B Direction of the force on a conductor in a solenoid

[Richie Smash]

Hi I have attached a picture here, of a solenoid with a copper block connected to a battery suspended in it.

The current moving is clockwise, So the magnetic poles would be as shown.

Now my problem always is this, I get so confused with this, I used Fleming's left hand rule to find the direction of the force acting on the copper, and I get upwards.

But something tells me it is downwards, is there some way I could do this more easily?

 

[BvU]

Is there a force on the copper block ?

 

[Richie Smash]

Yes there must be, because in the case I'm getting it from, the block is the conductor connected to a battery, and it is inside the magnetic field of the solenoid.

 

[BvU]

in the case I'm getting it from

What case is that ? A lab experiment, a textbook exercise, ... ?

Looks like you are short-circuiting the battery and the current is in the same direction as the magnetic field of the solenoid. Correct me if I interpret the picture in the wrong way.

 

[Richie Smash]

Hi, Sorry for being vague, it's from a question.

But I did not post in th ehomework section because, I completed the question, I just do not have a firm grasp on determining the direction of force on a current carrying conductor in a magnetic field.

I really do find it confusing, here is a photo of the original diagram, saying a copper rod connected to a battery is suspended in the centre of a magnetic field of a solenoid.

I think the force will go Up, but I have a feeling I'm wrong, all these right hand and left hand rules are confusing.

 

[jartsa]

Hi I have attached a picture here, of a solenoid with a copper block connected to a battery suspended in it.

The current moving is clockwise, So the magnetic poles would be as shown.

Now my problem always is this, I get so confused with this, I used Fleming's left hand rule to find the direction of the force acting on the copper, and I get upwards.

But something tells me it is downwards, is there some way I could do this more easily?

Yes. Currents pointing to same direction attract. Currents pointing to the opposite directions repel.

 

[Richie Smash]

So it would move downward then in this case?

 

[jartsa]

So it would move downward then in this case?

Yep.

 

[BvU]

all these right hand and left hand rules are confusing.

I sympathize, and propose to drop, forget and bury everything with "left hand" in it. For good.

Learn about a vector product (corkscrew rule works for me) and memorize two laws: the most important one is for the Lorentz force:$$
\vec F_L = q\;(\vec E+\vec v\times\vec B\,) $$ and the other is
$$ d\vec B = I \times d\vec l $$(this last one with some constants and some r dependency, of course )

Old thread here

 

[Richie Smash]

Thanks BVU, I'm going to have to learn about that in detail eventually, but from what I've read so far, I know the current in the picture i posted is then moving in the positive x direction, and the magnetic field the postive y direction, so therefore the resulting vector Force would be in the negative z direction?

 

[BvU]

Funny, I don't see no $x,y,z$ directions defined ... and my telepathic capabilities fail me altogether.
From your wording I suspect your coordinate system is different from what I would impose.
Please clarify.

 

[Richie Smash]

Hmm well ok, in the picture , A is the South Pole and B is the North pole, of the magnetic field.
So from the A to the B straight down the middle is the direction of the magnetic field, so this is the postive Y direction as i picture it.

So I believe the current, will also be going in the postive x direction from the battery at the top of the suspending wires.

Now Since the two are going in the postive, that means the resultant force is in the Negative Z, or down yes?

 

[BvU]

A is the South Pole and B is the North pole,

Contradicts

from http://onlinephys.haplosciences.com/magnetism.html (who took it from McGraw-Hill...)

the current, will also be going in the postive x direction

In the copper block it goes towards the viewer, so if you call that the positive x-axis, the direction of the z⁺ axis is downward.

 

[Richie Smash]

I had watched this video, and that''s how I made my assumption, as she said if the current in going clockwise, the South Pole is on the left hand side.

 

[BvU]

See and follow post #9
Clockwise and anticlockwise are confusing terms. In your picture the direction is opposite from that in the video.

 

[Richie Smash]

See and follow post #9
Clockwise and anticlockwise are confusing terms. In your picture the direction is opposite from that in the video.

Ok, I see was picturing the direction a lot in my head and perhaps I got it wrong, I will definitely spend a lot of time on post 9 then, I'm unfamiliar with the concepts so I will have to do it.