Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Ampere's Law
Name: Huang
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 1999 

Does Ampere's law (for steady currents) somehow assume the magnetic field of an infinitely long line of current as opposed to just a current element? Specifically, the magnetic field produced by just one infinitesimal current element alone (with no other current elements present in space) drops off in strength as 1/r^2, so it seems incompatible with Ampere's law if one chooses certain paths in a plane perpendicular to the current element. Why is this so?

No. Ampere's law relates the current passing through an area to the line integral of the magnetic field over the perimeter of the area. Sure, the field drops off as 1/r^2, but the distance over which the line integral is taken increases as r^2. So it doesn't matter where you draw the perimeter. You'll always get the same answer if the current is completely bounded by the perimeter.

Tim Mooney

Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory