# Finding Voltage from capacitance, current and time

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1. Nov 11, 2015

### Weightofananvil

1. The problem statement, all variables and given/known data
the current through a 2uF capacitor is show. sketch the capacitor voltage Vc between t = 0ms give that the voltage at t=0ms is 0V. Your graph must be correctly scaled with numerical values.

There is a graph included showing a current rise for 0-2 milliseconds. 0 current for 2-4 milliseconds negative current for 3 milliseconds.

3. The attempt at a solution
I understand this question is asking for the difference between graphing Ic and Vc. I have the graph drawn and understand the difference between Ic and Vc in a charging and discharging circuit.

Now, I must convert my current values into voltage values and then add it to my graph. This is where I am stuck.

I know that:

Vc=Vo(E^-t/rc) and ic =-Vo/R(E^-t/rc) are used when a capacitor is charging and discharging.
both of those have voltages in the formula, so I can't use them.

I know Q=CV
I also know coulomb is 1s/A
can I change that to be A= Δ Q/Δ T ??
and rearrange it to Q= Δ A x Δ t ??

I can't figure out how to get the current to voltage!

2. Nov 11, 2015

### Staff: Mentor

If you are given the current into or out of a capacitor you know the amount of charge moving into or out of the capacitor (since current is defined to be charge/time). Look up the relationship between charge and voltage for a capacitor. If you know the capacitance and charge at any instant then you can determine the voltage on the capacitor.

3. Nov 12, 2015

### CWatters

You are on the right lines by starting with Q=CV. Then differentiate both sides wrt time.