The phenomenon appers until state of the circuit changes steady-state after state change, is called Transient. It is one of the fundamental operations of pulse circuit.
When the switch in Figure 1a is opend, the capacitor C will be charged through the register R and voltage Vc will vary like shown in Figure 1b.
To change state of the circuit, changing the value of EMF E instead can also be thought that equivalent.
The relation between past time t and voltage VC is expressed in following formure.