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Ring Sequence: Chopping and Adding

Students who are just beginning upper-division courses are being asked to simultaneously learn physics concepts, use mathematical processes in new ways, apply geometric reasoning, and use extended multi-step problem solving. Having students successfully deal with a problem such as finding the magnetic field in all space due to a spinning ring of charge is a significant challenge. If we are to avoid doing the thinking for them and creating a template they can use, then we must create a sequence of learning opportunities that allow them to genuinely develop for themselves the ability to solve a problem like this.

We created a sequence of five small group activities that help students do deeper thinking while making each of the steps manageable. These five activities take roughly 30 to 60 minutes each and are designed to be used over the course of one or two months in conjunction with other forms of instruction such as lecture, individual homework, computer visualizations, and kinesthetic activities.

A more in-depth discussion of the rationale, student thinking, and the way these activities fit together can be found in a discussion of how these activities break the learning into manageable pieces


This activity is also part of the Power Series Sequence . Students use what they learned from The Distance Between Two Points - Star Trek and apply what they know about power series approximations to find a general expression and asymptotic solution to the electrostatic potential due to two point charges. Though the charge distribution students consider is elementary, the ideas and techniques used are applied later on to the more difficult ring of charge.

Many students coming into their junior have likely seen the electrostatic potential due to a ring of charge with high symmetry. In this small group activity, students aim to generalize the expression for the potential by applying what they have learned about charge densities, power series approximations, and various geometries. In order to accomplish this, students practice breaking the problem up into “manageable” pieces; finding a relationship for the charge density, understanding the geometry of the problem, setting up the correct integral, and then using power series approximations to evaluate the potential in some given region. Solving this problem is a big step for many students, and also prepares them for the more difficult problem of finding the electric field due to a ring of charge.

Now that students have had experience calculating the electrostatic potential due to a ring of charge, another layer of complexity is added to the problem. In this small group activity, students use the Coulomb definition of the electric field and work out the electric field due to a ring of charge. Again, students must break this problem up into multiple steps, making the problem as a whole more manageable; allowing them to develop a template for solving such problems. Moreover, this activity exemplifies the Paradigms “potential first” strategy; using techniques from the previous activity on the electrostatic potential, students tend to have an easier time making the transition from scalar fields to vector fields.


In many ways, this small group activity is simply the magnetic analogy to Electrostatic Potential Due to a Ring of Charge. By this time students are likely to see where the problem is headed: toward an integral which they will apply power series approximations to. However the set up for this problem is more complex, as students now must think about current densities and the inherent vector nature of the magnetic vector potential. Again, in order to solve this problem, students practice breaking the problem up in multiple steps, allowing them to develop a template approach for solving such problems.

Building from their experience with the previous exercises, by this small group activity many students have an idea on how to go about solving this problem. Using the techniques they have learned and the template they have been constructing from the four previous activities, students work out an expression for the magnetic field due to a spinning ring of charge, a feat for any junior-level physics major.

A Master's Project Paper describing the sequence of the five E&M activities:

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