Trouble understanding derivation of current density

In summary, the conversation discusses the formula for current density, which is I/A. This is derived from the equation I = (Q/t), where Q is the total charge and t is the time. It is then further simplified by substituting (nVq)/t for Q/t, where n is the number of charges, V is the volume, q is the average charge, and t is time. The discussion also addresses why the total charge is not simply equal to nq, but rather nVq, as n represents the number of charges per unit volume.
  • #1
PsychonautQQ
784
10
Current Density = (I/A)
I = (Q/t) =
(nVq)/t
where n is the number of charges, V is the volume, q is the average charge and t is time.

nVq/t = n(Ax)q/t = nAvq
where v is velocity.

Current Density = (I/A)
(nAvq)/A = nvq

My question is on the step where it goes Q/t = nVq/t
Why does Q (total charge) not just equal nq? Why is there a volume term thrown in there?
 
Physics news on Phys.org
  • #2
n must be the number of charges per unit volume, nV is the total number of charges, multiply by q and you get the total charge in the volume.
 

1. What is current density?

Current density is a measure of the flow of electric charge per unit area. It is represented by the symbol J and is typically measured in amperes per square meter.

2. How is current density calculated?

Current density is calculated by dividing the current flowing through a given area by the cross-sectional area itself. This can be represented by the equation J = I/A, where J is current density, I is current, and A is the cross-sectional area.

3. What is the significance of current density?

Current density is an important concept in electricity and magnetism as it helps us understand the distribution of charge and current in a given material. It can also be used to calculate the strength of magnetic fields and the resistance of materials.

4. How does current density relate to Ohm's Law?

According to Ohm's Law, the current through a material is directly proportional to the voltage and inversely proportional to the resistance. Current density is related to this by the equation J = σE, where σ is the electrical conductivity and E is the electric field strength.

5. Can current density be negative?

Yes, current density can be negative. This indicates that the current is flowing in the opposite direction of the chosen direction of measurement. However, the magnitude of the current density will still be a positive value.

Similar threads

Is Current in Superconductors Truly Infinite?
    • Electromagnetism
Replies
9
Views
1K
I Deriving expression for resistance in terms of current density
    • Electromagnetism
Replies
7
Views
1K
I On the derivation of Child-Langmuir law
    • Electromagnetism
Replies
6
Views
1K
Why is current defined as the rate of change of charge?
    • Electromagnetism
Replies
6
Views
1K
I Exploring the Impact of Dielectrics on Electric Flux Density
    • Electromagnetism
Replies
5
Views
912
Question about this Continuity Equation (electromagnetism)
    • Electromagnetism
Replies
3
Views
832
Definition of current density J?
    • Electromagnetism
Replies
4
Views
1K
B Ion Trap and Time-Varying Potentials
    • Electromagnetism
Replies
1
Views
719
An interesting question about the divergence of a current density
    • Electromagnetism
Replies
10
Views
3K
Calculating Spatial Current in Resonant Inductors
    • Electromagnetism
Replies
4
Views
855

Hot Threads

Recent Insights

Back
Top