Finding the Electric Potential at a Point Due to Multiple Charges

[eestep]

Homework Statement

A charge of +21.9 micro-Coulombs is placed at x=-3.3 m, y=0 m, a charge of -8.5 micro-Coulombs is placed at x=0 m, y=+3.8 m, and a charge of +15.6 micro-Coulombs is placed at x=+3.3 m, y=+3.8 m. If a charge of -15.4 micro-Coulombs is placed at origin, what is electrical potential of this charge in Joules? Answer is -1.04.

Homework Equations

$$\Delta$$U=U_f-U_i

The Attempt at a Solution

V_o=k(q₁/r₁+q₂/r₂+q₃/r₃)=(8.99 x 10⁹Nm²/C²)(21.9 x 10^-6C/3.3m+-8.5 x 10^-6C/3.8m+15.6 x 10^-6C/7.1m)=5.93 x 10⁴V
U_f=q₄V_o
q₄V_o-0=(-15.4 x 10^-6C)(5.93 x 10⁴V)

 

[tiny-tim]

hi eesteeestep!

… and a charge of +15.6 micro-Coulombs is placed at x=+3.3 m, y=+3.8 m
…
… =(8.99 x 10⁹Nm²/C²)(… +15.6 x 10^-6C/7.1m)

no, you've added 3.8 to 3.3 to get 7.1

(3.3,3.8) is a vector …

its magnitude is … ? ​