How do inductors store energy as a magnetic field?

[Puglife]

My last lecture that I have had, was in inductors. My professor said that they resist changes in current, because they store energy inside the inductor as a magnetic field. I know that as soon as you shut off the energy between the inductor, it starts to dissipate, slowly, until it is completely depleted.

My question is this, how does an inductor actually store energy in it as a magnetic field, and why does the magnetic field instantly dissipate the second the current is shut off across it?

 

[my2cts]

This is an excellent question.
A good discussion can be found in Feynman's Lectures part 2, chapter 27. See the link below.
The discussion is about a capacitor storing energy in the E-field, but a similar story can be made for an inductor and the magnetic field.
Feynman clearly states this part of the theory with a certain amount of dissatisfaction. I quote:
"So our “crazy” theory says that the electrons are getting their energy to generate heat because of the energy flowing into the wire from the field outside. Intuition would seem to tell us that the electrons get their energy from being pushed along the wire, so the energy should be flowing down (or up) along the wire. But the theory says that the electrons are really being pushed by an electric field, which has come from some charges very far away, and that the electrons get their energy for generating heat from these fields. The energy somehow flows from the distant charges into a wide area of space and then inward to the wire."
"Finally, in order to really convince you that this theory is obviously nuts, ..."
http://www.feynmanlectures.caltech.edu/II_27.html

 

[anorlunda]

Magnetic? Yes.

Instantly? No.

You just said that it resists changes. That rules out instant changes in current.

The voltage across an inductor is $L\frac{dI}{dt}$. It can become very large when you try to stop I. That is how we generate sparks for the spark plugs in cars.

 

[Puglife]

Magnetic? Yes.

Instantly? No.

You just said that it resists changes. That rules out instant changes in current.

The voltage across an inductor is $L\frac{dI}{dt}$. It can become very large when you try to stop I. That is how we generate sparks for the spark plugs in cars.

I realize that it releases it gradually, sorry if I didn't make that clear, what I don't know is why and how it store energy and slowly releases it, instead of just instantly discharging

 

[Puglife]

This is an excellent question.
A good discussion can be found in Feynman's Lectures part 2, chapter 27. See the link below.
The discussion is about a capacitor storing energy in the E-field, but a similar story can be made for an inductor and the magnetic field.
Feynman clearly states this part of the theory with a certain amount of dissatisfaction. I quote:
"So our “crazy” theory says that the electrons are getting their energy to generate heat because of the energy flowing into the wire from the field outside. Intuition would seem to tell us that the electrons get their energy from being pushed along the wire, so the energy should be flowing down (or up) along the wire. But the theory says that the electrons are really being pushed by an electric field, which has come from some charges very far away, and that the electrons get their energy for generating heat from these fields. The energy somehow flows from the distant charges into a wide area of space and then inward to the wire."
"Finally, in order to really convince you that this theory is obviously nuts, ..."
http://www.feynmanlectures.caltech.edu/II_27.html

Ok, so how does that particularly apply to the question I asked, thank you for the nice response time.

 

[Puglife]

Magnetic? Yes.

Instantly? No.

You just said that it resists changes. That rules out instant changes in current.

The voltage across an inductor is $L\frac{dI}{dt}$. It can become very large when you try to stop I. That is how we generate sparks for the spark plugs in cars.

Also, if you shut off the current to a inductor, wouldn't the change in current be some arbitrary number over zero time? So wouldn't the voltage be infinite across its leads? Why does that not instantly disapate all energy in the inductor?

 

[my2cts]

Ok, so how does that particularly apply to the question I asked.

You asked "how does an inductor actually store energy in it as a magnetic field".
Feynman answers to the very similar question "how does a capacitor actually store energy in it as an electric field".

 

[anorlunda]

Also, if you shut off the current to a inductor, wouldn't the change in current be some arbitrary number over zero time? So wouldn't the voltage be infinite across its leads? Why does that not instantly disapate all energy in the inductor?

