If you can understand the actions of a battery as described in the previous section, then the theory for capacitive circuits is just an extension of the same basic principles. Therefore it will be useful to consolidate this understanding with a more familiar analogy.
Consider a simple hand operated air pump for a bicycle. First consider pulling the piston back and then blocking the outlet, before applying a constant force to push the piston back in. As soon as you began to compress the trapped air, you would immediately experience an increasing resistance* to your actions. If you maintained the same force, then the motion of the piston would slow and then stop in a very short period of time. This is because an opposing force builds up from the increasing pressure of the trapped air that is being compressed.
(*Note. in this section we are referring to mechanical resistance, which you would experience as "stiffness" when pushing the piston in, (rather than electrical resistance))
[Also note. You could alternatively block the outlet when the piston is pushed fully in and then apply a constant force to pull it out. Your actions will similarly be opposed, this time by the suction created by the expansion of the trapped air.]
We can develop this analogy a little further, so it compares more closely to the actions we have described previously, for our imaginary battery. Instead of a bicycle pump, consider a hand operated rotary pump, as shown in the diagrams below. When the handle is rotated clockwise, the fan blades pulls air from the inlet on the left hand side, and pushes it out of the outlet on the right hand side. There are one way valves at the inlet and outlet, so that air can only flow one way through this pump.
Now consider blocking both the inlet and outlet. When we turn the handle, there will only be a brief period when air is pumped, before the suction at the inlet and the back pressure at the outlet, prevents you from turning the handle any further.
We have presented a simple description of a battery, as a device with internal chemical reactions, that cause electrons to be transferred from the positive to the negative terminal. We have then compared this to the actions of a hand operated rotary pump. The chemical reaction, is being compared to the effort you would apply when turning the handle and the movement of electrons, is being compared to the air that is pumped from the inlet to the outlet.
If the battery is not connected to a circuit, the electrons removed from the positive terminal, can only accumulate at the negative terminal. Imagining that we can start up the chemical reactions in a previously dormant battery, we would therefore start to build up positive and negative charge at the battery terminals, producing a voltage between them. As the charge builds up, the voltage increases and the internal current flow reduces. After a short period of time, the current flow stops completely and the terminal voltage is at its maximum value.
The equivalent scenario for the air pump, would be when the inlet and outlet are blocked. So the air removed from the inlet, can only accumulate at the output. This causes suction at the inlet and a pressure build up at the outlet, making it more difficult to turn the handle. After a short period of time, the pressure reaches a level which stops the handle from turning at all.
( Note: If instead of blocking the pump connections, a tube was connected between the inlet and outlet, then we could continually turn the handle to circulate air around the tubing. This is similar to connecting the battery to an electrical circuit, where the electrons at the negative terminal can pass through the circuit back to the positive terminal and hence a continuous current flow can be maintained. )
In summary, with the inlet and outlet blocked, then applying a constant force to rotate the pump creates a pressure, which resists this motion. When the pressure reaches a certain limit, it stops the handle turning. In the remainder of this section, we will look at ways that we can increase the amount of air transfered, before this pressure limit is reached. We will then compare this, with the ways we can transfer more charge on a capacitor, before the limiting voltage level is reached.
