Net Current from Mixed Carriers
Positive and negative carriers cross the same section at the same time, as they do in an electrolyte or a gas discharge. Each negative carrier has both its sign and its direction flipped before being added in — the step most students drop — and the contributions are then simply summed.
How to use this simulation
- Select opposite charges, opposite ways to see the two contributions reinforce
- Select opposite charges, same way to see them cancel
- Select like charges, opposite ways and like charges, same way to complete the four cases
- Read the running total at the section as each carrier crosses
What to look for
- Opposite charges travelling opposite ways produce the largest net current, not zero
- Opposite charges travelling the same way partly or wholly cancel
- Flipping a carrier's sign and its direction together leaves its contribution unchanged
- The net current can be zero while carriers are still crossing the section in both directions
- The chosen positive direction only sets the sign of the answer, never its magnitude
The physics behind it
Current is a signed quantity through a chosen section, so a positive direction has to be picked first. A positive carrier crossing along that direction contributes +Q/t; a negative carrier crossing along it contributes −Q/t, which is the same as a positive carrier crossing the other way. Flipping the sign and flipping the direction cancel each other, which is why negative carriers going one way and positive carriers going the other way add rather than subtract. The net current is the algebraic sum of those signed contributions.