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## Friday, September 21, 2012

### AP Physics B Electrostatics – A Free Response Practice Question

“This time, like all times, is a very good one if we but know what to do with it”
– Ralph Waldo Emerson

Today I give you a free response practice question meant for testing your knowledge, understanding and capacity for application of basic principles in electrostatics:
Two parallel flat metal plates P1 and P2 , each of area 0.16 m2, are arranged horizontally (Fig.) in air with a separation of 1.5×10–2 m. [Separation is shown exaggerated in the figure for convenience]. A constant potential difference of 60 V is established between the plates so that the electric field in the region between the plates is directed vertically downwards as indicated in the figure. Now, answer the following questions:
(a) If the potential of plate P1 is +10 V, what is the potential of plate P2? Justify your answer.
(b) Determine the magnitude of the electric field between the plates.
(c) Determine the capacitance of the parallel plate capacitor formed by the plates.
(d) Calculate the increase in the energy stored in the capacitor (formed by the plates P1 and P2) when additional charges equal to ten electronic charges.are added to it.
(e) A charged oil droplet of effective weight 3.2×10–14 newton is found to remain stationary in the space between the plates. Determine the charge on the oil droplet.
The above question carries 15 points. You may take up to 17 minutes for answering it. Try to answer it yourself within the stipulated time. I’ll come back shortly with a model answer for your benefit.