{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

# hw9 - Physics 21 Fall 2008 Solution to HW-9 26-41 In the...

This preview shows pages 1–2. Sign up to view the full content.

Physics 21 Fall, 2008 Solution to HW-9 26-41 In the circuit shown in the figure both capacitors are initially charged to 45.0 V. (a) How long after closing the switch S will the potential across each capacitor be reduced to 15.0 V? (b) What will be the current at that time? First find the effective capacitance and resistance of the circuit, then use those to determine the time constant. C eff = 15 . 0 µ F + 20 . 0 µ F = 35 µ F R eff = 50 Ω + 30 Ω = 80 Ω τ = R eff C eff = 2 . 8 ms (a) The voltage across the capacitors and the current through the resistors both decay exponentially with time constant τ : V ( t ) = V (0) exp( t/τ ) and I ( t ) = I (0) exp( t/τ ) . We can find t since we know V (0) and V ( t ). exp( t/τ ) = V ( t ) V (0) t = τ ln V (0) V ( t ) = 3 . 1 ms (b) The current goes down by the same factor as the voltage, and the initial value of the current is V (0) /R eff : I ( t ) = V ( t ) V (0) V (0) R eff = V ( t ) R eff = 0 . 19 A . 26-44 A 1.70 µ F capacitor is charging through a 14 . 0 Ω resistor using a 12.0 V battery. (a) What will be the current when the capacitor has acquired 1/4 of its maximum charge Q max ? (b) Will it be 1/4 of the maximum current?

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern