# Magnetic field of a current loop

## Main Question or Discussion Point

i know that a current loop creates some kind of a short bar magnet ,
but how ?
i mean in a short bar magnet , the magnetic field leaves the north pole and goes towards the south pole Of the MAGNET itself , but in the case of the solenoid or a coil , it just looks as if the magnetic field arises in air , do air molecules align themselves such as they become a magnetic bar ? or does the current just arise a magnetic circular field around the wire ?

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jtbell
Mentor
it just looks as if the magnetic field arises in air , do air molecules align themselves such as they become a magnetic bar ?
You don't need air in order to have a magnetic field around a wire.

VRT
...i mean in a short bar magnet , the magnetic field leaves the north pole and goes towards the south pole Of the MAGNET itself ...
It's not started on one pole and ended on the other one. It goes through the bar to make a closed loop. Magnetic field lines are always closed loops. One of Maxwell's equations is ∇B=0 what means there is no beginning and no end, or there is no "source" of magnetic lines.

well the thing here is that em forces are done by photons , they are like the little soldiers that make up the army.
whenever you have a current passing through a a coil no matter is it a loop or what you have also the em field.Now the em field made up by photons surrounds this conductor or the place where the action happens , there is a field around a coil without any additional metal but it's not as strong as it would be if there would be some metal inside the coil like an iron core , just like the one inductors and transformers have.
The thing is you could also use a as which is rich of electrons etc but the thing is a gas like air is very light , to really multiple the force you need something dense and not only something dense but something which is a good conductor, metals just happens to be good conductors both thermally and electrically it has to do with how their inner atoms are build up and that they have so called " free electrons" electrons that can be taken apart from their original atoms to form a conductive path for current for example , the same electrons also play role whenever that metal is subjected to a em field so the electrons align themselves and set up and multiple the field, that' s why you have metal cores inside the things I mentioned above.They would work without them just not that efficiently.

By the way the magnetic field can only arise from a source not from thin air, in the case of the coreless coil you mentioned the field arises from the coil because there is current that is passing through , when sufficient potential forms in clouds and dielectric discharge or lightning occurs the current travels from the sky through air towards ground and also a magnetic field and electric is formed , only this time the wire is air. whatever is a conductor and passes current through it can be taken to have a source of em field around it.

Drakkith
Staff Emeritus
i know that a current loop creates some kind of a short bar magnet ,
but how ?
i mean in a short bar magnet , the magnetic field leaves the north pole and goes towards the south pole Of the MAGNET itself , but in the case of the solenoid or a coil , it just looks as if the magnetic field arises in air , do air molecules align themselves such as they become a magnetic bar ? or does the current just arise a magnetic circular field around the wire ?
An electric current gives rise to a magnetic field around a wire because there are moving charges inside the wire. This works even when the wire is in a vacuum. When the wire is looped into a coil, the magnetic field from each loop adds together to form an electromagnet whose strength is far greater than the magnetic field of a single wire.

well the thing here is that em forces are done by photons , they are like the little soldiers that make up the army.
Whoa, easy buddy. Let's not confuse the guy by getting into virtual particles. That is WAAAY above his head.

whenever you have a current passing through a a coil no matter is it a loop or what you have also the em field.Now the em field made up by photons surrounds this conductor or the place where the action happens , there is a field around a coil without any additional metal but it's not as strong as it would be if there would be some metal inside the coil like an iron core , just like the one inductors and transformers have.
To my knowledge the EM field is not made of photons. It's not really made of anything. Also, most of your post is very confusing, even to me.

An electric current gives rise to a magnetic field around a wire because there are moving charges inside the wire. This works even when the wire is in a vacuum. When the wire is looped into a coil, the magnetic field from each loop adds together to form an electromagnet whose strength is far greater than the magnetic field of a single wire.

Whoa, easy buddy. Let's not confuse the guy by getting into virtual particles. That is WAAAY above his head.

To my knowledge the EM field is not made of photons. It's not really made of anything. Also, most of your post is very confusing, even to me.
actually i do understand photons :P and how they are not particles but rather quants of energy ,
i just happened to always have not-so-good physics teachers , so my basic physics is down , while i love to read advanced physics so its kind of weird how i understand some big stuff and misunderstand others

well the thing here is that em forces are done by photons , they are like the little soldiers that make up the army.
whenever you have a current passing through a a coil no matter is it a loop or what you have also the em field.Now the em field made up by photons surrounds this conductor or the place where the action happens , there is a field around a coil without any additional metal but it's not as strong as it would be if there would be some metal inside the coil like an iron core , just like the one inductors and transformers have.
The thing is you could also use a as which is rich of electrons etc but the thing is a gas like air is very light , to really multiple the force you need something dense and not only something dense but something which is a good conductor, metals just happens to be good conductors both thermally and electrically it has to do with how their inner atoms are build up and that they have so called " free electrons" electrons that can be taken apart from their original atoms to form a conductive path for current for example , the same electrons also play role whenever that metal is subjected to a em field so the electrons align themselves and set up and multiple the field, that' s why you have metal cores inside the things I mentioned above.They would work without them just not that efficiently.

