Two equal charges, of charge q each, experience a repulsive force of 4.3 N when separated by 15 cm. If q is positive, what is its value?

Question

Two equal charges, of charge q each, experience a repulsive force of 4.3 N when separated by 15 cm. If q is positive, what is its value?

q = ___ 𝜇C

I get 9.30e+10 C which is obviously not correct since the answer is supposed to be in 𝜇C but I'm fairly certain I solved the problem correctly.

I used coulomb's law of F=(k*q)/r^2, plugged in the given values, and solved for q (since q1=q2).

Answer 1

Hi Bri S.!

Just looking over your solution, it seems as though you forgot to square q:

F_q = k|q₁||q₂|/r²

=> q₁ = q₂

F_q = k|q₁||q₂|/r² = kq²/r² (NOTE: I dropped the absolute value bars because we know q > 0)

q² = F_q*r²/k => q = √F_q*r²/k

<=> √[(4.3 N(0.15 m)²]/(8.99x10⁹ N*m²*C^-2)

q = 3.28 μC

Let me know if that goes any better for you!

Cheers