Tuesday

July 22, 2014

July 22, 2014

Posted by **Bruce** on Wednesday, October 24, 2012 at 1:38pm.

I know energy is given by E(t)=q(t)^2, but beyond that I really don't know how to solve this. Any suggestions are appreciated.

Thank you!

- Physics -
**Elena**, Wednesday, October 24, 2012 at 1:59pmQ(t)=Q₀•exp(-t/RC)

E~Q² =>

E=E₀•exp(-2•t/RC)

100-80=20%

0.2•E₀=E₀•exp(-2•t/R•C)

ln0.2 =- 2•t/R•C

t= - ln0.2• R•C/2 =- (-1.6) •60•90•10⁻⁶/2 =0.0043 s.

- Physics -
**Damon**, Wednesday, October 24, 2012 at 2:02pmE = (1/2) c v^2

c = 90 * 10^-6

q = c v

so dq/dt = c dv/dt = -i

v = -i r = r c dv/dt

so

dv/dt = (-1/rc) v

v = Vi e^kt

dv/dt = k Vi e^kt = (-1/rc)Vi e^kt

k = -1/rc

v = Vi e^-t/rc

E = constant * v^2

so v^2 at t = .8 v^2 at t = 0

so

v at t = sqrt (.8) v at t = 0

v at t = .64 Vi

.64 = e^-(1/rc)t

ln .64 = -.446 = -(1/rc) t

t = .446 * 90 * 10^-6 * 60

- misread -
**Damon**, Wednesday, October 24, 2012 at 2:07pmsqrt .2 not .8

so

.04 = e^-(1/rc) t

-3.22 = -(1/rc) t

t = 3.22 * 90 * 10^-6 * 60

- Physics -
**Bruce**, Wednesday, October 24, 2012 at 11:02pmthank you!

**Related Questions**

physics - A fully charged 98 uF capacitor is discharged through a 125 ohms ...

Physic II - Suppose a capacitor having a capacitance of C fully charged to a ...

Physics - How long will it take a charged 80-ìF capacitor to lose 20 percent of ...

physics - A 20.0 \mu F capacitor initially charged to 20.0 \mu C is discharged ...

Physics - In a portable deﬁbrillator, energy is stored in a capacitor ...

Physics - A capacitor of capacitance C= 10mF, is charged using a battery of ...

physics - A resistor with 780 is connected to the plates of a charged capacitor ...

Physics - In the following circuit R_1=3 Ohm, R_2=10 Ohm, C=0.003 Farad, V=5 ...

Physics - Find the Time Constant for a 50 k resistor in a series with a 60 uF ...

Physics - An RC-circuit has a time constant of 6x10-3 seconds. The capacitor ...