We don't get infinities in real life. When the voltage gets high, a spark jumps across the contacts of the switch and the current decreases in finite time.

A common error is to mix up ideal components with real life. The clue to that mix up, is when you get infinity for any answer. In real life, no R, L, C, V, I, or switch is purely ideal.

 

[tech99]

An inductor is analogous to a flywheel, having lots of inertia. It is hard to get the current flowing and hard to stop it. When you try to suddenly switch off, it is akin to applying a brake to a flywheel. It tries not to respond, but eventually the energy is dissipated as heat in whatever brake you use.

 

[Puglife]

An inductor is analogous to a flywheel, having lots of inertia. It is hard to get the current flowing and hard to stop it. When you try to suddenly switch off, it is akin to applying a brake to a flywheel. It tries not to respond, but eventually the energy is dissipated as heat in whatever brake you use.

I understand what it actually does, I just don't understand why? I don't see why the second you shut off the current across an inductor, how it is still able to maintain a magnetic field inside itself?

 

[tech99]

I understand what it actually does, I just don't understand why? I don't see why the second you shut off the current across an inductor, how it is still able to maintain a magnetic field inside itself?

If you think about a heavy flywheel spinning, it cannot maintain any momentum once you apply enough braking force to stop it. But it tries not to stop. Similarly, the inductor tries to stop you from switching off, by generating a high voltage across the switch gap. This is like the mechanical reaction against braking (F=-MA). But once you force the current down to zero, actually be applying a reverse voltage with the switch, the magnetic field has gone.

 

[Puglife]

If you think about a heavy flywheel spinning, it cannot maintain any momentum once you apply enough braking force to stop it. But it tries not to stop. Similarly, the inductor tries to stop you from switching off, by generating a high voltage across the switch gap. This is like the mechanical reaction against braking (F=-MA). But once you force the current down to zero, actually be applying a reverse voltage with the switch, the magnetic field has gone.

where does it get the energy to create that strong of a opposite voltage, if it is not connected to any source. My main issue is I don't see how it physically stores energy, rather, i see it as it allows energy to go through it, and that energy self inducts itself, and opposes itself, thus opposing current change. The issue with that way of thinking (which is probably totally wrong), is that the second the current stops flowing through it, the field should sease to exist, and thus the self induction instantly stops (i know this isn't what really happens now, but I don't know why at all). I would like to know the reasoning behind why it does what it does, instead of just know what it does. Thank you all very much for all of your help, you all are very kind to be helping me

 

[tech99]

where does it get the energy to create that strong of a opposite voltage, if it is not connected to any source. My main issue is I don't see how it physically stores energy, rather, i see it as it allows energy to go through it, and that energy self inducts itself, and opposes itself, thus opposing current change. The issue with that way of thinking (which is probably totally wrong), is that the second the current stops flowing through it, the field should sease to exist, and thus the self induction instantly stops (i know this isn't what really happens now, but I don't know why at all). I would like to know the reasoning behind why it does what it does, instead of just know what it does. Thank you all very much for all of your help, you all are very kind to be helping me

When a magnetic field is built, it is a store of energy. Maxwell originally used a mechanical analogy to develop his theories and it used spinning cells having inertia and momentum. The magnetic field will not collapse to nothing until you physically force the current down to zero. The inductor will not let you do this without a struggle.

 

[Puglife]

When a magnetic field is built, it is a store of energy. Maxwell originally used a mechanical analogy to develop his theories and it used spinning cells having inertia and momentum. The magnetic field will not collapse to nothing until you physically force the current down to zero. The inductor will not let you do this without a struggle.

why will it not let you force the current down to zero, and why is a electromagnetic field stored energy?

 

[meBigGuy]

Let's say you are passing 1 amp through an inductor with a parallel 1 ohm resistor and remove the supply.

The inductor has a magnetic field, and wants to maintain the 1 amp flow, so it will then create 1 volt to "send" the 1 amp through the 1 ohm resistor, reducing the magnetic field as it flows . The 1 amp reduces exponentially in time because a change in current is required to create the voltage across the resistor. This continues until the field is depleted.