By the way the magnetic field can only arise from a source not from thin air, in the case of the coreless coil you mentioned the field arises from the coil because there is current that is passing through , when sufficient potential forms in clouds and dielectric discharge or lightning occurs the current travels from the sky through air towards ground and also a magnetic field and electric is formed , only this time the wire is air. whatever is a conductor and passes current through it can be taken to have a source of em field around it.
that was very very very helpful :D made me understand alot of things , also , do photons move in a circular path * loop * ? or are they just just spread out inside the magnetic field ?

Drakkith
Staff Emeritus
actually i do understand photons :P and how they are not particles but rather quants of energy ,
i just happened to always have not-so-good physics teachers , so my basic physics is down , while i love to read advanced physics so its kind of weird how i understand some big stuff and misunderstand others
Trust me. You do not understand photons. If you did you wouldn't have asked the following question.

that was very very very helpful :D made me understand alot of things , also , do photons move in a circular path * loop * ? or are they just just spread out inside the magnetic field ?
No, this is entirely wrong. The magnetic field is not made of photons. There are no photons moving anywhere. Forget you ever heard of photons for now. Photons are a Quantum Mechanical idea, NOT classical. Learn classical physics first, then move on to learning Quantum Mechanics.

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Trust me. You do not understand photons. If you did you wouldn't asked the following question.

No, this is entirely wrong. The magnetic field is not made of photons. There are no photons moving anywhere. Forget you ever heard of photons for now. Photons are a Quantum Mechanical idea, NOT classical. Learn classical physics first, then move on to learning Quantum Mechanics.
okay okay :P what is the classical explanation then if i am not ready for photons ? a wire just builds a magnetic loop around it perpendicular to it ? just like a bar magnet ?
without explanations or anything ? i guess i will have to take it as it is until i get to study physics at the university

Trust me. You do not understand photons. If you did you wouldn't asked the following question.

No, this is entirely wrong. The magnetic field is not made of photons. There are no photons moving anywhere. Forget you ever heard of photons for now. Photons are a Quantum Mechanical idea, NOT classical. Learn classical physics first, then move on to learning Quantum Mechanics.
i dont understand how photons present the magnetic field , but i do know how photon exchange happen between charged particles * in laymans terms * , also how they are released from falling of excites electrons at specific wavelengths

Drakkith
Staff Emeritus
okay okay :P what is the classical explanation then if i am not ready for photons ? a wire just builds a magnetic loop around it perpendicular to it ? just like a bar magnet ?
without explanations or anything ? i guess i will have to take it as it is until i get to study physics at the university
Without getting into the math and equations and all, yes. A moving charge generates a magnetic field. So when you have current flow through a conductor a magnetic field is generated around it thanks to the charges moving through it.

just like a bar magnet ?
No actually. A wire will have a magnetic field with the field lines forming loops around the wire. A permanent magnet has field lines that loop from one end and loop back to the other end.

Drakkith
Staff Emeritus
i dont understand how photons present the magnetic field , but i do know how photon exchange happen between charged particles * in laymans terms * , also how they are released from falling of excites electrons at specific wavelengths
That's fine, but none of that will help you understand a magnetic field.

okay thanks alot for the explanation :)

or does the current just arise a magnetic circular field around the wire ?
Yes, this one, it is a manifestation of Ampere's law.

Forget photons, this is magnetostatics and has nothing to to with photons or even radiation. That post was just really confusing.

Ok if you say my last post was confusing then lets make this one as simple and robust as possible.
To the OP.
Some simple things.
A magnetic field just is , like you just are (the complete roots of either us and the field are still unknown so lets not get into them as others have suggested)

Magnetic field is like drakkith said wherever a charge is moving , as long as a charge moves whoula you have a field around it. In a conductor many charges move so the field is stronger , add a metal bar around that conductor and make it a coil and whoula the field is even more magnified due to the free charges that the metal offers aligning themselves to form the field.

Want the field stronger ? easy, increase current and conductor diameter if necessary
Want a weaker field ? decrease current , the simple way is add a resistor , even though there are more advanced ways which I will not go into.

About the bar magnet , a permanent magnet is magnetized when it's made (you can search google on how a magnet or a piece of metal become magnetized , search for magnets how they are made etc)
So in a permanent magnet you have the electrons aligned so the net field forms around the piece of metal, in an electro magnet you just have a normal piece of metal with a piece of wire around it which is also just metal , usually different kind of like copper , now the electrons in both the wire and the iron core are each on their own so no net field forms , put some current through the wire and the magnetic field aligns the electrons so now they resemble the permanent magnet and so you have a net field

Ok if you say my last post was confusing then lets make this one as simple and robust as possible.
To the OP.
Some simple things.
A magnetic field just is , like you just are (the complete roots of either us and the field are still unknown so lets not get into them as others have suggested)

Magnetic field is like drakkith said wherever a charge is moving , as long as a charge moves whoula you have a field around it. In a conductor many charges move so the field is stronger , add a metal bar around that conductor and make it a coil and whoula the field is even more magnified due to the free charges that the metal offers aligning themselves to form the field.

