Physics problems I can not understand.?

Question

2. Two identical conducting spheres are placed with their center 0.30m apart. One is given a charge of +12 x 10^-9C and the other is given a charge of -18 x 10^-9C.

a. Find the electric force exerted on one sphere by the other.

3. A small cork with an excess charge of +6.0uC is placed 0.12m from another cork, which carries a charge of -4.3uC.

a. What is the magnitude of the electric force between the corks?

c. How many excess electrons are on the negative cork?

I understood my other homework problems, but these I am stuck on and our teacher is not in today to answer questions. I would appreciate an explanation, as I already know the answers from the key but can not get them using Coulomb's Law.

Answer 1

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2.

Force

F = k q1 q2 / r^2

k= 9x10^9 Nm^2/C^2 is Coulomb's constant

q1 and q2 are charges, r is distance

F = 9x10^9 * 12x10^-9 * -18x10^-9 / 0.30^2

= -2.16x10^-5 N

sign "-" is for attractive force (repulsive is "+")

3.

Same as above

q1 = 6x10^-6 C

q2 = -4.3x10^-6 C

r = 0.12m

F = 9x10^9 * 6x10^-6 * -4.3x10^-6 / 0.12^2

= -16.125 N

To find how many excess electrons are on the negative cork, we need to divide its charge with charge of one electron, e=-9.109x10^-19 C

n = = -4.3x10^-6 / -9.109x10^-19 = 4.72x10^12 electrons