Let's say we have a simple circuit consisting of a power supply and a resistor, and currently the input voltage is 0V. We now apply a voltage of 5V to the circuit (like a step increase - instantaneously). The voltage across the resistor changes instantaneously to 5V. If a capacitor is introduced into this circuit, it will gradually charge until the the voltage across it is also 5V, and the current in this circuit will become zero. My question: What is now preventing us from suddenly changing the voltage from 5V to let's say 10V (again like a step increase - instantaneously)? We could do it before the capacitor was introduced, but why not now? The answer I have thus far always gotten is that for that to happen, the current flowing in the circuit must be infinite, and since that cannot happen, the voltage cannot be changed instantaneously. The problem is that we ARE changing the voltage of the power source instantaneously, just like before the capacitor was introduced. The introduction of the capacitor has not somehow taken away our ability to change the voltage of the power source, has it??? If we continue according to my understanding (i.e. that we can change the voltage instantaneously), that will mean that the current in the circuit will shoot up as if going to infinity, but that clearly cannot ever happen in ANY circuit, so doesn't that violate i=C*(dv/dt)? Don't we change voltage first, and then happens whatever happens? How can anything restrict an 'external variable'?