# Indefinate Energy storage in inductor coil

#### balnag555

Joined Oct 17, 2011
2
HI,
I am thinking about storing energy in an inductor coil for indifinate time. My idea is to charge inductor coil through a battery connected in series with the coil. Now when i remove the battery from the circuit, the inductor cannot discharge the magnetic field as it is an open circuit. Inductor should discharge this energy only when the circuit is closed. So the magnetic field energy should remain for any period of time. please explain if i am wrong?

#### MikeML

Joined Oct 2, 2009
5,444
You are wrong! The current flow through the coil creates the magnetic flux (NI or Ampere-turns). At the instant the circuit is switched off (disconnected from your battery), the magnetic flux collapses, inducing a very large voltage (L dI/dt) across the opening switch, ionizing the air and creating an arc across the switch contacts. All of the energy stored in the inductor is consumed in the arc (which acts as a resistor) in a few ms.

#### crutschow

Joined Mar 14, 2008
25,686
Mike is correct in that you can't open the coil to store energy. But energy can be stored in a coil if you short the turns together after you store the energy in the inductor current (E = 1/2 LI$$^{2}$$). This energy can be stored for an indefinite time if the coil is made from superconducting wire as the current will keep circulating forever. For a normal coil the energy is rapidly dissipated in the coil wire resistance.

Capacitors are much better at storing energy long term then a standard inductor since they can be made with very low leakage.

#### Richie121

Joined Jan 12, 2014
27
Crutschow is correct as I learned this is how the huge MRI scanners operate and keep their strong magnetic field for years, but when discharged there is virtually no field at all.

#### wayneh

Joined Sep 9, 2010
16,402
...This energy can be stored for an indefinite time if the coil is made from superconducting wire ...
I understand and agree, but I must say I'm a bit befuddled by fields.

I love a good energy balance, so here's a thought experiment. Setting the current in motion in the superconductor takes energy, and that energy is stored in the field. Now an observer comes along and discovers your magnet and observes (only) its field. To the observer, it looks no different than a permanent magnet. How does the observer account for the energy? Would you say any magnet contains the field energy?

As I move a conductor past a magnet's field, I have to do work to overcome the field and the eddy braking. The magnet does no work and loses no energy. I've never really been able to reconcile that.

#### crutschow

Joined Mar 14, 2008
25,686
I understand and agree, but I must say I'm a bit befuddled by fields.

I love a good energy balance, so here's a thought experiment. Setting the current in motion in the superconductor takes energy, and that energy is stored in the field. Now an observer comes along and discovers your magnet and observes (only) its field. To the observer, it looks no different than a permanent magnet. How does the observer account for the energy? Would you say any magnet contains the field energy?

As I move a conductor past a magnet's field, I have to do work to overcome the field and the eddy braking. The magnet does no work and loses no energy. I've never really been able to reconcile that.
A static magnetic field does indeed contain energy. But you can't extract that energy unless you change the field intensity. If you do that with a permanent magnet, you destroy the magnet.

You can indeed generate energy in a conductor in the form of an electric current by passing a wire through the field but the energy is provided by the work required to move the wire through the magnetic field. The field is just a passive agent in the process with the field unchanged after the wire passes through.

#### nsaspook

Joined Aug 27, 2009
7,752
I understand and agree, but I must say I'm a bit befuddled by fields.

I love a good energy balance, so here's a thought experiment. Setting the current in motion in the superconductor takes energy, and that energy is stored in the field. Now an observer comes along and discovers your magnet and observes (only) its field. To the observer, it looks no different than a permanent magnet. How does the observer account for the energy? Would you say any magnet contains the field energy?

As I move a conductor past a magnet's field, I have to do work to overcome the field and the eddy braking. The magnet does no work and loses no energy. I've never really been able to reconcile that.
The 'work' that was used to produce the structure of the matter (dipole electron spin alignments in the atomic lattice is a QM effect that can't be explained in classical physics) in the magnet is expressed in the magnetic field of a permanent magnet. That field in isolation is not potential energy, it's really a part of a force can transfer 'work' as a expression of the change in the state of matter in space over time.

http://www.tcd.ie/Physics/Magnetism/Guide/understanding.php

#### balnag555

Joined Oct 17, 2011
2
Thankyou MikeML and crutschow. you have cleared my doubts. Probably that is why sparking occurs at wire joints/cracks. I am wondering if we do the experiment in absolute vacuum, due to the absence of air there is no chance of ionization and energy cannot be dissipated.

I am thinking of some way to stop discharging this built up magnetic field. Basically when the magnetic field is being discharged the inductor acts as a current source. So as the current flows in the coil it produces a magnetic field opposite to the existing magnetic field and cancelling the very existing magnetic field which is causing the flow of this current. Probably by using another coil (antiparallel winding like the case of transformer) which generates back emf due to flow of current in the existing coil and opposes the flow of current in the existing coil during discharging phase we might stop the discharge of magnetic field.

#### t_n_k

Joined Mar 6, 2009
5,455
You can't beat nature. Heard of the vacuum arc?

#### crutschow

Joined Mar 14, 2008
25,686
The energy will still be dissipated in a vacuum by thermal radiation and RF energy from the arc.

If you cancel out the field with another field then you end up with a net energy of zero.

A magnetic field is cause by movement of electric charges (in a permanent magnet it's the movement of the electrons orbiting the atoms aligned in the same direction). You can't get around that. If you stop the charge flow or cancel it with an opposing flow then you stop the magnetic field.

#### t_n_k

Joined Mar 6, 2009
5,455
So as the current flows in the coil it produces a magnetic field opposite to the existing magnetic field and cancelling the very existing magnetic field which is causing the flow of this current.
This is a misconception. The magnetic field is not cancelled by the ongoing current flow in the "discharging" state. The current flow and hence the field direction are the same as they were prior to the interruption of the primary energy source used in charging the coil. The coil emf, induced by the now decreasing / collapsing magnetic field, drives the (commensurately decreasing) current in the same direction until such time as the stored energy in the field is fully expended in the load and coil losses.

Perhaps you are confusing this situation with that of the ampere-turns balance in the mutually coupled windings of a transformer transferring energy from source to load.