The underlying differential equation for this very minimal model is a non-dimensional version of the equation for an RC circuit, with charge measured in units of C*emf, and time measured in units of RC.

The [

**charge**] of the capacitor is a stock.
The [

**current**] flows into the capacitor increasing the [**charge**].
The [

**current**] depends on the instantaneous [**charge**] of the capacitor: The greater the [**charge**] already deposited in the capacitor, the smaller the [**current**]. Specifically, the [**current**] is equal to 1 - [**charge**].
As a result, the [

**charge**] grows exponentially to its maximum value of 1, while the [**current**] decreases exponentially to zero.