# Electromagnetism

#### AWSariti

Joined Nov 27, 2004
7
For us Newbies;

Often students are told that so-and-so is the case but not really why. For experienced physicists, not a problem, but for some newcomers just knowing that something is the case sometimes leaves us hanging. I realize that sometimes the explanation may be too complicated. But here is one re: electromagetism and mmf that I think I have puzzled out for myself. Maybe self-evident, but for a studnet I still think it needs be said.

In Chapter 14 we are told "the amount of magnetic field force generated by a coiled wire is proportional to the current through the wire multiplied by the number of "turns" or "wraps" of the wire in the coil." Great -- but why? How?

My take: the very small mmf generated by current through a straight wire over X distance (say 10 feet) is squeezed into a much shorter length when coiled, say to just 1 foot. The magnet field of the 10-ft wire is now piggy-backed on itself. The total mmf is the same, but it's now a flux stretching just over a 1-foot in length, so all that mmf of the ten feet (but coiled up) is having a bigger impact.

One thing I still can't understand--why is there no N and S pole in the single wire? When it moves the compass, does it move it randomly, first one way, then the other? It seems to me if there is a N/S in the coil, there has got to be a N/S in the wire. My guess would be it's too weak to be measured...but then, again, how just does it move the compass, and if it doesn't move it in a random pattern, it's got to have the N/S.

So, anyone out there who can comment on this?

#### Brandon

Joined Dec 14, 2004
306
Originally posted by AWSariti@Dec 30 2004, 03:06 PM
For us Newbies;

Often students are told that so-and-so is the case but not really why. For experienced physicists, not a problem, but for some newcomers just knowing that something is the case sometimes leaves us hanging. I realize that sometimes the explanation may be too complicated. But here is one re: electromagetism and mmf that I think I have puzzled out for myself. Maybe self-evident, but for a studnet I still think it needs be said.

In Chapter 14 we are told "the amount of magnetic field force generated by a coiled wire is proportional to the current through the wire multiplied by the number of "turns" or "wraps" of the wire in the coil." Great -- but why? How?

My take: the very small mmf generated by current through a straight wire over X distance (say 10 feet) is squeezed into a much shorter length when coiled, say to just 1 foot. The magnet field of the 10-ft wire is now piggy-backed on itself. The total mmf is the same, but it's now a flux stretching just over a 1-foot in length, so all that mmf of the ten feet (but coiled up) is having a bigger impact.

One thing I still can't understand--why is there no N and S pole in the single wire? When it moves the compass, does it move it randomly, first one way, then the other? It seems to me if there is a N/S in the coil, there has got to be a N/S in the wire. My guess would be it's too weak to be measured...but then, again, how just does it move the compass, and if it doesn't move it in a random pattern, it's got to have the N/S.

So, anyone out there who can comment on this?
[post=4361]Quoted post[/post]​
There is a N and S on a single wire, but it depends on your point of reference. Normally you don't worry about a magnetic field on a single wire. Its when you have more than 1 wire you have an issue. But I will explain.

We'll start with using your right hand. It is the key. Make a fist and stick your thumb out. Your thumb represents the direction of current flow. Your curling fingers represent the direction of magnetic flux generated by the flow of current. If you were to take a small flat bar and place it in line with a portion of your curled fingers, one end of the bar would be N the other end would be S. N and S only have to do with magnetic flux leaving a gausian surface or entering that surface. if you don't make a surface, you don't have poles. If you think about how a barmagnet is set up, the Mag field goes from one end to the other, very much like a battery. Think of a magnet just like a battery. In the end, thats what it is, a magfield storage device. In one end out the other. It why when you break a magnet you always will have a N and a S.

A good topic to read about to understand more about single wire magnetics is the Rail gun. I operates off of a single line of massive current to generate a mag field to accelerate an ionized conductor to fire some form of projectile. Neat neat stuff.

Now on to coils.

We did you thumb as the current, now let your fingers be the current. If you took that straight line with tons of disks around it and coiled the wire up, in the center of the coil of wire you have all those magnetic fields from every point of the wire being generated all folded into a single point. If you need a visual, take a few coins and stand them up in some clay or something and make a ring (donut)out of the coins. in the center they all touch. Since the current in the wire flows in one direction, all the magentic flux flows in one direction. Therefor that 1 coil of wire has its entire magnetic flux at a single point in the center ALL going in the same direction. For sake of argument, lets say it goes out the top and curves back down to go into the bottom.

