# Diff bet short&long bar magnet,Magnetic field due to current in loop

## Main Question or Discussion Point

What is the difference between a short bar magnet and a long bar magnet ?
How is the magnetic field passing through circular loop similar to short bar magnet? And if a number of circular loops are connected to diff sources of current but attached together , do they form a solenoid ? Or are the same rules of circular loop magnetic field applied on them ?
I know that current in loop produces magnetic field in center ,but there is also magnetic field on the horizontal plane , that makes 2 mag fields , is there a net mag field ? And how is tis simlar to a bar magnet ? If a bar manet was like this , won't this leave it having 4 poles ? Please I am in desperate need of help !

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Please describe to me magnetic fields of short bar and long bar magnets if possible

rude man
Homework Helper
Gold Member
For a bar magnet the only measurable quantity is its moment. The moment is found by finding the torque exerted on the bar magnet when suitably aligned in a known B field.

τ = m x B
τ = torque, newton-m
m = magnetic moment, newton-m/T
B = magnetic field, Tesla

So what you do is align the bar magnet perpendicular to the B field and determine the torque exerted on the magnet about its center.

A single coil of area A carrying current i can be shown to have a magnetic moment m = i A. In that sense a bar magnet and a coil can be compared.

I don't know what you mean by "two magnetic fields". There is only one magnetic field, whether it's set up by a single-turn coil or a solenoid. The difference is that for a single coil it's extremely difficult to determine the B field anywhere but along its principal axis. It's easy to determine the (approximate) B field everywhere inside a solenoid but more difficult to determine the B field along its axis outside the solenoid.

I'm still new to all this that's why my questions may seem a bit irrational . In my text book ,the picture depicting the magnetic field due to current in circular loop goes like this (please try to image with me ,can't seem to attach pic.s,so pla bear with my primitive ways) a surface on which iron fillings are sprayed ,2 openings through which a wire is passed ,the 2 pt.s are collinear(horizontally)the wire emerges from opening one and is bent in the middle going through opening two forming an arch,the two end of the wire are connected to a source of current under the surface.
When a current is passed through the coil,the iron fillings are arranged such that : part of them form groups of concentric circles around each end of the wire (this is what I meant by the first magnetic field becase each pooint will form a magnetic pole ),the other part takes the shape of parallel lines in the middle of the surface , the lines are parallel to the transverse axis of the tube.There are also concentric circles around the middle part of the coil (perpendicular to its longitudinal axis ,around the peak of the arch) this part also has poles (this is what I meant by magnetic field 2)

HallsofIvy
Homework Helper
There really is no difference between "short" and "long" bar magnets, except, of course, scale. I suspect you are looking at pictures which show the field lines (as shown by iron filings) that look "rounded" for the short magnet, more parallel to the magnet for the long magnet.

But that is only because you are looking, comparatively, closer to the long magnet than for the short magnet. If you were to "blow up" the picture of the short magnet so it appeared to be as long as the long magnet, there would be, though not shown in your picture, additional field lines, not "rounded" close to the magnet and if you were to look further out in the picture of the field lines for the long magnet, you would see them just as rounded as in the picture of the short magnet.

There really is no difference between "short" and "long" bar magnets, except, of course, scale. I suspect you are looking at pictures which show the field lines (as shown by iron filings) that look "rounded" for the short magnet, more parallel to the magnet for the long magnet.

But that is only because you are looking, comparatively, closer to the long magnet than for the short magnet. If you were to "blow up" the picture of the short magnet so it appeared to be as long as the long magnet, there would be, though not shown in your picture, additional field lines, not "rounded" close to the magnet and if you were to look further out in the picture of the field lines for the long magnet, you would see them just as rounded as in the picture of the short magnet.
Thank you , I think I got the whole matter wrong ,so the magnetic field in the solenoid is the same as the circular loop ,but its much more concentrated , that's why its more rounded right ? But how can a short bar agnet have more intensity than the long bar one ?

Totally messed up , there are no two magnetic fields XD I guess my question would be , why are the iron fillings in the middle arranged in the form of straightlines