If you do it with a 10 ohm resistor, it will initially generate 10 times the voltage to produce the initial 1 amp.

 

[NascentOxygen]

why will it not let you force the current down to zero, and why is a electromagnetic field stored energy?

It will, but you have to cause its current to reach zero before all its energy has been drained.

A magnetic field around a solenoid represents stored energy, because you can use it to power an electric or electronic circuit after all other voltage sources have been removed.

 

[my2cts]

why will it not let you force the current down to zero, and why is a electromagnetic field stored energy?

You are now ready to read the Feynman Lecture and see that even he did not understand it !

 

[Puglife]

It will, but you have to cause its current to reach zero before all its energy has been drained.

A magnetic field around a solenoid represents stored energy, because you can use it to power an electric or electronic circuit after all other voltage sources have been removed.

How does the magnetic field not instantly shut off as soon as you pull the switch? Like how does it physically store the energu

 

[Puglife]

You are now ready to read the Feynman Lecture and see that even he did not understand it !

If he does not know the answer, does anyone? Is their an actual concrete answer? And if he didn't know it, then what is the lecture on? And how will it help me?

 

[Puglife]

When a magnetic field is built, it is a store of energy. Maxwell originally used a mechanical analogy to develop his theories and it used spinning cells having inertia and momentum. The magnetic field will not collapse to nothing until you physically force the current down to zero. The inductor will not let you do this without a struggle.

So does it have yl do with maxwell theories on electromagnetic waves? Does it propionate because it turned into a em wave?

 

[NascentOxygen]

How does the magnetic field not instantly shut off as soon as you pull the switch? Like how does it physically store the energu

As others have pointed out, it is not possible to instantly halt current in an inductor. If you try to, by flipping open a switch, a high voltage is created and this jumps across the switch contacts, ionizing the air and heating the air and switch and delivering the inductor's stored energy as the equivalent in heat. The faster you try to operate the switch, the greater the voltage spark produced so it is still able to jump across the wider air gap. Some of the energy radiates away as radio waves, you hear this as static on your AM radio when there's an electrical spark nearby.

inductor voltage = L.$\frac {di} {dt}$

 

[Puglife]

As others have pointed out, it is not possible to instantly halt current in an inductor. If you try to, by flipping open a switch, a high voltage is created and this jumps across the switch contacts, ionizing the air and heating the air and switch and delivering the inductor's stored energy as the equivalent in heat. The faster you try to operate the switch, the greater the voltage spark produced so it is still able to jump across the wider air gap. Some of the energy radiates away as radio waves, you hear this as static on your AM radio when there's an electrical spark nearby.

inductor voltage = L.$\frac {di} {dt}$

I understand what it physically does, and exactly what happened in the real world, I just don't know why it does that. I know that it resists current change, what i don't know is why, is it that it produces a electromagnetic induction in the opposite direction of current flow, thus producing an opposing voltage? I also know that after the current stops completely, it will turn into a supply of voltage to maintain its previous current, I just don't know the reasoning behind why it does that (other than it resists changes to current, which isn't the answer I am looking for). I also do not know how it is possible for it to "store" energy as a magnetic field, the magnetic.
Thank you all for your help though, I appreciate you all spending the time to help me.

 

[NascentOxygen]

I also know that after the current stops completely, it will turn into a supply of voltage to maintain its previous current, ✘

You haven't got it right. Current doesn't reach zero until all stored energy has been delivered.

 

[Puglife]

You haven't got it right. Current doesn't reach zero until all stored energy has been delivered.

I see where the confusion in what I was trying to say came from, and it is this, when you shut off the current you are supplying to the inductor, it physically supplys its own voltage to maintain its own current, until the energy stored goes away (that was totally my bad, I worded it in a ridiculous manner), but that didnt answer any of my other questions, not help me in any way.

Also thank you for your good response time, its really appreciated!

