# Voltage and Current in RC circuits

## Homework Statement

A circuit includes a spark gap that connects to the ground. The width of the gap is adjusted so that the capacitor discharges to the ground each time the voltage reaches 99% of the maximum.

(The circuit is basically a simple series circuit with an 80v power supply, connected to a 2500$\Omega$ resistor. Then comes a junction, one way leads to a 400$\mu$F capacitor, and the loop starts again in the power supply. The other way leads to the grounded spark gap).

Find the current flowing through the resistor at time t=$\tau$

## Homework Equations

V=IR
V=V0(1-e-t/$\tau$)

## The Attempt at a Solution

First I found the constant $\tau$ for the circuit by doing (2500)(400X10-6)=1

Then found V at time $\tau$ by doing

V=80(1-e-$\tau$/$\tau$)=51

Then I thought that simply dividing 51 by 2500 would do it, but apparently not, any ideas?
Thanks...

gneill
Mentor
The voltage across the resistor is not the voltage across the capacitor.

HINT: You can modify your voltage formula slightly to give the voltage across the resistor.

The voltage across the resistor is not the voltage across the capacitor.

HINT: You can modify your voltage formula slightly to give the voltage across the resistor.