Charge on Capacitor After 4.00ms: Solving the Problem

[Ryo124]

A capacitor is charged to 1 coulomb; the capacitance is 9.00×10-5 farads.
Then a switch is closed which puts the capacitor in a closed circuit with a resistor; the resistance is 9.00 ohms.

Calculate the charge on the capacitor after 4.00 milliseconds. (1 ms = 0.001 s).

I know some equations relating capacitance, charge, and voltage, but I don't understand how to approach this problem.

 

[Doc Al]

Look up "RC time constant" and the associated equation for a discharging capacitor.

 

[Ryo124]

Time Constant is T=RC;

but I don't know how to use this equation to solve my problem.

 

[Doc Al]

That's just the definition of the time constant, not the capacitor discharge equation that uses it. Read this: Charging a Capacitor (Charging and discharging are inverse operations.)

 

[Ryo124]

Still not making sense. I don't know the value of Q0 or V. Someone please help.

 

[Doc Al]

Q0 is given in your first post; you don't need V. Don't you have a textbook?

$$Q = Q_0 e^{{-t}/{RC}}$$