How many electrons must be removed from an initially neutral disk to give it a n

Question

How many electrons must be removed from an initially neutral disk to give it a net charge of +2.1 C?

Answer:  

2. [5pt]
A metal sphere has an initial charge of +6.0 C. What is the net charge of the sphere after 4.7×1013 electrons have been placed on it?

Answer:  

3. [5pt]
A plate carries a charge of 2.4 C, while a rod carries a charge of -1.6 C. How many electrons must be transferred between the plate and the rod so that they have the same charge?

Answer:  

4. [5pt]
A system of two charges, q1 = 8.3 C and q2 = 2.1 C has the charges a distance 113.0 cm apart. Calculate the magnitude of the electrostatic force between these two charges.

Explanation / Answer

Part 1:

The charge of an electron is:
q = 1.6*10-19 C

So, the number of electrons needed is:
n = Q/q = (2.1*10-6 C)/(1.6*10-19 C)

n = 1.3*1013 electrons

Part 2:

The net charge of the metal sphere will be : [Initial charge - final charge]
Initial Charge = +6*10-6 C

Final charge = 4.7*1013*q

where, q is the charge of one electron = -1.6*10-19 C

Final charge = 4.7*1013 * [-1.6*10-19] = - 7.52*10-6

Initial charge - final charge =+6*10-6 C - 7.52*10-6

The net charge of the metal sphere will be = - 1.52 C

Part 3:

Electrons transfer until the charge on the plate equals the charge on the rod. The total charge of plate and rod together, i.e. before the charge transfer, is 0.8C, so after transfer the charge on both will be 0.4 C. Hence 2.0 C of charge got transferred from plate to rod.

The charge of an electron is 1.602*10-19C,

so, the number of electrons transferred = 2.0 * 10-6 C / 1.602*10-19 C

number of electrons transferred between plate and rod = 1.25*1013 electrons.

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