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 Electricity and Magnetism Electricity and Magnetism Physics Help Forum Apr 2nd 2015, 09:24 AM #1 Junior Member   Join Date: Oct 2014 Posts: 18 electric field of a charged disc on an axial point. I need to find the electric field of a uniformly charged disc at a point on its axis through the centre. At first I calculated the potential Φ. Φ=(σ/2ε0)[( a^2 + x^2)^(1/2) - |x| ] where a is the radius of disc, x the distance of axial point from centre of the disc and σ the charge density. I am following nayfeh and brussels pg 53- 54 along with purcell berkley physics 55-56 and university physics young zemansky 12th ed pg 732 example 21.12.So far the books are not conflicting with the answer. Now to find the electric field I am using the following formula E = - dΦ/dx (x^) Now |x|=(x^2)^(1/2).So d|x|/dx=x/|x| And d( a^2 + x^2)^(1/2) /dx= x( a^2 + x^2)^(-1/2) So E=(σ/2ε0)[x/|x| - x( a^2 + x^2)^(-1/2)](x^) inserting -ve. Now if x> 0 then E=(σ/2ε0)[1 - x ( a^2 + x^2)^(-1/2)](x^) As x/|x|=1.x=+x This is also consistent with the books. If x <0 E=-(σ/2ε0)[1 - x( a^2 + x^2)^(-1/2)](x^) As x/|x|=-1.x=-x This is what I am getting.But in nayfeh the following answer is given for x<0. E=-(σ/2ε0)[1 + x( a^2 + x^2)^(-1/2)](x^).How is the term x( a^2 + x^2)^(-1/2) be positive?? Also if I substitute random arbitrary values to a and x say a=5units x= 10 units then for x> 0 E=0.106σ/2ε0(x^).By principle at an equal distance at the back of the disc E=-0.106σ/2ε0(x^). But if were to consider nayfeh then E=-1.89σ/2ε0(x^).So who is right?? Did I do something wrong ?  Tags axial, charged, disc, electric, field, point Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post romakarol Electricity and Magnetism 0 Feb 24th 2016 06:03 PM blazerqb11 Electricity and Magnetism 0 Oct 2nd 2014 08:23 PM yungman Advanced Electricity and Magnetism 6 Jun 5th 2011 10:55 PM ILoveCollege Advanced Electricity and Magnetism 3 Jan 24th 2010 12:17 PM rice4lifelegit Electricity and Magnetism 2 Sep 7th 2009 05:52 PM