Why is a magnet not an infinite source of energy?

In summary: Thin enough that the magnet is still attracted to the door strongly. Would the magnet still hold itself up or slide to the floor? It's the friction between the magnet and the fridge door that holds it up not the magnetic field. Gluing something to the fridge door is just another way of increasing the friction. The paint on the fridge door doesn't need an energy source to stop itself falling off either :-)
  • #1
aspaceo
1
0
Hi Guys

Could someone please point out where I'm going wrong with the following problem.

If I place a magnet on a vertical surface such that the magnet must resit the force of gravity in order to hold itself in place, will this magnet stay in place forever? If it does why does this not mean the magnet is supplying infinite energy to resit the force of gravity. My thinking was that the magnet would naturally degrade over time thus getting out of the infinite energy problem, however, I'm not at all convinced this is correct.

Any help you can give me on this problem would be hugely appreciated.

Thanks
 
Physics news on Phys.org
  • #2
Hi aspaceo! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Your kitchen table is holding itself in place, too, without the expenditure of energy.

Practical magnets do degrade with time, but measured in years if you don't knock them about.
 
Last edited by a moderator:
  • #3
aspaceo said:
Hi Guys

Could someone please point out where I'm going wrong with the following problem.

If I place a magnet on a vertical surface such that the magnet must resit the force of gravity in order to hold itself in place, will this magnet stay in place forever? If it does why does this not mean the magnet is supplying infinite energy to resit the force of gravity. My thinking was that the magnet would naturally degrade over time thus getting out of the infinite energy problem, however, I'm not at all convinced this is correct.

Any help you can give me on this problem would be hugely appreciated.

Thanks

This is all about the difference between Force and Energy. To 'get' Energy out of a device, a Force has to be moved through a Distance. Once a magnet has attracted a piece of iron (or a ball has rolled down a hill, spring being stretched etc. etc.) then more Energy (in the form of Work) needs to be transferred to go back to the start situation. A Force, exerted for millions of years, even, is doing no work if there is no movement.

In the case of an Electromagnet, you need to supply Energy (Electrical, this time) to generate the Magnetic Field and the field 'holds' energy, if you like. In an electromagnet, the (mechanical) Force times Distance is in a different (electromagnetic) form but you still have to put in Energy to set things up.

Perpetual Motion is a verboten subject on PF but many of the cranky ideas and suggestions about PM are based on a lack of appreciation of the difference between Force and Work. It can be an interesting exercise to de-bunk some of those loopy ideas you can find elsewhere on the Internet.
 
  • #4
aspaceo said:
Hi Guys

Could someone please point out where I'm going wrong with the following problem.

If I place a magnet on a vertical surface such that the magnet must resit the force of gravity in order to hold itself in place, will this magnet stay in place forever? If it does why does this not mean the magnet is supplying infinite energy to resit the force of gravity. My thinking was that the magnet would naturally degrade over time thus getting out of the infinite energy problem, however, I'm not at all convinced this is correct.

Any help you can give me on this problem would be hugely appreciated.

Thanks

Who needs magnets? Why isn't a postage stamp or a piece of adhesive tape 'an infinite source of energy'? Both items stick to vertical surfaces.
 
  • #5
If you look up the basic definitions of energy, force, and work you will see why a magnet does not have infinite energy.
 
  • #6
What Drakkith said.

Work or Energy = force x distance.

The magnet doesn't move so the distance is zero and the energy expended is zero.

Consider what would happen if you put a thin bit of ice or oil between a fridge magnet and a fridge door. Thin enough that the magnet is still attracted to the door strongly. Would the magnet still hold itself up or slide to the floor?

It's the friction between the magnet and the fridge door that holds it up not the magnetic field. Gluing something to the fridge door is just another way of increasing the friction. The paint on the fridge door doesn't need an energy source to stop itself falling off either :-)
 
  • #7
CWatters said:
Work or Energy = force x distance.

The magnet doesn't move so the distance is zero and the energy expended is zero.
That is only true in the rest frame of the magnet. There are inertial frames where the magnet does move. The key, to why the net work is zero, is not movement (which is relative) but the zero net force on the magnet.
 
Last edited:
  • #8
CWatters said:
What Drakkith said.

Work or Energy = force x distance.

The magnet doesn't move so the distance is zero and the energy expended is zero.

Consider what would happen if you put a thin bit of ice or oil between a fridge magnet and a fridge door. Thin enough that the magnet is still attracted to the door strongly. Would the magnet still hold itself up or slide to the floor?

It's the friction between the magnet and the fridge door that holds it up not the magnetic field. Gluing something to the fridge door is just another way of increasing the friction. The paint on the fridge door doesn't need an energy source to stop itself falling off either :-)
Not a very good example what if the door was horizontal,friction won't be holding it up then!
 

Related to Why is a magnet not an infinite source of energy?

1. Why can't we use magnets as an unlimited source of energy?

Magnets are not an infinite source of energy because they follow the laws of thermodynamics, which state that energy cannot be created or destroyed, only transformed. This means that magnets can only convert energy from one form to another, and cannot create new energy.

2. Can't we just continuously repel and attract magnets to create unlimited energy?

While it may seem like continuously repelling and attracting magnets could generate unlimited energy, the reality is that this process would eventually slow down and stop due to friction and other physical limitations. Additionally, the energy used to separate the magnets in the first place would need to be replenished, making it unsustainable as an infinite energy source.

3. Why do magnets eventually lose their magnetic properties?

Magnets can lose their magnetic properties over time due to factors such as exposure to high temperatures, physical damage, and exposure to other magnetic fields. As a result, the amount of energy that can be extracted from a magnet decreases over time, making it unsuitable as an infinite energy source.

4. Can magnets be used to generate energy at all?

Yes, magnets can be used to generate energy through processes such as electromagnetic induction. However, this energy is not unlimited and still follows the laws of thermodynamics. For example, a generator that uses magnets to generate electricity will eventually stop working once the energy stored in the magnets is depleted.

5. Is it possible to create a perpetual motion machine using magnets?

No, it is not possible to create a perpetual motion machine using magnets or any other means. As stated by the laws of thermodynamics, perpetual motion is impossible because it would require an infinite source of energy, which does not exist. Therefore, a magnet cannot be used as a component of a perpetual motion machine.

Similar threads

  • Electromagnetism
2
Replies
59
Views
4K
  • Electromagnetism
Replies
7
Views
1K
Replies
1
Views
1K
Replies
15
Views
3K
Replies
6
Views
2K
Replies
10
Views
1K
Replies
16
Views
2K
  • Electromagnetism
Replies
18
Views
2K
Replies
8
Views
2K
Replies
3
Views
726
Back
Top