# Simple question (electricity)

1. Mar 13, 2004

### broegger

I have a somewhat simple question, that I for some reason can't figure out..

suppose you have a circuit with a source of emf (e.g. a battery).. if you put a single resistor in this circuit there is a potential drop equal in size to the emf (i neglect internal resistance in the source).. now, if you place a second, identical resistor in the circuit the potential drop over the first resistor will be exactly half of it's initial value (that is, 1/2*emf).. my question is: how does the electrons "know" only to put half their energy in the first resistor and save the other half for the second - how does they detect the presence of the new resistor?

I'm sure I should know this, but my book doesn't offer any explanation..

2. Mar 13, 2004

### Staff: Mentor

All the electrons know about is the electric field that pulls them along. They don't "save" energy. Imagine gazillions of electrons being jerked along through the resistor by the electric field. Each electron doesn't get far before it smacks into a lattice molecule, thus transfering its energy to thermal energy of the resistor. But then the electron once again gets accelerated by the field... until it smacks into the lattice again. Etc.

3. Mar 13, 2004

### broegger

Thank you.. so the presence of the new resistor somehow alters the electric field, so that the dissipated energy in the first resistor drops to the value corresponding to 1/2*emf??

4. Mar 13, 2004

### zeker711

Maybe another simpler way to visualize this is to think about the analogy of water flowing. If you dump a bucket of water out of a window, let say 100 meters above the ground, it falls easily to the ground .. SPLASH!

But, if you take that bucket of water to a height of 100 meters and pour it on a rocky slope, it takes some time to reach the ground, but it eventually does (ignoring evaportaiton of course).

So, if this rocky slope is in TWO portions, with a free fall between them, you have little trouble visualizing the water flowing through the first part .. falling freely to the second stage, and resuming its journey until it is all at the bottom.

All it KNOWS is the potential difference and the flwo dynamics result from that alone.

Regards

5. Mar 13, 2004

Right.