# Capcitance Analogy

1. Oct 9, 2005

### allergic

My Physics TA said that a good way to think about resistors is to also think about pipes:
the circuit is a pipe with water flowing thru it (water being charge) and a resistor acts like a section of pipe that is smaller than the rest... it slows down the current of charge and water.
using a similar situation, is there a similar way to think about capacitors?
just wondering :)

2. Oct 9, 2005

### mezarashi

Probably not. When dealing with capacitors and inductors you have fields involved, namely the electric and magnetic and energy is being stored. Then you have frequency responses. The analogy with the water and pipes is VERY rough. Once you "kind of" understand what is going on there. Move on and develop a stronger analog.

3. Oct 10, 2005

### vanesch

Staff Emeritus
There is some way to see analogies (but as with all analogies, they are limited).
Voltage can be modelled by the height of the water level (pressure), and current can be modelled by the flow of water. A resistor can indeed be modelled with a pipe (except that the pressure difference over a pipe usually goes as square of the flux (current) and not proportional to it...). A capacitor can be modelled as a reservoir. A big capacitor is a reservoir with big cross section, and a small capacitor as a reservoir with small cross section. The charge is the amount of water in the capacitor (the higher the water column, the higher the voltage and the charge for a given cross section).

Inductors are more difficult. There is an intrinsic "induction effect" due to the inertia of the motion of the water. But it is hard to see how to "increase" the self induction, except with a pump and a flywheel. Transformers could be modelled by pumps connected together mechanically. I even had a professor who had modelled a "water transistor" :-)

The problem is that this becomes so involved that it is in fact easier to master the original concepts in electricity

cheers,
Patrick.