Energy Loss From Permanent Magnets

[Eugbug]

Here's a question I posted about 10 years ago as a member of the forum. No one seemed to provide a satisfactory answer then so maybe someone might have some ideas on the subject:

When a permanent magnet attracts an object it does work and loses energy.
When the object is pulled away is the magnetic field rejuvenated to some extent and energy transferred back to the magnet?
I often wondered about this because the magnetic catches on cupboard doors never seem to wear out which would happen if all the magnetic potential energy was used up.
The magnet is in a fixed position and exerts a force on the piece of metal on the door. The door is accelerated by the force so it gains kinetic energy and the only source of energy in the system is from the magnetic field of the magnet.
There must be energy transfer from the magnet in order to accelerate the door and give it kinetic energy.

This is the conversation in the forum from then:

http://physicspost.com/physicsforums/topic.asp-ARCHIVE=&TOPIC_ID=4161.htm

 

[Drakkith]

The force from a magnet does not require an expenditure of energy to function. In a similar way, gravity doesn't require it either. Note that you cannot get repeatable work from a permanent magnet if all you are doing is allowing an object to be attracted to it in such a manner.

Edit: I don't know for sure, but it would appear to me to be similar to potential energy related to gravity. The potential energy is converted to kinetic and then lost when the object impacts the magnet. But again, I'm not sure.

 

[Eugbug]

When the door is moved close to the magnet and is let go, the magnet does work as it moves the mass of the door through a distance. That is the definition of work. The energy to do this must come from the magnet.
Presumably the same thing would happen if a distant object is moved into the influence of Earths gravity. The gravitational force would attract the body giving it velocity and kinetic energy. On impact, some of energy would be dissipated as heat.

 

[Meir Achuz]

The energy does not come from the magnet. The magnetic field energy changes.
It is the same when when a proton attracts an electron, the field energy changes, but not the proton.

 

[sardar]

I can provide some insight into this question about energy loss from permanent magnets. Firstly, it is important to understand that permanent magnets do not actually lose energy when they attract objects. The energy used to attract the object comes from the magnetic potential energy stored in the magnet itself. This potential energy is a result of the alignment of the magnetic domains within the magnet, which creates a magnetic field.

When an object is attracted to a permanent magnet, it is not the magnetic field that is doing work, but rather the magnetic force exerted by the field on the object. This force is conservative, meaning that it conserves energy and no energy is lost in the process. The magnet itself does not lose any energy in this process.

Now, when the object is pulled away from the magnet, the magnetic field does not get "rejuvenated" or regain energy. The magnetic field remains the same and the object simply moves out of the field. The energy used to pull the object away comes from the kinetic energy of the object, which was gained when it was attracted to the magnet.

In the case of a cupboard door, the magnet is in a fixed position and exerts a force on the metal object. This force accelerates the object, giving it kinetic energy. The source of this energy is not from the magnetic field of the magnet, but rather from the person pulling the door open. The magnet simply provides the force to overcome the object's inertia.

In summary, permanent magnets do not lose energy when they attract objects, and the energy used to pull an object away from a magnet comes from the object's own kinetic energy. The magnetic field of a permanent magnet remains constant and does not require any energy transfer to maintain its strength.