7) An ideal but air-filled parallel-plate capacitor has round plates and carries

Question

7) An ideal but air-filled parallel-plate capacitor has round plates and carries a fixed amount of equal opposite charge on its plates. A ll the geometric parameters of the capacitor (plate diameter and plate separation) are now DOUBLED. If the original energy density between the plates was u0, what is the new energy density? A) 16u0 B) 4u0 E) u /16 0 8) For the eircuit shown in the figure, determine the current in (a) the 1.0- resistor. (b) the 3.0- resistor. (c) the 40- resistor. 2.0 Ww 3.0 500 1.0

Explanation / Answer

7) Option E)

Energy density = Energy/Volume = uo/16

8) 3 ohm, 4 ohm and 5 ohm resistors are in parallel, so their net resistance = 1/R’ = 1/3 + ¼ + 1/5

                                               R’ = 1.28 ohm

Then R’ and 1 ohm and 2 ohm resistors are in series =======> R = 4.28 ohm

Total current = I = 12/4.28 = 2.8 A

Current across 1.0 ohm resistor = 2.8 A

Current across R’ = 2.8 A

Voltage across 1.0 ohm resistor = 2.8 V

Voltage across 2.0 ohm resistor = 2.8*2 = 5.6 V

Voltage across resistor R’ = 2.8*1.28 = 3.6 V

Voltage across 3.0 ohm = Voltage across 4.0 ohm = Voltage across 5.0 ohm = 3.6 V

Current across 3.0 ohm resistor = 3.6/3 = 1.2 A

Current across 4.0 ohm resistor = 3.6/4 = 0.9 A

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