Name: PHYS122 Quiz 4 1. Robert Millikan received the Nobel Prize for determining the charge on the electron. To do this, he set up a potential difference between two horizontal parallel metal plates. He then sprayed drops of oil between the plates and adjusted the potential dif-ference until drops of a certain size remained suspended at rest between the plates, as shown above. Suppose that when the potential difference between the plates is ad-justed until the electric field is 10 000 N/C downward, a certain drop with a mass of 3.27 × 10-16 kg remains suspended. (a) What is the magnitude of charge on this drop? (b) The electric field is downward, but the electric force on the drop is upward. Explain why. (c) The oil drop slowly evaporates while the drop is being observed, but the charge on the drop remains the same. Indicate whether the drop remains at rest, moves upward, or moves downward. Explain briefly. •March 10 -2- 2. A small plastic ball with a mass of and with a charge of is suspended from an insulating thread and hangs between the plates of a capacitor (see the drawing). The ball is in equilibrium, with the thread making an angle of 30.0° with respect to the vertical. What is the magnitude of the electric field between the plates?March 10 -3- 1. A charge Q1 = −16 × 10-6 coulomb is fixed on the x-axis at +4.0 meters, and a charge Q2 = +9 × 10-6 coulomb is fixed on the y-axis at +3.0 meters, as shown in the diagram above. (a) i. Calculate the magnitude of the electric field E1 at the origin due to charge Q1. ii. Calculate the magnitude of the electric field E2 at the origin due to charge Q2. −16 × 10-6 C Q1 x (m) y (m) Q2 +9 × 10-6 CMarch 10 -4- iii. On the axes below, draw and label vectors to show the electric fields E1 and E2 due to each charge, and also indicate the resultant electric field E at the origin. (b) Calculate the electric potential V at the origin A charge Q3 = −4 × 10-6 coulomb is brought from a very distant point by an external force and placed at the origin. (c) On the axes below, indicate the direction of the force on Q3 at the origin. (d) Calculate the work that had to be done by the external force to bring Q3 to the origin from the distant
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