c Electric Potential /and Electric field of a point charge distribution
c
      dimension x(100), y(100), q(100)
      equivalence (epsi,dx,dy)
      real k
      k=8.99E9
      pi=2.*asin(1.)
      epsi=1.e-4
50    print*,'n points charges'
      read * , n
      do 10 i=1,n
      print*,' enter  x,y,q for point I',i
      read *,x(i),y(i),q(i)
10    continue
      print*,' coordinates of P (observation point)    xp , yp '
      read*,xp,yp
      rp=sqrt(xp**2+yp**2)
      v=0.
      vdx=0.
      vdy=0.
      do 20 i=1,n
      r=sqrt((xp-x(i))**2 +(yp-y(i))**2)
      rdx= sqrt((xp+epsi-x(i))**2 +(yp-y(i))**2)
      rdy=sqrt((xp-x(i))**2 +(yp+epsi-y(i))**2)
      v=v+k*q(i)/r
      vdx=vdx+k*q(i)/rdx
      vdy=vdy+k*q(i)/rdy
20    continue
      ex=-(vdx-v)/dx
      ey=-(vdy-v)/dy
      er=(ex**2+ey**2)**.5
      theta= atan(ey/ex)
      print 110,xp,yp,rp,v
      print 100, er,ex,ey,theta*180./pi
100   format(1x,'er,ex,ey, theta(degrees)=',4(2x,e11.4))
110   format(1x,'xp,yp,rp,V=',4(2x,e11.4))
      print*,'  '
      print*,'enter 0=no or 1=yes for another calculation'
      read*,id
      if(id.eq.0)goto 200
      if(id.eq.1)goto 50
200   stop
      end