Answer:
The third charged particle must be placed at x = 0.458 m = 45.8 cm
Explanation:
To solve this problem we apply Coulomb's law:
Two point charges (q₁, q₂) separated by a distance (d) exert a mutual force (F) whose magnitude is determined by the following formula:
Formula (1)
F: Electric force in Newtons (N)
K : Coulomb constant in N*m²/C²
q₁, q₂: Charges in Coulombs (C)
d: distance between the charges in meters (m)
Equivalence
1μC= 10⁻⁶C
1m = 100 cm
Data
K = 8.99 * 10⁹ N*m²/C²
q₁ = +5.00 μC = +5.00 * 10⁻⁶ C
q₂= +7.00 μC = +7.00 * 10⁻⁶ C
d₁ = x (m)
d₂ = 1-x (m)
Problem development
Look at the attached graphic.
We assume a positive charge q₃ so F₁₃ and F₂₃ are repulsive forces and must be equal so that the net force is zero:
We use formula (1) to calculate the forces F₁₃ and F₂₃
![F_{13} = \frac{k*q_1*q_3}{d_1^2}](https://tex.z-dn.net/?f=F_%7B13%7D%20%3D%20%5Cfrac%7Bk%2Aq_1%2Aq_3%7D%7Bd_1%5E2%7D)
![F_{23} = \frac{k*q_2*q_3}{d_2^2}](https://tex.z-dn.net/?f=F_%7B23%7D%20%3D%20%5Cfrac%7Bk%2Aq_2%2Aq_3%7D%7Bd_2%5E2%7D)
F₁₃ = F₂₃
We eliminate k and q₃ on both sides
![\frac{q_1}{d_1^2}= \frac{q_2}{d_2^2}](https://tex.z-dn.net/?f=%5Cfrac%7Bq_1%7D%7Bd_1%5E2%7D%3D%20%5Cfrac%7Bq_2%7D%7Bd_2%5E2%7D)
![\frac{q_1}{x^2}=\frac{q_2}{(1-x)^2}](https://tex.z-dn.net/?f=%5Cfrac%7Bq_1%7D%7Bx%5E2%7D%3D%5Cfrac%7Bq_2%7D%7B%281-x%29%5E2%7D)
We eliminate 10⁻⁶ on both sides
![(1-x)^2 = \frac{7}{5} x^2](https://tex.z-dn.net/?f=%281-x%29%5E2%20%3D%20%5Cfrac%7B7%7D%7B5%7D%20x%5E2)
![1-2x+x^2=\frac{7}{5} x^2](https://tex.z-dn.net/?f=1-2x%2Bx%5E2%3D%5Cfrac%7B7%7D%7B5%7D%20x%5E2)
![5-10x+5x^2=7 x^2](https://tex.z-dn.net/?f=5-10x%2B5x%5E2%3D7%20x%5E2)
![2x^2+10x-5=0](https://tex.z-dn.net/?f=2x%5E2%2B10x-5%3D0)
We solve the quadratic equation:
![x_1 = \frac{-b+\sqrt{b^2-4ac} }{2a} = \frac{-10+\sqrt{10^2-4*2*(-5)} }{2*2} = 0.458m](https://tex.z-dn.net/?f=x_1%20%3D%20%5Cfrac%7B-b%2B%5Csqrt%7Bb%5E2-4ac%7D%20%7D%7B2a%7D%20%3D%20%5Cfrac%7B-10%2B%5Csqrt%7B10%5E2-4%2A2%2A%28-5%29%7D%20%7D%7B2%2A2%7D%20%3D%200.458m)
![x_2 = \frac{-b-\sqrt{b^2-4ac} }{2a} = \frac{-10-\sqrt{10^2-4*2*(-5)} }{2*2} = -5.458m](https://tex.z-dn.net/?f=x_2%20%3D%20%5Cfrac%7B-b-%5Csqrt%7Bb%5E2-4ac%7D%20%7D%7B2a%7D%20%3D%20%5Cfrac%7B-10-%5Csqrt%7B10%5E2-4%2A2%2A%28-5%29%7D%20%7D%7B2%2A2%7D%20%3D%20-5.458m)
In the option x₂, F₁₃ and F₂₃ will go in the same direction and will not be canceled, therefore we take x₁ as the correct option since at that point the forces are in opposite way .
x = 0.458m = 45.8cm