# Constant current for series resistance

1. Sep 3, 2010

### tasnim rahman

How do resistors of different resistances in series maintain constant current throughout a circuit? I mean is not the current supposed to decrease (within the resistor only) while going through the resistors? Need quick reply.

2. Sep 3, 2010

### bapowell

The total current flowing in a series circuit cannot change from one point to the next, since the current is conserved (the electrons don't flow out of the circuit). It is true that the total equivalent resistance of the circuit determines the current that a given voltage will output; but this current then flows through the circuit, equally through each individual resistor. What changes at each resistor is the voltage drop across it -- this is how a constant current is maintained.

3. Sep 3, 2010

### tasnim rahman

Thanks a lot for the quick reply Bapowell. From the equation I=nAve, which one, the drift velocity or the electron density, is affected by resistors, for series circuit resistance? How does the input of a single resistor in a circuit reduce the whole current available in the circuit? Does current density have anything to do with it?

4. Sep 3, 2010

### bapowell

No problem, happy to help. In the expression I = nAve, the drift velocity is the quantity relevant to this discussion. The drift velocity is determined by the applied voltage, the constant of proportionality being the resistance. For a given voltage, say a 9 V battery, hooked up to series circuits with different equivalent resistances, we would find different currents flowing through each circuit, on account of the different drift velocities. Intuitively, this makes sense: the battery, or voltage source, pushes the electrons through the circuit. A 20V battery can push more electrons more easily than a 9V battery. Meanwhile, resistance impedes the flow of electrons, making it more difficult for the battery to push the electrons through the circuit. For a fixed battery, increasing the resistance of the circuit makes it more and more difficult for the battery to push the electrons, resulting in slower electron drift velocity, and hence, lower current.

5. Sep 3, 2010

### tasnim rahman

Thanks a lot again. I think I got it now.