19. In the figure, ?1, ?2, and ?3 are charges on a two-dimensional plane. We take . If the force on q1 points in the −x-direction, what is the magnitude of ?3? (A) Q (B) (C) (D) (E)

9. According to the First Law of Thermodynamics, if a closed system absorbs heat Q from its surroundings and does work W on its surroundings, what is the change in the internal energy of the system, ΔU? (Sign convention: heat absorbed is positive, work done by the system is positive) (A) ΔU = Q + W (B) ΔU = Q - W (C) ΔU = W - Q (D) ΔU = -Q - W

19. In the figure, ?1, ?2, and ?3 are charges on a two-dimensional plane. We take . If the force on q1 points in the −x-direction, what is the magnitude of ?3?

(A) Q
(B)
(C)
(D)
(E)

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