Lesson: Ampere's Law

Contributed by: VU Bioengineering RET Program, School of Engineering, Vanderbilt University

A shortwave loop antenna.
Ampere's law can be used to calculate the magnetic field around a current loop.
Copyright © https://en.wikipedia.org/?title=Loop_antenna#/media/File:Loop_antenna.jpg


The lesson begins with a demonstration introducing students to the force between two current carrying loops, comparing the attraction and repulsion between the loops to that between two magnets. After a lecture on Ampere's law, students begin to use the concepts to calculate the magnetic field around a loop. This is applied to determine the magnetic field of a toroid, imagining a toroid as a looped solenoid.

Engineering Connection

Ampere's law enables engineers to calculate the magnetic field around a loop which is useful in studying the magnetic field produced by the MRI magnet. In addition to calculating the magnetic field, engineers use Ampere's law to determine the correct amount of current and voltage that is necessary to create a functioning circuit board to perform the necessary tasks. Students will use ampere's law to solve the attached homework.

Educational Standards

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  • Results of scientific inquiry--new knowledge and methods--emerge from different types of investigations and public communication among scientists. In communicating and defending the results of scientific inquiry, arguments must be logical and demonstrate connections between natural phenomena, investigations, and the historical body of scientific knowledge. In addition, the methods and procedures that scientists used to obtain evidence must be clearly reported to enhance opportunities for further investigation. (Grades 9 - 12) Details... View more aligned curriculum... Give feedback on this alignment...
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Learning Objectives

After this lesson, students should be able to

  1. Apply Ampere's law to calculate magnetic fields in symmetric situations.
  2. Describe the magnetic field of a torroid.


In order to solve the MRI problem, we need to understand and measure the magnetic field given by current loops. Engineers continue to study magnetic fields, and they will work with safety engineers to see if they can design a more powerful MRI machine around 7 Tesla, opposed to the conventional 1.5 Tesla used in most imaging facilities.

Demo: Force between Two Current Loops

Objective: The goal of this demonstration is to show the force between two current carrying loops. The attraction and repulsion between the loops closely resembles the force between magnets and can help students begin to visualize the force between current loops as related to a current within the solid or to the magnetic moment.


  • 2 small-diameter coil
  • 2 30W or higher 5V DC power supply
  • 2 lab stands
  • 1 thin str aluminum foil
  • Wires with alligator clip leads 4
  • tape, 1 roll

*This demo was tested with a 2-inch diameter coil of 100 turns using 20 gauge magnet wire held together with duct tape.

Two ring stands are next to each other, both with bars attached. A metal ring is hanging from each one such that the rings are parallel and at the same height.
Figure A

Before presenting the demonstration, pose the problem of two short segments of wire aligned in a parallel fashion and have the students determine the direction of the magnetic field in each segment produced by the other segment. Then have them determine the direction of the magnetic force on each segment. Then consider many of these segments forming a loop in which each segment of the loop is attracted to the corresponding segment on the other loop. Have the students determine the magnetic moment vector of each coil to conclude that loops with parallel magnetic moments will attract.

Set up the demonstration as shown in the diagram above. Connect each loop to its own power supply, but do not complete the circuit until you are ready to start the demonstration as the wires may overheat. Do not leave the circuits connected for more than a few seconds. Momentarily complete the circuit to both coils and watch them attract. Reverse the current in one of the loops to see them repel.

Lesson Background and Concepts for Teachers

Legacy Cycle Information

This lesson fits into the research and revise phase of the legacy cycle where students will be provided with additional information enabling them to revise their initial ideas for solving the challenge. The research aspect will consist of a demonstration on the force between two current loops and a formal lecture on Ampere's law and its applications.

Ampere's Law

Although the total magnetic flux through a closed surface must be zero, the sum of the magnetic field around a closed loop does not have to be zero. More specifically, if a closed loop is imagined in space, we can create a vector at each tiny segment called dl pointing in the direction of the loop with a magnitude equal to the length of the segment. Then we will look at the magnetic field at that point, and take the dot product

and think of this as the magnetic field along that segment of the loop. The integral of these dot products around the loop,
, is a mathematically precise way of considering the magnetic field around a closed loop.

A loop of wire has a battery and the current traveling counterclockwise. There is a magnetic field line pointing right and a ":dl" vector pointing down and to the right.
Figure B
Copyright © 2006 Eric Appelt, Vanderbilt University

As you might imagine, the magnetic field around a loop would be greater if a wire carrying current was passing through the interior of the loop. In fact, the exact value of the field around the loop is

where I is the total current passing through the loop.

This result is named Ampere's law, after its discoverer.

Much like Gauss law, it is useful in determining the magnitude of the magnetic field in highly symmetric situations.

Example: Determine the magnetic field outside of an infinitely long straight wire.

Solution: Consider a closed circular loop of radius R around the wire centered on the axis of the wire, as shown in the diagram. You can see that the diagram is rotationally symmetric, so that the magnetic field must have a constant magnitude around the loop.

A wire is horizontal on the page with current traveling to the right. A circular loop of wire is centered around the axis of the wire.
Copyright © 2006 Eric Appelt, Vanderbilt University



Now by Ampere's law,


This result is in agreement with the Biot-Savart law. Note that this would appear to work for a finite segment of wire and give the same result, contradicting the result from the Biot-Savart law saying that

equation B =
. Experimentally, the Biot-Savart law has been shown to be correct. This discrepancy can be understood by realizing that the segment of wire must be part of a larger circuit, breaking the symmetry and invalidating the use of Ampere's law, or that the current must be a non-steady flow from one conductor to another. From this, we can surmise that Ampere's law must be valid only for steady-state currents.


A line drawing of a donut-shaped loop of wire, with smaller loops making up the big loop.

A toroid is basically a solenoid that is bent into a circle, as shown. This doughnut shaped figure has an inner radius a, an outer radius b, a current I, and a total number of turns N. To determine the magnetic field using amperes law, we will imagine a circular loop of radius r sharing a center with the toroid in the plane of the toroid.

Case 1: r < a

If r < a, then there is no current passing through the loop, so

. Since the diagram possesses rotational symmetry, the magnetic field must have equal magnitude anywhere along the loop, so B = 0 everywhere inside the inner radius.

Case 2: a < r < b

In this case, each turn passes through the loop in one direction, as shown in the cross sectional view to the right.

Two identical circular loops are shown near each other with current traveling in opposite directions. Segment a is labeled as the distance from the inside of one  loop to the midpoint between the loops, b is the distance from the outside of a loop to the midpoint, and c is from the center of the loop to the midpoint.
Copyright © 2006 Eric Appelt, Vanderbilt University



Case 3: r > b

In this case, for each inner turn passing current I through the loop in one direction, there is an outer turn passing current I through the loop in the other direction, so that the total current through the Amperian loop is zero, thus B = 0 everywhere outside the outer radius.


toroid: A surface generated by the revolution of any closed plane curve or contour about an axis lying in its plane.



Homework: Assign students to complete the Ampere's Law Homework questions to assess their progress in comprehending the concepts.


"MRI." Dictionary.com. Random House Unabridged Dictionary, Random House. Accessed June 23, 2008. http://dictionary.reference.com/browse/mri


Eric Appelt


© 2013 by Regents of the University of Colorado; original © 2006 Vanderbilt University

Supporting Program

VU Bioengineering RET Program, School of Engineering, Vanderbilt University


The contents of this digital library curriculum were developed under National Science Foundation RET grant nos. 0338092 and 0742871. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.