Ferromagnetism and Work done by a bar magnet.

[siddharth5129]

I was just doing a read-through of my freshman griffith's electrodynamics textbook ( I found my comprehension of electrodynamics slipping again ... always gets me edgy ) and I find my self flummoxed yet again. So he goes through an example of magnetic forces lifting a weight, and shows how it's actually the source of emf that's doing the work against magnetic forces to keep the weight moving upwards. This makes perfect sense. So I find that it would make sense when applied to say, two bar magnets attracting one another, if the magnetic forces somehow acted against the microscopic dipole currents and reduce their strength. If the magnet was reinforced with a electric coil, then the source emf would be doing the work against the magnetic forces, no problem there. But if it isn't, do the dipole currents reduce in strength ( in this case, alignment, seeing as the dipole moments that contribute to ferromagnetism are quantized ), as i am forced to conclude? Or am i missing something here? An iron magnet doesn't have an in built source of emf that does the work does it ?

 

[Jano L.]

On which page of Griffiths' book on electrodynamics is the example described? From you post it is not cleat what the situation looks like.

 

[siddharth5129]

Section 5.1.3 of the text. Example 5.3, page 210 and 211. Thanks for your time.

 

[Jano L.]

emf of the battery does not do work against magnetic forces - those cannot give or accept work. The emf does work against electric field in the source (battery) to maintain voltage and electric field driving the current along the wire. If the current is strong enough, the magnetic force will accelerate the metal upwards. This acceleration goes with induced electric field of the source and the wire itself (due to current distribution changing its position in space) which acts back on the wire and does work against the gravity force. The gravitational potential energy comes from the energy of the battery.

In case of two magnets with solid magnetization, the attraction is due to magnetic forces, but as soon as the magnets are released and accelerate, electric forces kick in and do the work to increase kinetic energy of the magnets. This is at the expense of the original mutual magnetic energy of the magnets.