 

[marcusl]

My main issue is I don't see how it physically stores energy

Energy storage in an electric field works like this. It takes energy to move and hold charges in place to create an electric field. The work done in assembling the configuration and building the field is not lost--it is said to be stored in the field, and can be recovered (again as work) by allowing the charges to move away back to their original locations. The energy was thus "stored" in the field.

To generate a current-based magnetic field, work must be done to hold charges together and to cause them to move in a coordinated way that we call a current. That work is, likewise, stored in the field and may be recovered if the current is allowed to decay. The recovered energy can be expended as work done on resistances in a circuit that is connected to the field/current assembly (assuming that it involves wires). The work is driven by a potential (a.k.a. EMF or voltage) that appears as the current decays. A voltage or potential, in fact, has units of energy (indeed, high energy particle physicists measure energy with the unit "electron volts"), so everything is consistent.

 

[tech99]

Energy storage in an electric field works like this. It takes energy to move and hold charges in place to create an electric field. The work done in assembling the configuration and building the field is not lost--it is said to be stored in the field, and can be recovered (again as work) by allowing the charges to move away back to their original locations. The energy was thus "stored" in the field.

To generate a current-based magnetic field, work must be done to hold charges together and to cause them to move in a coordinated way that we call a current. That work is, likewise, stored in the field and may be recovered if the current is allowed to decay. The recovered energy can be expended as work done on resistances in a circuit that is connected to the field/current assembly (assuming that it involves wires). The work is driven by a potential (a.k.a. EMF or voltage) that appears as the current decays. A voltage or potential, in fact, has units of energy (indeed, high energy particle physicists measure energy with the unit "electron volts"), so everything is consistent.

I don't think "allowed to decay" is quite accurate. To stop the current, we have to apply a force, a reverse voltage, just like stopping a flywheel, and the energy stored in the inertia of the system does work against our force by producing movement or current.

 

[Puglife]

Energy storage in an electric field works like this. It takes energy to move and hold charges in place to create an electric field. The work done in assembling the configuration and building the field is not lost--it is said to be stored in the field, and can be recovered (again as work) by allowing the charges to move away back to their original locations. The energy was thus "stored" in the field.

To generate a current-based magnetic field, work must be done to hold charges together and to cause them to move in a coordinated way that we call a current. That work is, likewise, stored in the field and may be recovered if the current is allowed to decay. The recovered energy can be expended as work done on resistances in a circuit that is connected to the field/current assembly (assuming that it involves wires). The work is driven by a potential (a.k.a. EMF or voltage) that appears as the current decays. A voltage or potential, in fact, has units of energy (indeed, high energy particle physicists measure energy with the unit "electron volts"), so everything is consistent.

Doesn't the heat dissipation, as well as the current into a resistor equaling the current out of a resistor, or in this case wire, conserve it's energy? How does the production if an electromagnetic field, continuously follow the conservation of energy. What is the energy loss in the circuit, that physically causes the electromagnetic field to form? where does the energy to do the initial work to create the field come from, because isn't the entirety of the energy drop from resistance dissipated through heat?

 

[anorlunda]

Doesn't the heat dissipation, as well as the current into a resistor equaling the current out of a resistor, or in this case wire, conserve it's energy? How does the production if an electromagnetic field, continuously follow the conservation of energy. What is the energy loss in the circuit, that physically causes the electromagnetic field to form? where does the energy to do the initial work to create the field come from, because isn't the entirety of the energy drop from resistance dissipated through heat?

There are such things as superconducting electromagnets with zero resistance, they store energy too. So it is better to divorce resistance from inductance in your thinking.

 

[Puglife]

There are such things as superconducting electromagnets with zero resistance, they store energy too. So it is better to divorce resistance from inductance in your thinking.

ok, in that case, where does the energy physically come from, in order to produce the magnetic field, if their is no energy loss in terms of electricity in the circuit? does it not apply to conservation of energy for some reason?

 

[anorlunda]

The energy comes from the external circuit that is forcing the current and supplying the voltage. P=V*I.