# CH23 - PART 4 ELECTROMAGNETISM ELECTRIC CHARGE FORCE AND...

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PART 4 ELECTROMAGNETISM CHAPTER 23 ELECTRIC CHARGE, FORCE, AND FIELD Problem 1. Suppose the electron and proton charges differed by one part in one billion. Estimate the net charge you would carry. Solution Nearly all of the mass of an atom is in its nucleus, and about one half of the nuclear mass of the light elements in living matter (H, O, N, and C) is protons. Thus, the number of protons in a 65 kg average-sized person is approximately 1 2 27 28 65 167 10 2 10 ( ) ( . ) kg kg = × × - ¼ , which is also the number of electrons, since an average person is electrically neutral. If there were a charge imbalance of q q e proton electron - = - 10 9 , a person’s net charge would be about ± × × × - 2 10 10 28 9 16 10 3 2 19 . . , × = ± - C C or several coulombs (huge by ordinary standards). Problem 2. A typical lightning flash delivers about 25 C of negative charge from cloud to ground. How many electrons are involved? Solution The number is Q e = = = × = × - 25 16 10 156 10 19 20 C C . . . Problem 5. If the charge imbalance of Problem 1 existed, what would be the approximate force between you and another person 10 m away? Treat the people as point charges, and compare the answer with your weight. Solution The magnitude of the Coulomb force between two point charges of 3.2 C (see solution to Problem 1), at a distance of 10 m, is kq r 2 2 9 2 2 2 8 9 10 3 2 10 9 22 10 = = = × = × ( / )( . ) . N m C C m N. This is approximately 1.45 million times the weight of an average-sized 65 kg person. Problem 8. How far apart should an electron and proton be so the force of Earth’s gravity on the electron is equal to the electric force arising from the proton? Your answer shows why gravity is unimportant on the molecular scale! Solution The electric force between a proton and an electron has magnitude ke r 2 2 = , while the weight of an electron is m g e . These are equal when r ke m g e = = × × × = - - 2 9 2 2 19 2 31 2 9 10 16 10 911 10 9 8 5 08 = ( / )( . ) ( . )( . / ) . N m C C kg m s m (almost fifty billion atomic diameters). Problem 9. Two charges, one twice as large as the other, are located 15 cm apart and experience a repulsive force of 95 N. What is the magnitude of the larger charge?

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CHAPTER 23 541 Solution The product of the charges is q q r F k 1 2 2 2 9 2 2 10 2 015 95 9 10 2 38 10 = = × = × - Coulomb m N N m C C = = ( . ) ( ) ( / ) . . If one charge is twice the other, q q 1 2 2 = , then 1 2 1 2 10 2 38 10 q = × - . C and q 1 218 = ± . . C μ Problem 11. A proton is on the x- axis at x = 16 . nm. An electron is on the y -axis at y = 0 85 . nm. Find the net force the two exert on a helium nucleus (charge + 2 e ) at the origin. Solution
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CH23 - PART 4 ELECTROMAGNETISM ELECTRIC CHARGE FORCE AND...

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