Digital systems
Thermal and electric systems relationship

Apparently, there is an analogy between electric circuit described above, and a thermal system, where a room with heat losses is heated, or water boiling in an electric kettle. The difference is, that in electric analogy, a discharging is done by zero voltage at the input, and the capacitor is discharged partly via shunt R2 and partly via input resistor R1 back to the source – effectively, it discharges via parallel combination of both resistors,

to the zero voltage. The system is symmetric, thus linear.

On the other hand, thermal systems are generally significantly asymmetric, thus nonlinear. Interruption of heating represents disconnection of the source, i.e. disconnection of thermal flow between source and system; the system then cooles down only by heating loss. More realistic analogy to the thermal system is thus a tank with simultaneous filling and drainage or an electric circuit with disconnection of supply source, as shown in following Figure. At the end of charging, the current is cut off and the capacitor discharges only through resistor R2.

This example matches much better the behavior of heating system; however, it is not clear, what the zero voltage (temperature) means. More realistic model is shown on following Figure, which takes arbitrary ambient temperature into account, represented here as second voltage source u2.

If the voltage of the second source u2 is lower than capacitor voltage, the current flows back to the source, resembling a heating loss. In contrary, if the ambient temperature is higher than a room temperature, heat flows into the room, what corresponds to capacitor charging from source u2.