Question

Three identical metallic conducting spheres carry the following charges: q11 = +8.20 μC, q22 = +7.80 μC, and q33 = −2.80 μC. The spheres that carry the charges q11 and q22 are brought into contact. Then they are separated. After that, one of those two spheres is brought into contact with the third sphere that carries the charge q33; those two are then separated as well.
a) What is the final charge on the third sphere?

b)How many excess (or deficiency) electrons make up the final charge on the third sphere?

Answer 1

(a)

When  spheres that carry the charges q11 and q22 are brought into contact,

Net charge on each sphere = Q,

According to conservation of charges,

2Q = q11 + q22

so, Q = (q11 + q22) / 2

Q = (8.20 + 7.80) / 2

Q = 8 uC

After that, one of those two spheres is brought into contact with the third sphere.

  final charge on the third sphere,

Q' = (8 - 2.80) / 2

Q' = 2.6 uC

(b)

We know that,

Q = ne

number of  excess (or deficiency) electrons make up the final charge on the third sphere,

n = Q' / e

n = 2.6*10^(-6) / 1.6*10^(-19)

n = 1.62*10^13 electrons