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Electrostatics The Atom • All matter is composed of atoms that contain: • Protons (positively charged particles) • Neutrons (neutrally charge particles) • Electrons (negatively charged particles) Every atom has a positively charged nucleus surrounded by negatively charged electrons. Electric charge, “q”, is measured in Coulombs, C. • All electrons are identical in mass and quantity of charge : q = - 1.6 x 10-19 C • All protons are identical to other protons and although they are more massive than electrons, they have the same quantity of positive charge as electrons have negative charge: q = + 1.6 x 10-19 C Compare the mass difference! Electrical forces explain bonding in molecules. • All neutrons are identical to other neutrons, have more mass than protons, and have neither positive or negative charge. • Atoms typically have the same number of protons as electrons so the atom has a net charge of zero. • When an atom has lost or gained electrons, it is charged and called an ION. Charles Coulomb, mid 1700’s, studied and published papers about the electrostatic force between 2 charged objects. The fundamental rule of elctricity: Like charges repel; opposites attract. A neutral object will attract both positive and negative charges Ben Franklin was the first to use the terms “positive” and “negative” to describe electrical charge. Mid 1700’s Hmmm.. +++ --- Coulomb’s Law calculates the electrical force between charges: where k is a constant, k = 9 x 109 N·m2/C2 Sample problem using the formula • sample problem Sample Problems using logic • The electrostatic force between two charges located 2 meters apart is 0.10 N. What will the force between these charges when they are located 1 meter apart? Solution: The distance has halved. The force will become stronger if the charges are closer. • To get the factor, put ½ in the place of d (remember to square it). • 1/(1/2)2 = 1/(1/4) = 4; now multiply the original force times the factor • (.10)(4) = .40N • A 2-C charge and a 4-C charge attract each other with 10 N of force. How much will a 2-C charge and an 8-N charge attract each other when placed the same distance apart? • Solution: one of the charges doubled. The force will be stronger since force and charge are directly related. • The force will double: 10 N X 2 = 20 N