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Electricity

Blazing goIITian

Joined:	21 Apr 2007
Post:	453

2 Feb 2012 04:13:03 IST

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Capacitors

Physics

A parallel-plate capacitor has an area of 25 cm2, and a separation distance of 3.50 mm. A charge of 25

µC is placed on the capacitor.

(a) Calculate the capacitance of the capacitor, the voltage across the capacitor, and the electric field between the plates. Show your work below, and record your answers in the boxes provided (don’t forget the units!).

Now, a sheet of plastic 3.50 mm thick is placed between the plates, thereby completely filling the space between them. The dielectric constant of the sheet is κ = 4.0. The charge on the plates remains the same as in part (a).

(b) With the dielectric inserted, calculate the capacitance, the voltage across the capacitor, and the electric field between the plates. Show your work below, and record your final answers in the boxes provided (don’t forget the units!).

Now, the dielectric from part (b) is removed, and a new sheet, also with dielectric constant κ = 4.0 but with half the thickness (1.75 mm) is placed inside the capacitor, thereby filling only half the space between the plates, as shown in the figure. Again, the charge on the plates remains fixed.

(c) Calculate the new voltage between the plates. (Hint: You know the electric field at all points between the plates, right? Define a point C to be the left hand edge of the sheet, and then calculate VAC and VBC!)

(d) Calculate the energy stored on the capacitor in all 3 cases, and summarize your results in the boxes provided.

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