Published On: Sun, Jul 14th, 2019

## Solved Problems in Electrostatics

1.When two objects attract each other, must be charged.

Sol:

No.

A charged object can produce a separation of charge in a nearly uncharged object because atomic electrons can shift around to some extent even without leaving their parent atoms.

2. An iron atom has 26 protons in its nucleus. How many electrons does this atom contain?

Sol.

Since a normal atom is electrically neutral, the number of negatively charged electrons it contains equals the number of positively charged protons in its nucleus. Then iron atom therefore contains 26 electrons.

3. A pith ball has a charge of + 10̄̄̄̄ ̄¹² C. How many such electrons are involved?

Sol:

The charge of an electron is

e  = 1,6 x 10 ̄¹⁹ C. Hence

Ne = q/e = 10̄̄̄̄ ̄¹² C / 1,6 x 10 ̄¹⁹ C/el. = 6,25 x 10⁶ electrons.

4. What is the magnitude and direction of the force on a charge of +40 x 10‾⁹ C that is 5 cm from a charge of +50 x 10⁻⁸ C?

Sol.

F = k x (q1 x q2)/(r ²)

= ((9 x 10⁹Nm²/C²) x (40 x 10‾⁹ C) x (50 x 10⁻⁸ C) / (5 x 10 ̄² m)² = 14,4 x 10 ̄⁴ N

5. Two charges, one of +50 x 10⁻⁷ C and the other -20 x 10⁻⁷ C, attract each other a force of 100 N. How far apart are they?

Sol:

r = √(k x q1 x q2)/F

= √(9 x 10⁹Nm²/C² x 50 x 10⁻⁷ C x 20 x 10⁻⁷ C)/ 100 N

= √90 x 10 ‾⁶ m

= 30 x 10‾³ m   = 30 mm