# Q & A: constant current in resistors

Most recent answer: 08/31/2013
Q:
How doesn't the electric current change although the kinetic energy of electerons decreases and changes to internal energy while moving in the resistor?Doesn't the speed change?And why the size of the wire section isn't important to calculate electeric current?
- Sarah (age 17)
Iran
A:

There's a good reason why the current through a wire or resistor is the same through each cross-section, even if the width changes. If the current weren't constant, that would mean that different amounts of current would be entering and leaving some section. In that case charge would be building up in that section. The repulsive force between like charges is very strong, so that sort of process very quickly stops. Thus the current density must adjust so that the net current through any cross-section is the same, regardless of the width.

With regard to the average kinetic energy of the electrons, it's true that they lose kinetic energy through collisions (especially with phonons) in the conductor. However, there's an electric field on the conductor, which accelerates the electrons. On the average, the effects cancel, keeping the kinetic energy constant, at least until the lost kinetic energy starts to heat the whole thing up.

Mike W.

(published on 08/31/2013)