• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Finding the magnetic field of a loop at far distances

1. Homework Statement
Loop of current ##I## sitting in the xy plane. Current goes in counter clockwise direction as seen from positive z axis. Find:

a) the magnetic dipole moment
b) the approximate magnetic field at points far from the origin
c) show that, for points on the z axis, your answer is consistent with the exact field (Ex. 5.6), when z R.

2. Homework Equations
##\vec{m} = I \int d\vec{A}##
##\vec{A}_{dip} (\vec{r}) = \frac{\mu_0 \vec{m} \times \hat r}{4 \pi r^2}##
##\vec{B}=\nabla \times \vec{A}##

3. The Attempt at a Solution
a) I got that this is ##I\pi R^2 \hat z##
b) I got ##\vec{A}_{dip} (\vec{r}) = \frac{\mu_0 I R^2}{4 r^2}(\hat z \times \hat r)##, but I don't know how to interpret ##(\hat z \times \hat r)##. I tried taking the vector product by treating them as cylindrical coordinates and using the conversion to cartesian, which resulted in ##\hat \phi##. So it says that the vector potential curls around the z axis, which doesn't make sense...

Then I know I have to do ##\vec{B}=\nabla \times \vec{A}##, but that's contingent on the previous part being correct.

c) ???

Any tips?
 
Last edited:

Charles Link

Homework Helper
Insights Author
Gold Member
2018 Award
4,345
1,833
Hello again. Suggest you google "magnetic dipole". The Wikipedia has a good explanation. One item here that might create some confusion: In some SI units, I have seen the authors write ## B=\mu_o H +\mu_o M ## instead of ## B=\mu_o H +M ##. In any case, try reading Wikipedia. I think the answer is often computed in spherical coordinates rather than cylindrical, but it appears Wikipedia's answers are independent of coordinate system. ## \\ ## editing... Also, there is a formula for the curl of a cross-product of two vectors. It can often be found on the cover or appendix of an E&M textbook. ## \\ ## editing... additional item... A magnetic dipole can also be modeled as a "+" pole at one end and a "-" pole at the other and the inverse square law can be used for the ## H ## from each pole. The calculations using the pole model are a little simpler than Biot-Savart type integrals around the loop. I believe about a month or two ago, I verified the Wikipedia formulas using the pole method when someone else posted a question about the magnetic field of a magnetic dipole. ## \\ ## Here is a "link" to that discussion: https://www.physicsforums.com/threads/equation-of-magnetic-field-produced-by-a-solenoid.888895/
 
Last edited:

Want to reply to this thread?

"Finding the magnetic field of a loop at far distances" You must log in or register to reply here.

Related Threads for: Finding the magnetic field of a loop at far distances

Replies
1
Views
758
Replies
1
Views
3K
  • Posted
Replies
1
Views
646
Replies
0
Views
1K
  • Posted
Replies
1
Views
3K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top