Singh, C. (2006). Student understanding of symmetry and Gauss's law of electricity. American Journal of Physics, 74(10), 923-936.

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000074000010000923000001&idtype=cvips&gifs=yesProblem 7 in Appendix B

In Fig. 4, a point charge +Q1 is at the center of an imaginary spherical surface and another point charge +Q2 is outside it. Point P is on the surface of the sphere. Let ?

_{S} be the net electric flux through the sphere and E

_{P} be the electric field at point P on the sphere. Which one of the following statements is true?

(a) Both charges +Q1 and +Q2 make nonzero contributions to ?

_{S} but only the charge +Q1 makes a nonzero contribution to E

_{P}.

(b) Both charges +Q1 and +Q2 make nonzero contributions to ?

_{S} but only the charge +Q2 makes a nonzero contribution to E

_{P}.

(c) Only the charge +Q1makes a nonzero contribution to ?

_{S} but both charges +Q1 and +Q2 make nonzero contributions to E

_{P}.

(d) Charge +Q1 makes no contribution to ?

_{S} or E

_{P}.

(e) Charge +Q2 makes no contribution to ?

_{S} or E

_{P}.

The answer given is (c), based on the applet visualize E field lines and equal Potential lines ,i got (d), any idea what i got wrong?

Oh, i was looking to the wrong thing, i was looking at the E field lines spacing, i should just look at the number for V=? and E on the sphere.

in critical examination of the applet, i think the answer is c

i understand how to use the apple now. not to worry.

Maybe to enhance the usability of the applet, create a object to act as the detector so students can explore the readings on a particular position in space? this will promote the attention focusing on what the students in moving on the applet space.

example

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1602.0