1.03 Transient Current Flow Analogy.

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 immediately slow down and then stop in a relatively short period of time. This is because an opposing force builds up from the increasing pressure of the trapped air that is being compressed. (This is very similar to what you would experience if you compressed a stiff spring by hand while maintaining a constant force)

bicycle pump

(*Note. In this statement we are referring to mechanical resistance, which you would experience as "stiffness" when pushing the piston in, (not 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 that it compares more closely with the behaviour of our imaginary battery described previously. 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 pull air from the inlet on the left hand side, and push it through the outlet on the right hand side. Note: There are one way valves at the inlet and outlet, so that air can only flow one way through the pump.

rotary pump rotary 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, prevent the handle being turned any further.

rotary pump rotary pump rotary pump


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 being 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. This causes a build up of positive and negative charge at the respective 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 voltage between the terminal reaches 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 will create a back pressure, which resists this motion. When the pressure reaches a certain limiting value, 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 build up more charge on a capacitor, before the equivalent limiting voltage level is reached.