# Extra magnetic energy

• B
A long thin current-carrying conductor have a magnetic field around it in the direction according the RHR. If a compass needle was placed beside the wire it moves to point in the direction of the magnetic field. Now the current in the wire doesn't diminish and can still light a bulb in its way. Correct? if so, then how did the compass needle moved since energy can't be created. The magnetic effect gives the needle an initial kinetic energy before coming to rest as it aligns with the field. Where is the energy from?

Think about the magnet as a very small wire with current. But this wire is free to move

Now recall that two wires can attract or repel. But if this small wire (magnet) is fixed at its center, this wire will experience torque from both of its sides (actually, each point in the magnet). Then, as time flows. The magnet reach an equilibrium and stops moving. This equilibrium position is the direction of the magnetic field from the large wire.

Where does this energy come from? from the field of course. You can think of each molecules in the small wire and the large wire being attracted or repelled by each others, but the large wire wins. Remember Newton's third law!. You probably did or studied the experiment of two wires repelling and attracting (Both bends slightly, but since they are fixed they do not attach to each other nor repel to infinity).

#### Attachments

• 2000px-Earths_Magnetic_Field_Confusion.svg.png
113.2 KB · Views: 266
Nugatory
Mentor
Where is the energy from?
There are two sources of this energy. Their relative contribution depends on the details of your setup.
1) Energy comes from whatever is generating the current in the wire: the power requirement increased slightly for a moment as you moved the magnetic needle into place.
2) When you move the compass needle into place you're applying some small force so doing some small amount of work and adding energy to the system.

Last edited:
phinds, Zaya Bell and CWatters
Where does this energy come from? from the field of course.
Okay, but does that mean it will cost the battery(for example) an "extra" energy for that attraction or perhaps whether or not there's another object being attracted, their is always that energy loss due to the magnetic field

Energy comes from whatever is generating the current in the wire: the power requirement increased slightly for a moment as you moved the magnetic needle into place.
Oh! So whenever it is being attracted, there's a cost in the battery (for an example)

A good practical example of this effect:

Discrete solenoid valves operate by energizing a coil to magnetically move a 'slug' to impede (or allow) flow - not fundamentally different from your compass needle. For test of critical devices ( spacecraft thrusters, for example) the current/voltage to the coil is often monitored - the difference between a 'moving' slug and a 'stopped' slug is very clear.

Okay, but does that mean it will cost the battery(for example) an "extra" energy for that attraction or perhaps whether or not there's another object being attracted, their is always that energy loss due to the magnetic field

For the needle: When the needle spin to align in the direction of the magnetic field from the current, there is work done. The work and kinetic energy cease when it stops (v=0, Δd=0).

For the wire: The needle exhibits magnetic field in the space. If you are considering the wire as a uniformly charged object, disregarding that it is in fact a flow of particles, you would say there is no work done on the current since it is not moving. But if you considered the current as a flow of particles, each particle is displaced by Δd and move with velocity v in the presence of the magnetic field of the needle and so there is work done by the magnetic.

So I would say: it does affect the current flow. But I am not sure how would this affect the longevity of the battery, as the battery output energy is fixed.

For the needle: When the needle spin to align in the direction of the magnetic field from the current, there is work done. The work and kinetic energy cease when it stops (v=0, Δd=0).

For the wire: The needle exhibits magnetic field in the space. If you are considering the wire as a uniformly charged object, disregarding that it is in fact a flow of particles, you would say there is no work done on the current since it is not moving. But if you considered the current as a flow of particles, each particle is displaced by Δd and move with velocity v in the presence of the magnetic field of the needle and so there is work done by the magnetic.

So I would say: it does affect the current flow. But I am not sure how would this affect the longevity of the battery, as the battery output energy is fixed.
Thanks, I think I get it now

For the needle: When the needle spin to align in the direction of the magnetic field from the current, there is work done. The work and kinetic energy cease when it stops (v=0, Δd=0).

For the wire: The needle exhibits magnetic field in the space. If you are considering the wire as a uniformly charged object, disregarding that it is in fact a flow of particles, you would say there is no work done on the current since it is not moving. But if you considered the current as a flow of particles, each particle is displaced by Δd and move with velocity v in the presence of the magnetic field of the needle and so there is work done by the magnetic.

So I would say: it does affect the current flow. But I am not sure how would this affect the longevity of the battery, as the battery output energy is fixed.
I may have to experiment to know about battery life.