Physics Help Forum Electrostatic Force

 Jan 16th 2009, 06:53 PM #1 Junior Member   Join Date: Apr 2008 Posts: 21 Electrostatic Force Four particles form a square. The charges are $\displaystyle q_1 = q_4 = Q$ and $\displaystyle q_2 = q_3 = q$. Just for your info, the particles are arranged as such: 1 2 3 4 (a)What is $\displaystyle \frac Qq$ if the net electrostatic force on particles 1 and 4 is zero? Here's my solution for part a: (My notation for Force for particles is this, say I want to say the force of particle 2 on 1, I write $\displaystyle F_{21}$) $\displaystyle \sum F_1 = F_{21} + F_{31} + F_{41}$ This says that the net force on 1 is the sum of the forces on 1 from 4, 3, and 2. $\displaystyle \sum F_1 = k\frac{qQ}{a^2} + k\frac{qQ}{a^2} + k\frac{Q^2}{(\sqrt{2}a)^2}$ $\displaystyle = 2k\frac{qQ}{a^2} + k\frac{Q^2}{2a^2}$ $\displaystyle 2k\frac{2qQ + Q^2}{2a^2} = k\frac{2qQ + Q^2}{a^2}$ Turns out I get the same thing for $\displaystyle \sum F_4$ because the net force for Q is 0. So, now I get the net force on Q: $\displaystyle \sum F_Q = 2k\frac{2qQ + Q^2}{a^2} = 0$ $\displaystyle \frac{2k}{a^2}(2qQ + Q^2) = 0$ Since k and a are obviously not 0, then $\displaystyle (2qQ + Q^2)$ must be 0, so: $\displaystyle 2qQ + Q^2 = 0$ $\displaystyle Q^2 = -2qQ$ $\displaystyle Q = -2q$ $\displaystyle \frac Qq = -2.0$ Is that right? If so, here's part b. (b) Is there any value of q that makes the net electrostatic force on each of the four particles 0? If my answers aren't right, could you please correct them and explain so I can better understand how to do other problems of this nature. Thanks.
 Jan 19th 2009, 03:25 AM #2 Junior Member   Join Date: Jan 2009 Posts: 3 Force can not be added as such .they need to be addded vectorially.so the solution may be wrong.

 Tags electrostatic, force

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post ems Electricity and Magnetism 3 Mar 17th 2014 06:22 AM harimakenji Electricity and Magnetism 0 Aug 8th 2010 08:49 AM jasonlewiz Electricity and Magnetism 0 Jul 23rd 2010 10:19 PM treetheta Advanced Electricity and Magnetism 1 Oct 20th 2008 01:08 AM chopper22 Advanced Electricity and Magnetism 0 Oct 17th 2008 01:34 AM