Current density and drift velocity

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SUMMARY

The discussion centers around the relationship between current density (J), charge carrier density (n), and drift velocity (v) in a system with both positive and negative ions. The equation J = ne(v) is established, where n represents the total number of charge carriers per unit volume. It is clarified that the total carrier density n is the sum of the densities of positive and negative ions, leading to the conclusion that their current densities add due to their opposite drift velocities. The confusion arises regarding the addition versus subtraction of current densities in different scenarios.

PREREQUISITES
  • Understanding of vector equations in physics
  • Familiarity with electric fields and their effects on charged particles
  • Knowledge of charge carrier density and its implications
  • Basic concepts of current density in conductive materials
NEXT STEPS
  • Study the principles of charge carrier mobility in electric fields
  • Explore the concept of drift velocity in semiconductor physics
  • Learn about the mathematical derivation of current density equations
  • Investigate scenarios where current densities may subtract, such as in opposing electric fields
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Students and professionals in physics, electrical engineering, and materials science who are looking to deepen their understanding of current density and drift velocity in charged particle systems.

auk411
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Homework Statement



J = ne(v). (yes. it's a vector equation and I haven't indicated that, but just ignore that. It's not important to my question.)

Let's say you have some positive ions traveling downward, and some negative ions traveling upward. The electric field is downward. Let's say you also know J. You know n, the number of carriers per unit volume for the positive ions and negative ions (which need not be the same n for each one).

To solve for v, it's pretty clear that you simple need to do: J/ne.

One of the things that is tripping me up is that I'm supposed to know that n here equals:
n (for the positive ions) PLUS n (for the negative ions).

An equivalent statement (and supposed to be explanation) is that: The positive and negative singly charged ions have drift velocities in opposite directions, so their current densities ADD.

But I don't get it. Why do they add and not subtract? Secondly, when would they subtract?
 
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Look at your earlier thread.
 
SammyS said:
Look at your earlier thread.

I did and more responses are needed.
 

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