## Solved Problems in Electrostatics

Author: Radoje Jankovic

**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