Value of resistance?

Question

Hi,

A 40 micro F capacitor that had been charged to 30 V is discharged through a resistor. The capacitor voltage as a function of time is shown in Figure P31.73, in which each interval on the horizontal axis equals 2 ms. What is the value of the resistance?

Figure 31.73:
http://i137.photobucket.com/albums/q208/infinitbelt/p31-73alt.gif

Any ideas?


Thanks!

Answer 1

the voltage in the capacitor will follow the equation
v(t)=vi/e^(t/T)

where T= R*C
and e is Euler's constant
2.7183

Looking at the graph, at t= 4 ms, V=10
so
10=30/e^(4ms/T)
4ms/T=ln(30/10)
ln is the natural log
T=4ms/ln(3)
T=.003641
T=R*C
R=.003641/40*10^-6
R=91 ohm

j