Question

1. Three point charges are arranged on a line. Charge q3 = +5.00nC and is at the origin. Charge q2 = -4.00 nC and is at x = 4.00 cm . Charge q1 is at x= 2.50 cm .

(Part A) What is q1 (magnitude and sign) if the net force on q3 is zero?

2. Two small spheres spaced 20.0 centimeters apart have equal charge.

(Part A) How many excess electrons must be present on each sphere if the magnitude of the force of repulsion between them is 4.57×10−21newtons?

Answer 1

1)

In order to get net electric force at q3 to be 0, electric field at origin due to q1 must be equal to electric field due to q2

k q1 / r1^2 = k q2 / r2^2

q1 /2.5^2 = 4 / 4^2

q1 = 1.5625 nC

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2)

F = k q^2/r^2

4.57 x 10^-21 = (9 x 10^9)(q^2)/(0.2^2)

Charge, q = 1.43 x 10^-16 C

Number of electrons, n = (1.43 x 10^-16)/(1.6 x 10^-19) = 890.73

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