I have been going through a lot of the clicker-type conceptual physics questions that I have collected over the years from other physics teachers who have been gracious enough to share them, when I came across a sequence of four questions related to the potential and potential energy of a three charge system.

I really like the set of questions, but I’m looking for some feedback from other teachers on how to best use them.

Here’s the system:

Three charges form a triangle at points 1, 2 and 3. The electrical potential at q₃ has some value V₃. (This setup of the system is the same for all four questions below.)

Here are the questions:

1. If the charge of q₃ is doubled, the value of the electrical potential at the point 3 will:
A.) increase by a factor of two
B.) increase by a factor of four
C.) decrease by a factor of two
D.) decrease by a factor of four
E.) remain the same
2. If the charges q₁, q₂, and q₃ is doubled, the value of the electrical potential at point 3 will:
A.) increase by a factor of two
B.) increase by a factor of four
C.) increase by a factor of eight
D.) decrease by a factor of four
E.) remain the same
3. If the charge of q₃ is doubled, the value of the electrical potential energy of q₃ will:
A.) increase by a factor of two
B.) increase by a factor of four
C.) decrease by a factor of two
D.) decrease by a factor of four
E.) remain the same
4. If the charges q₁, q₂ and q₃ is doubled, the value the value of the electrical potential energy, Utot, of the entire system of particles will:
A.) increase by a factor of two
B.) increase by a factor of four
C.) increase by a factor of eight
D.) decrease by a factor of four
E.) remain the same

I really like the first two questions because they get to the heart of what I have observed to be a common conceptual difficulty amongst physics students — separating the cause of the potential (in this case q₁ and q₂) from the effect that the potential has on another charge. In this case, the point is that the third charge does not affect the potential, so the first answer should be E and the second answer should be B.