric current; usually this means an electromagnet comprising a coil connected to a DC power source and a ferrous (iron) core.
 

Offline dlorde

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I'd say an electromagnet is a kind of temporary magnet that uses an electric current to generate a magnetic field. Perhaps another kind of temporary magnet is a metal that acts as a magnet when in contact with another magnet, but not on its own.
 

Offline evan_au

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There are "soft" and "hard" magnetic materials.

In a "soft" magnetic material, if you apply a strong external magnetic field (eg from a coil of wire with DC current flowing*), the magnetic field in the material will almost totally** disappear as soon as you remove the external magnetic field. This is a temporary magnet.

In a "hard" magnetic material, if you apply a strong external magnetic field (eg from a coil of wire with DC current flowing*), much of the magnetic field will be retained in the material, even after the external field is removed. This makes a permanent magnet.

*The external magnetic field does not need to come from a coil or wire - it can come from stroking a magnetic material with a permanent magnet, or even from beating the material in the Earth's (fairly weak) magnetic field.

**How much of the magnetic field is retained after the external field is removed can be seen from the hysteresis curve of the magnetic material. Soft magnetic materials have a "skinny" hysteresis curve and retain very little of the external field, while hard magnetic materials have a "wide" hysteresis curve, and retain most of the external field (high remanence).
« Last Edit: 26/09/2013 00:59:37 by evan_au »
 

Offline chris

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Evan - iron satisfies both doesn't it, but when the DC is switched off in an electromagnet the iron demagnetises does it not?
 

Offline evan_au

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Pure iron can be used as the core of an electromagnet, and it is a temporary magnet. So the two terms are not mutually exclusive.

Coercivity determines whether a material is considered magnetically "hard" or "soft". http://en.wikipedia.org/wiki/Coercivity

Pure iron has low coercivity=magnetically "soft": You can turn it into an electromagnet by winding a coil around it and putting a current through it. When the current stops, most of the magnetic field is lost (ie it is a temporary magnet). The iron retains a weak remanent magnetic field (ie it is a weak permanent magnet).

But most kinds of iron are alloys, which can have quite different magnetic properties.
Tables of remanent magnetic field stregths can be found here: http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_6.html

Because it is hard to change the direction of magnetism in a "hard" magnetic material, they are often formed by heating them above their Curie temperature (where ferromagnetic effects disappear), and letting them cool inside a strong electromagnet. They then take on the magnetic field of the electromagnet.

So iron can be different kinds of magnet, depending on its precise chemical composition, crystal structure, history of exposure to magnetic fields, its history of thermal and mechanical treatment - and how closely you look...
 

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