☰ Contents · Physics

Charging of bodies. Electric charge

Lessons 1–2 · 2 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, J.E. Usarov, K.T. Suyarov, M.K. Yuldasheva. Physics, textbook for Grade 8 of general secondary schools, revised and enlarged 3rd edition. “O‘qituvchi” Publishing and Printing Creative House, Tashkent, 2019
2

Electric charge

Textbook: pp. 9–11
GoalKnow the structure of the atom, the idea of electric charge, the coulomb and the elementary charge (q = N·e).
New words
electric charge (q) · elektr zaryad (q)coulomb (C) · kulon (C)elementary charge (e) · elementar zaryad (e)proton, neutron, electron · proton, neutron, elektron
Explanation

At the centre of an atom lies the nucleus, which holds positively charged protons and uncharged neutrons; negatively charged electrons move around it. In a neutral atom the number of electrons equals the number of protons (hydrogen has 1, helium 2, carbon 6). The physical quantity that describes how strongly a body is electrified is called electric charge; it is denoted q and measured in coulombs (C) in the SI. When bodies are rubbed, not atoms but only some electrons pass from one body to the other: the body that gains electrons becomes negative, the one that loses electrons becomes positive. A charge equal to the magnitude of the electron’s charge is called the elementary charge: e = 1.6·10⁻¹⁹ C. The charge of any body is a multiple of e: if N electrons were transferred, q = N·e. The electron’s mass is 9.1·10⁻³¹ kg, the proton’s 1.67·10⁻²⁷ kg.

Worked examples
5·10¹⁰ electrons passed from a glass rod to the silk. The rod’s charge is q = N·e = 5·10¹⁰ · 1.6·10⁻¹⁹ C = 8·10⁻⁹ C (positive), and the silk’s charge is –8·10⁻⁹ C.
A body carries an excess negative charge of 4.8·10⁻⁹ C. The number of extra electrons is N = q : e = 4.8·10⁻⁹ : 1.6·10⁻¹⁹ = 3·10¹⁰.
Class activity

Draw models of the hydrogen, helium and carbon atoms in your notebook: show the numbers of protons and neutrons in the nucleus and of electrons around it, and explain why each atom is neutral.

Practice
1
How many electrons does a neutral carbon atom (6 protons) have?
2
2·10⁹ electrons passed from one body to another. What charge does the first body get (C)?
3
About how many times heavier is a proton than an electron? (m_p = 1.67·10⁻²⁷ kg, m_e = 9.1·10⁻³¹ kg)
4
Why are the charges of two rubbed bodies equal in size but opposite in sign?