Published On: Sun, Jul 14th, 2019

Solved Problems in Electrostatics

Author: Radoje Jankovic

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



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?


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?


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?


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?


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

About the Author

- Radoje - Rade Jankovic Electrician, Electrical Technician, Electrical Engineer, PhD, Ecologist, Environmentalist, Designer, Educator, Investigator... Today PROFESSIONAL TECHNICAL WRITER AND DOCUMENTATOR Massive practical experience in almost all fields of electrical engineering over 40 years. My International education group in Facebook: EEEW - ELECTRICAL & ELECTRONIC EDUCATION WORLD where published more than 8000 small or bigger articles, lessons, technical advice, projects, technical calculations, test questions with and without answers, illustrated test questions with and without answers; all my original works and few thousands of may original photos from practice, drawings, circuit diagrams, environmental lessons and examples from everyday practice etc., etc.

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Solved Problems in Electrostatics