problem #1(adsbygoogle = window.adsbygoogle || []).push({});

Two point charges of +3.0uC and -7.0uC are placed at x=0 and x=.20m, respectively. What is the magnitude of the electric field at the point midway between them (x=.10m)?

E3 = (kq)/r^2 = [(9x10^9 )(3x10^-6 C)]/.10m^2 = 2.7x10^6 N/C

E7 = (kq)/r^2 = [(9x10^9 )(7x10^-6 C)]/.10m^2 = 6.3x10^6 N/C

E=E3+E7 = 9x10^6 N/C

solutions manual has the answer as 9x10^5 N/C

problem #2

If the distance between two closely spaced parallel planes is halved, the electric field between them: becomes 4 times as large.

E=4/A

E=4/d^2

if d=2, E=1

if d=1, E=4

so becomes 4 times as large

solutions manual has the answer as remains the same

problem #3

A 12-V battery is connected to a parallel-plate capacitor with a plate area of 0.40 m^2 and a plate seperation of 2.0 mm. How much energy is stored in the capacitor?

A=0.40 m^2

d=2.0 mm = .002 m

V= 12V

find Uc

C=(EoA)/d = (8.85x10^-12 x .40 m^2)/.002 m

C= 1.77x10^-9 F

Uc=1/2cv^2 = 1/2(1.77x10^-9 F)(12v)^2 = 1.27x10^-7 J

solutions manual has the answer as 2.5x10^-7 J

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