The water analogy is very commonly used in textbooks. Basically, if you have a certain amount of resistance in a hose, then to increase the flow (GPM) you need greater pressure (PSI).

A copper conductor has a certain amount of resistance. To increase the flow of current (amps), you also need a higher pressure (volts).

Here's a quick diagram which might help you visualize the internal resistance angle a little better (apologies for the poor quality -- I just drew it "freehand" very quickly):
[Linked Image]
The part within the dotted blue box represents the battery with its positive and negative terminals. It's shown represented as the actual source of EMF and a series resistance.

Assume that the source of EMF is the constant 12.6V we've been using in the example above. Whenever we draw current from the battery there has to be a voltage dropped across that internal resistance. In our example, that internal resistance is 0.4 ohm, so if you draw 2 amps of current you "lose" 0.8V across that internal resistance.

The EMF remains the same, but because we can only get to the two terminals of the battery, we can get only 11.8 volts to the load.

Does that make it any clearer?

In reality, the EMF and internal resistance cannot be separated in this way -- The resistance is an integral part of the chemical composition of the battery. This representation just makes it a little easier to understand what's happening inside the battery.


[This message has been edited by pauluk (edited 08-05-2003).]