Want the field stronger ? easy, increase current and conductor diameter if necessary
Want a weaker field ? decrease current , the simple way is add a resistor , even though there are more advanced ways which I will not go into.

About the bar magnet , a permanent magnet is magnetized when it's made (you can search google on how a magnet or a piece of metal become magnetized , search for magnets how they are made etc)
So in a permanent magnet you have the electrons aligned so the net field forms around the piece of metal, in an electro magnet you just have a normal piece of metal with a piece of wire around it which is also just metal , usually different kind of like copper , now the electrons in both the wire and the iron core are each on their own so no net field forms , put some current through the wire and the magnetic field aligns the electrons so now they resemble the permanent magnet and so you have a net field
so the nail in the coil is just a better medium for propagation of magnetic field , thats great
now is the attracting objects different than aligning them ?
i know a magnet creates a magnetic field which aligns electrons in the same direction
but is it the same as the force pulling objects ? are they two different things ?

the actual physical way of how electrons align is not directly related to which way the the magnetic field attracts and how the field lines go.
In everyday engineering nobody does that basically because all you need to know to understand this is that you have a piece of wire say now and depending on to which way the current flows yopu can determine the magnetic field lines.
Remember that electric field lines and magnetic field lines are perpendicular to each other or you could say 90 degrees out of phase.

yes the nail is just a better medium for the field to propagate than air because the nail is iron which has much higher density and better atomic structure , electrons etc.

well basically if you have a magnet which has magnetic field around it every magnetically conducting object will be attracted to this magnet , the closer they will get the stringer a force they will feel.
This is because the closer a neutral metal gets to a magnet the more its electrons get aligned so it comes closer and closer until it reaches the magnet.

Drakkith
Staff Emeritus
Magnetic field lines are not perpendicular to electric field lines, that's the magnetic and electric field vectors in an em wave. I could charge a magnet and have both types of lines come out of it on each end.

Magnets don't produce a magnetic field due to electrons being aligned, it's due to the magnetic dipoles being aligned. Magnetisation is defined as the volume density of magnetic dipoles.

Magnetic field lines are not perpendicular to electric field lines, that's the magnetic and electric field vectors in an em wave. I could charge a magnet and have both types of lines come out of it on each end.
yes this has been confusing me , because in real life a magnet can attract an object in a direction parallel to the direction of the magnetic field * if the object is opposite to the north pole for instance , the direction of the magnetic field is so close to being parallel with the direction in which the force is applied to attract the object *

Drakkith
Staff Emeritus
yes this has been confusing me , because in real life a magnet can attract an object in a direction parallel to the direction of the magnetic field * if the object is opposite to the north pole for instance , the direction of the magnetic field is so close to being parallel with the direction in which the force is applied to attract the object *

that attraction is perpendicular to the magnetic field , but as i mentioned before , at some points the attraction force is very close to being parallel to the magnetic force * like at points facing the north , south pole of the magnet * , so it was confusing for me

ehild
Homework Helper
i know that a current loop creates some kind of a short bar magnet ,
but how ?
Ask the opposite: how bar magnets can exist?

The bar magnets are created by atomic current loops. The electrons orbiting around the nuclei are like current loops, so some magnetic momentum can be assigned to the atoms: they behave as small "bar magnets". These interact and align with each other making magnetic domains. These domains can also aligned and you get a piece of magnet.

ehild

that attraction is perpendicular to the magnetic field , but as i mentioned before , at some points the attraction force is very close to being parallel to the magnetic force * like at points facing the north , south pole of the magnet * , so it was confusing for me
The Lorentz force is indeed perpendicular to the magnetic field, but the magnetic field in the proximity of the pole of a permanent magnet is very inhomogeneous, and any current loop by its nature has a finite dimension.

BxI will therefore point slightly towards or away from the centre of the bar magnet (but always perpendicular to B itself), and it is the vector sum of BxI over the loop itself which will point towards or away from the magnet.

Watch for 5 minutes from 11:56:

The Lorentz force is indeed perpendicular to the magnetic field, but the magnetic field in the proximity of the pole of a permanent magnet is very inhomogeneous, and any current loop by its nature has a finite dimension.

BxI will therefore point slightly towards or away from the centre of the bar magnet (but always perpendicular to B itself), and it is the vector sum of BxI over the loop itself which will point towards or away from the magnet.

Watch for 5 minutes from 11:56: