18 H. Phy students coulombs law cutnell (1).docx

18 H. Phy students coulombs law cutnell (1).docx - Chapter...

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Chapter 18 Electrical Forces and Electrical Fields 18.1 THE ORIGIN OF ELECTRICITY A. ELECTRONS 1. Electric charge, like mass, is a fundamental property of an object. 2. Charge can be positive or negative (unlike mass, which is always non-negative) 3. Electrical forces are much stronger than gravity. They are less noticeable because positive and negative forces often balance each other out. 4. Electrons can be transferred from one object to another, so charge can be transferred. 5. Charge is measured in units of Coulomb. a. Electric charge is always conserved. b. Electric charge is quantized. That is, it always comes in integer multiples of a fundamental charge. The charge of 1 electron e = - 1.60 × 10 −19 C. 6. All macroscopic objects get their charge from the electrons and protons that constitute them. Finding the charge of an object: q = N (e) q = charge (coulombs) N = number of charged particles usually electrons e = - 1.60 × 10 −19 Coulomb ( charge per 1 electron)[ or + 1.60 × 10 −19 Coulomb ( charge per 1 proton)] a. You have 6.70 x10 5 excess electrons on a balloon. What is the charge? q = N (e) q =6.70 x10 5 (- 1.60 × 10 −19 C) q= - 1.07 x 10 −13 C b. You have lost 3.22 x 10 6 electrons on a balloon. What is the charge? q = N (e) q = - 3.22 x 10 6 (- 1.60 × 10 −19 C) q= 5.15 x 10 −13 C c. How many electrons are in -1 coulomb of charge?
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q = N (e) - 1 C = N (- 1.60 × 10 −19 C) N = - 1 C/ (- 1.60 × 10 −19 C) N = 6.25 x 10 18 electrons B. Example problems 1.(1) How many electrons must be removed from an electrically neutral silver dollar to give it a charge of +3.4 μ C? q = N (e) +3.4 μ C = N (- 1.60 × 10 −19 C) N = +3.4 x 10 -6 C/ (- 1.60 × 10 −19 C) N = - 2.2 x 10 13 electrons (are lost) 2. How many electrons must be removed from an electrically neutral silver dollar to give it a charge of +6.1 μ C? q = N (e) +6.1 μ C = N (- 1.60 × 10 −19 C) N = + 6.1x 10 -6 C/ (- 1.60 × 10 −19 C) N = - 3.8 x 10 13 electrons (are lost) 3.(2) A rod has a charge of -2.6 μC. How many electrons must be removed so that the charge becomes +3.5 μC? difference in charge = final charge - initial charge difference in charge = +3.5 μC - (-2.6 μC) difference in charge = +6.1 μC q = N (e) +6.1 x 10 −6 C =N (- 1.60 × 10 −19 C) - 3.8 x 10 13 electrons (lost)= N 4. A rod has a charge of -1.8 μC. How many electrons must be removed so that the charge becomes +7.1μC? difference in charge = final charge - initial charge difference in charge = +7.1 μC - (-1.8 μC) difference in charge = +8.9 μC q = N (e) +8.9 x 10 −6 C =N (- 1.60 × 10 −19 C) - 5.6 x 10 13 electrons (lost)= N 5.(3) A metal sphere has a charge of of +6.0 μC. What is the net change after
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8.0 x 10 13 electrons have been placed on it? q = N (e) q = 8.0 x 10 13 (- 1.60 × 10 −19 C) q = - 1.28 × 10 −5 total charge = initial charge + added charge total charge = 6.0 x 10 −6 C + - 1.28 × 10 −5 total charge = - 6.8 x 10 −6 C 6. A metal sphere has a charge of of +9.0 μC. What is the net change after 2.3 x 10 13 electrons have been placed on it? q = N (e) q = 2.3 x 10 13 (- 1.60 × 10 −19 C) q = - 3.68 × 10 −6 C total charge = initial charge + added charge total charge = 9.0 x 10 −6 C + - 3.68 × 10 −6 total charge = 5.3 x 10 −6 C HW page 552 # 1-3 18.2 CHARGED OBJECTS AND THE ELECTRIC FORCE A. Law of Conservation of Electric Charge 1. Electrons are neither created nor destroyed, simply transferred.
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  • Winter '19
  • Rene Bishop
  • Electric charge, Law of Conservation of Electric Charge

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