Magnetic Field due to Opposing Coils

[mitchy16]

Homework Statement

Describe the magnetic field you would observe if two coils were connected with magnetic fields pointing in opposite directions.

Homework Equations

No equation

The Attempt at a Solution

If the magnetic field was pointing in opposite directions, would it mean that it would cancel out as a result of the opposing directions? I'm not sure if the question meant current, or if they are actually asking magnetic field. Thanks!

 

[berkeman]

Homework Statement

Describe the magnetic field you would observe if two coils were connected with magnetic fields pointing in opposite directions.

Homework Equations

No equation

The Attempt at a Solution

If the magnetic field was pointing in opposite directions, would it mean that it would cancel out as a result of the opposing directions? I'm not sure if the question meant current, or if they are actually asking magnetic field. Thanks!

Are you familiar with that the external magnetic field looks like for a coil that is carrying a current?

BTW, is there a figure that goes with this question? You can upload the PDF or JPEG figure with the UPLOAD button in the lower right of the Reply window. The answer is a bit different for short coils versus long solenoid coils...

 

[mitchy16]

Are you familiar with that the external magnetic field looks like for a coil that is carrying a current?

BTW, is there a figure that goes with this question? You can upload the PDF or JPEG figure with the UPLOAD button in the lower right of the Reply window. The answer is a bit different for short coils versus long solenoid coils...

As in the magnetic field wraps around the current carrying coil? Yes.
And no, sorry, there is no figure with this problem. It pertains to a lab, and in the lab the radius of the two coils was 10.5cm and they were approximately 15cm apart, if that helps any! Thank you!

 

[berkeman]

As in the magnetic field wraps around the current carrying coil? Yes.
And no, sorry, there is no figure with this problem. It pertains to a lab, and in the lab the radius of the two coils was 10.5cm and they were approximately 15cm apart, if that helps any! Thank you!

You left out a dimension...

 

[berkeman]

Homework Equations

No equation

Also, this is not quite right. What is the equation for the B-field generated by a coil (of a yet-to-be-specified length)?

 

[mitchy16]

You left out a dimension...

The radius of loops is 10.5cm, distance between is 15cm, and distance to was 20cm?
I am not sure what else I'm missing, sorry I'm a bit confused!

Also, this is not quite right. What is the equation for the B-field generated by a coil (of a yet-to-be-specified length)?

Would it be the following?
https://www.physicsforums.com/file:///C:/Users/rajinder/AppData/Local/Temp/msohtmlclip1/01/clip_image002.png

I have only been provided with this equation for the coils, I am not sure if there is a different one I should be using.

 

[berkeman]

The radius of loops is 10.5cm, distance between is 15cm, and distance to was 20cm?
I am not sure what else I'm missing, sorry I'm a bit confused!

Sorry to be confusing, I was just fishing to see if you realized that the length of the coil was not being specified. If the coils are very short and compact, that is different from if they are long solenoids. I guess I'll assume they are short coils for this question?

Would it be the following?

i'd have to check, but that looks reasonable. So if the coils are short, each one looks like the following, right? And when two short coils are close together, you just add the fields vectorially. So what are your answers to the questions now, based on the vector fields of short coils adding vectorially?

http://onetesla.com//media/wysiwyg/coil1_1_.gif

 

[berkeman]

Here's a better plot of the external B-Field of a short coil. Can you sketch your solution to the problem based on 2 coils spaced a bit apart with their generated B-fields in opposition?

http://www.a-levelphysicstutor.com/images/fields/mag-f-coil-diag.jpg

 

[mitchy16]

Here's a better plot of the external B-Field of a short coil. Can you sketch your solution to the problem based on 2 coils spaced a bit apart with their generated B-fields in opposition?

http://www.a-levelphysicstutor.com/images/fields/mag-f-coil-diag.jpg
View attachment 219655

Okay, so, just for clarification would they attract? Which would produce a larger magnetic field?

 

[berkeman]

Okay, so, just for clarification would they attract? Which would produce a larger magnetic field?

The original problem statement said the magnetic fields were pointing in opposite directions, so it's like having 2 bar magnets with the North poles facing toward the other bar magnet. Do they attract or repel?

Homework Statement

Describe the magnetic field you would observe if two coils were connected with magnetic fields pointing in opposite directions.

So getting back to the original problem statement, sketch two short coils like that last figure I posted, and separate them by about 2 diameters on a common axis. Have the North poles (the pointy ends of the B-field vectors) face each other.

So since they are on the same axis, and the axial B-field goes straight out, you can imagine that the two B-fields will cancel on the coaxial axis right in the middle half-way between the two coils, right? But as you move along that middle axis between the coils one way or the other, the B-field will not be zero in general, right? Because the B-field of a short coil falls off with some power of the distance, the B-field will be strongest right in the middle of the source coil, and will fall off as you get farther away.

Can you start that sketch of the 2 coils spaced 2 diameters apart on the same axis? Start sketching the vector combination of the 2 B-fields that are shown in that last sketch I posted. Just add up the magnitude and directions of the B-field vectors at different locations, and ask yourself what the resultant direction and magnitude would be at each point due to the combinations of the two source coils.

Can you start that sketch and Upload it so we can check it?