Physics
posted by Taj
A charged capacitor,C=10e6 F, with an initial stored energy of 3.5J, is discharged across a resistor, R. The charge on the capacitor drops to 40% of its maximum value in time, t=8.2s, from the instant the switch is closed (t=0).
Find the current through the resistor at a time t=10s.
How much thermal energy is dissipated across the resistor after two time constants?
Determine the electrostatic energy on the capacitor after two time constants.

Damon
C = Q/V
V = (1/C) integral i dt
dV/dt = (1/C) i
but
i = V/R
so
dV/dt = (1/C) V/R
or
dV/V = (1/RC) dt (I bet you knew that)
so
ln V = (1/RC) t
V = Vi e^(t/RC) (which you also know)
and Q = CV = CVi e^(t/RC)
at t = 8.2
Q(8.2) = Cv = .4 CVi = CVi e^(8.2/RC)
so
.4 = e^(8.2/{R*10^5} )
ln .4 = 8.2*10^5/R
solve for R
but how much energy is stored at V ?
E = (1/2) C V^2
that should get you started
Respond to this Question
Similar Questions

Physics
Took a leaf out of Eileen's book for the picture corresponding to the problem (you have to delete the spaces in 'h t t p :'): h t t p ://img444.imageshack.us/img444/1057/circuit6pk4.png In the circuit that is in the link above, the … 
physics
Having trouble with this one: A capacitor having a capacitance of 10uF is charged by 100v battery. A.)Calculate the charge on the capacitor? 
SCience
An RCcircuit has a time constant of 6x103 seconds. The capacitor stores a maximum energy of 8 J, when charged using a 3Volt battery. Using the same battery, we want to increase tenfold the amount of stored energy, while keeping … 
Physics
An RCcircuit has a time constant of 6x103 seconds. The capacitor stores a maximum energy of 8 J, when charged using a 3Volt battery. Using the same battery, we want to increase tenfold the amount of stored energy, while keeping … 
Physics
Could someone please tell me what equations i am suppose to use to work out these problems. A capacitor in RC circuit has two overlapping plates of width 3.40 m and length 8.90 m separated by 2.60 cm. A dielectric of dielectric constant … 
physics
A fully charged 98 uF capacitor is discharged through a 125 ohms resistor. How long will it take for the capacitor to lose 75% of its initial energy 
physics
How to calculate the potential energy in a CR circuit with a resistor and 10microfarrad capacitor before the switch is closed. the capacitor has a charge of 100 v and two seconds after the switch is closed looses 37% of its original … 
Physics
A fully charged 90µF capacitor is discharged through a 60Ω resistor. How long will it take for the capacitor to lose 80% of its initial energy? 
physics
A resistor with 780 is connected to the plates of a charged capacitor with capacitance 4.32 . Just before the connection is made, the charge on the capacitor is 7.80 1)What is the energy initially stored in the capacitor? 
Physics
A solenoid has N=1760.0 turns, length d=40 cm , and radius b=0.4 cm, (b<<d) . The solenoid is connected via a switch, S1 , to an ideal voltage source with electromotive force ϵ=7 V and a resistor with resistance R=25 Ohm …