Direction of electron flow in solenoid

[BobRoss]

I'm having a bit of confusion applying the left hand rule to find the direction of electron flow in a coil. I'm given the following pictures.

1.)

To use the left hand rule here, I would wrap my hand around the object with my thumb pointing towards the north pole, so my fingers would indicate that the electrons are flowing from right to left. Is that correct?


2.)

For this second one, the electrons are flowing from left to right?



Next, I am asked to indicate the location of the north pole on the following diagrams.

3.)

Is the north pole on the right side here?



4.)

Here, the electrons leave the short bar and travel from the bottom to the top. So then the north pole is located at the top of the bar?

 

[gneill]

The direction of the wrapping of the coil winding matters, not the flow from "top to bottom" or "left to right". When you use the left hand rule, your fingers should point in the direction that the current is flowing in the wires making up the turns on the side of the coil where your fingers make contact.

 

[BobRoss]

So how would you place your hand on the conductor in the second diagram to figure out which direction the electrons are moving?

 

[gneill]

So how would you place your hand on the conductor in the second diagram to figure out which direction the electrons are moving?

Thumb points to north pole. Grab the coil. What direction do your fingers wrap? What direction are they pointing where they touch the wires?

 

[BobRoss]

With my thumb towards the north pole, my fingers wrap over the conductor and point towards the north pole. When I move my fingers to be touching the wires though, they point towards the south pole...

 

[gneill]

With my thumb towards the north pole, my fingers wrap over the conductor and point towards the north pole. When I move my fingers to be touching the wires though, they point towards the south pole...

How can your fingers point in the same direction as your thumb?

Your thumb should be sticking out (like you are hitch-hiking). Thumb direction and finger direction should be perpendicular!

 

[BobRoss]

Well they don't point in exactly the same direction. There is a drawing in my textbook that shows the electron flow to be in the direction that my knuckles are pointing, which is around the conductor and to the right. So then what do I say for the direction of the electrons in the second diagram? I'm not getting this for some reason.

 

[gneill]

Your fingers wrap in the direction of the current flow in the wires. Imagine that your fingers are carrying current (from knuckles to tips).

 

[BobRoss]

For the diagram 2 you said my thumb points towards the north pole when I grab the coil. Then if I imagine the current flowing through my fingers from knuckles to tips, the current is flowing towards the right?

 

[gneill]

For the diagram 2 you said my thumb points towards the north pole when I grab the coil. Then if I imagine the current flowing through my fingers from knuckles to tips, the current is flowing towards the right?

Nope. The current is flowing UP through the wires on the nearside and DOWN the through the wires on the other side --- the same way your fingers wrap around the solenoid. Trace the current in the wire to see which way the spiral progresses. You'll see that it makes its way towards the South pole in this case.

 

[BobRoss]

Okay, so for the first diagram I initially posted, the current is flowing down through the wires on the nearside and up through the wires on the other side? So the current direction would be towards the north pole for that one?

 

[gneill]

Okay, so for the first diagram I initially posted, the current is flowing down through the wires on the nearside and up through the wires on the other side? So the current direction would be towards the north pole for that one?

Yup. I think you've got it.

 

[BobRoss]

I think I was just confused by how my book explained the left hand rule. Just to be sure, for the third diagram the north pole is located at the left side and for the fourth diagram the north pole is on the bottom?

 

[gneill]

I think I was just confused by how my book explained the left hand rule. Just to be sure, for the third diagram the north pole is located at the left side and for the fourth diagram the north pole is on the bottom?

That looks right.

 

[BobRoss]

Great, thanks for your help!