physics 1.pdf

# A true b false 10 true or false the units of coulom b

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A True B False 10. True or False? The units of Coulom b s constant are C N m 2 2 \$ . A True B False 11. True or False? Two source charges, each equal to + Q , are fixed in position, a distance d apart. At the point that is a distance d /2 from each of these charges, the electric field must be zero. A True B False Physics 180B — Fall 2017 — TEST 1: FORM A Solutions — page 2 of 4 See Sheet 1, Problem 2. The charge of this atom is 21(+ e ) + 24(0) + 18(- e ), which equals +3 e . This is +3(1.6 ◊ 10 -19 C) = +4.8 ◊ 10 -19 C, not +3 C. See Sheet 1, Problem 6. Particle 2 is repelled by Particle 1 (like charges repel), so Particle 2 will move away from Particle 1. So the distance r between them increases , which means the force (and thus the acceleration) will decrease . If 5 electrons are transferred from one neutral object to another, then one of the objects is left with a charge of +5 e , and the other object acquires a charge of -5 e . So, the magnitude of the electric force between them will be k (5 e )(5 e )/ r 2 = 25 ke 2 / r 2 . [ ] [ ] [ ] [ ] [ ][ ] [ ] C m N m C N F k q k r q F k r q q k 2 2 2 1 2 2 2 2 1 2 ( ( ( \$ \$ = = = = See Sheet 2, Problem 6(d). At the midpoint between the two source charges, the E - field due to one charge has the same magnitude but points in the opposite direction from the E - field to the second charge. These two individual vectors add up to zero. See Sheet 2, Problem 6(b). The E - field at x = - d due to the positive charge points to the left and has less magnitude (because it’s farther away from its source charge) than the E - field at x = - d due to the negative charge (which points to the right). Therefore, the (net) E - field points to the right.

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Questions 12 through 18 are worth 8 points each. 12. Calculate the magnitude of the electric force between a particle of charge +8 nC and a particle of charge -6 n C if the particles are separated by a distance of 6 mm. A 12 N B 7.2 ◊ 10 -6 N C 1.2 ◊ 10 -6 N D 72 N E 1.2 ◊ 10 -3 N 13. A charged particle is placed at a certain point where the electric field is 3 ◊ 10 6 N/C, pointing downward. If the particle experiences an electrostatic force of 1.2 ◊ 10 -3 N upward, what is the charge of this particle? A -2.5 ◊ 10 -10 C B +4 ◊ 10 -10 C C -2.5 ◊ 10 -2 C D -4 ◊ 10 -10 C E +2.5 ◊ 10 -10 C 14. If q 1 = -12 q and q 2 = -3 q in the figure, determine the net electrostatic force on q , in terms of k , q , and d . (Assume that q is positive.) A k d q 12 25 2 2 , downward B k d q 12 7 2 2 , upward C 0 D k d q 12 7 2 2 , downward E k d q 12 25 2 2 , upward 15.
• Fall '17
• Steven Leduc

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