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=yes

Problem 7 in Appendix B
[img]http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=319.0;attach=3423;image[/img]
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 ?[sub]S[/sub] be the net electric flux through the sphere and E [sub]P[/sub] 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 ?[sub]S[/sub] but only the charge +Q1 makes a nonzero contribution to E [sub]P[/sub].
(b) Both charges +Q1 and +Q2 make nonzero contributions to ?[sub]S[/sub] but only the charge +Q2 makes a nonzero contribution to E [sub]P[/sub].
(c) Only the charge +Q1makes a nonzero contribution to ?[sub]S[/sub] but both charges +Q1 and +Q2 make nonzero contributions to E [sub]P[/sub].
(d) Charge +Q1 makes no contribution to ?[sub]S[/sub] or E [sub]P[/sub].
(e) Charge +Q2 makes no contribution to ?[sub]S[/sub] or E [sub]P[/sub].

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 [img]http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=319.0;attach=3426;image[/img]