This 1 coil of wire has a N (out) and a S (in) since it has been coiled up. Out the top, in the bottom. The air located in the coil is the surface (a round disk) with the center being the point of maximum M field. One side of this surface is N, the other side is S. The more you coil, the more overlap of M field you get at the center for each coil. Like your stacking one magnet after another on top of each other, which you can do. Just like putting batteries in series.

As for the compass and how is would deflect would depend on how the mag field produced by the wire intersected with the compass. Back to the right hand. The Magfield flows in 1 direction around the wire. If you place that wire above the compass then below the compass it will act totally different. In one position the mag field enters the top of the compass then out the bottom. The other way is the exact opposite. Lay a coin down on a table and rotate it clockwise while it is 'above' say another coin. Now take the rotating coin and slide is 'below' the other coin and spin in clockwise again. You will see that the stationary coin sees the same rotation speed of the other coin but in the opposite direction. If the other coin were a compass it would react differently to each.

Hope I have helped in some way. Its been a while since I did into magnetics and was trying to remember how my professor passed it down to us.

#### AWSariti

Joined Nov 27, 2004
7
Originally posted by Brandon@Dec 30 2004, 03:50 PM
There is a N and S on a single wire, but it depends on your point of reference. Normally you don't worry about a magnetic field on a single wire. Its when you have more than 1 wire you have an issue. But I will explain.

We'll start with using your right hand. It is the key. Make a fist and stick your thumb out. Your thumb represents the direction of current flow. Your curling fingers represent the direction of magnetic flux generated by the flow of current. If you were to take a small flat bar and place it in line with a portion of your curled fingers, one end of the bar would be N the other end would be S. N and S only have to do with magnetic flux leaving a gausian surface or entering that surface. if you don't make a surface, you don't have poles. If you think about how a barmagnet is set up, the Mag field goes from one end to the other, very much like a battery. Think of a magnet just like a battery. In the end, thats what it is, a magfield storage device. In one end out the other. It why when you break a magnet you always will have a N and a S.

A good topic to read about to understand more about single wire magnetics is the Rail gun. I operates off of a single line of massive current to generate a mag field to accelerate an ionized conductor to fire some form of projectile. Neat neat stuff.

Now on to coils.

We did you thumb as the current, now let your fingers be the current. If you took that straight line with tons of disks around it and coiled the wire up, in the center of the coil of wire you have all those magnetic fields from every point of the wire being generated all folded into a single point. If you need a visual, take a few coins and stand them up in some clay or something and make a ring (donut)out of the coins. in the center they all touch. Since the current in the wire flows in one direction, all the magentic flux flows in one direction. Therefor that 1 coil of wire has its entire magnetic flux at a single point in the center ALL going in the same direction. For sake of argument, lets say it goes out the top and curves back down to go into the bottom.

This 1 coil of wire has a N (out) and a S (in) since it has been coiled up. Out the top, in the bottom. The air located in the coil is the surface (a round disk) with the center being the point of maximum M field. One side of this surface is N, the other side is S. The more you coil, the more overlap of M field you get at the center for each coil. Like your stacking one magnet after another on top of each other, which you can do. Just like putting batteries in series.

As for the compass and how is would deflect would depend on how the mag field produced by the wire intersected with the compass. Back to the right hand. The Magfield flows in 1 direction around the wire. If you place that wire above the compass then below the compass it will act totally different. In one position the mag field enters the top of the compass then out the bottom. The other way is the exact opposite. Lay a coin down on a table and rotate it clockwise while it is 'above' say another coin. Now take the rotating coin and slide is 'below' the other coin and spin in clockwise again. You will see that the stationary coin sees the same rotation speed of the other coin but in the opposite direction. If the other coin were a compass it would react differently to each.

Hope I have helped in some way. Its been a while since I did into magnetics and was trying to remember how my professor passed it down to us.
[post=4363]Quoted post[/post]​

Thanks Bradon. That's the full explanation I was looking for. Tony

#### Brandon

Joined Dec 14, 2004
306
Originally posted by AWSariti@Jan 5 2005, 08:40 AM
Thanks Bradon. That's the full explanation I was looking for. Tony
[post=4424]Quoted post[/post]​