An uncharged capacitor is connected in series with a battery, a switch and a resistor, as shown in the figure below.
The battery has e.m.f. 
and negligible internal resistance. The capacitance of the capacitor is 
. The switch is closed at time 
. During the time interval to 
, the charge passing through the resistor is 
. Determine, for the capacitor at time , the potential difference 
across the capacitor.
.
