### Group 4 Solving for Potential of a Charged Ring: Concepts with which They Wrestle

1) Choosing a coordinate system (G4,1:21)

2) Dealing with all the different “r's” :

b) r as position vector of “voltmeter”;

c) r' as position vector to dQ on ring;

d) r - r' as the vector between dQ and “voltmeter”; (G4,1:22)

e) script r as | r - r'|; (G4,1:25)

f) r as in V( r), simply refering to a variable that varies across space.

3) dr vs d(theta) (G4, 1:26-1:30)

4) The problem of “solving for potential in all space” and realizing that solving for a single point does this.

5) Determining what is held constrant during integration

6) The relation between V = kQ/r and the integral form of the equation (G4,1:22-1:24)

7) Determining where the charge is. There is confusion about whether there is a piece of charge at the location at which the potential is measured (the location of the “voltmeter”) (G4,1:28)

8) Whether to integrate in one, two, or three dimensions (one for a linear density, two for the area of the plane in which the ring is located and three for all space). Included in this is recognizing a circle as an inherently one-dimensional object

9) The nature of electric potential as different from electric field (e.g. the potential for the ring is not zero at the origin and there are no cancelling vectors)

10) Needing to put r - r' into rectangular coordinates

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