# I Importance of Current density?

1. May 31, 2017

### Rajeswar Panja

Why we do use electrical current density in place of total electrical current? Actually I want to know what is the advantage of using electrical current density?

2. May 31, 2017

### Staff: Mentor

3. May 31, 2017

### Rajeswar Panja

Yes, I read on wiki article but my question is that why we do use current density which means current/unit area rather than the total current?

4. Jun 1, 2017

### sophiecentaur

We don't always use Current Density. In fact, in many years of EE, Current Density hasn't figured very highly in any of my work. Talk to a power Engineer and you may get a different answer.
What is the context of your question?

5. Jun 1, 2017

### Staff: Mentor

We use whichever one we need for a given problem

6. Jun 1, 2017

### cosmik debris

Current density is a vector, current is a scalar?

7. Jun 3, 2017

### Mgcini Keith Phuthi

Current density is a local property (For a point), whereas current is a global property (e.g. for an entire wire). Both are useful depending on the problem but I guess I'll talk a bit more about current density. Densities in general are useful because they allow you to look at the effect of complicated distributions of the quantity in question (e.g. current,charge,mass) by adding up all the little current densities (Numerically or otherwise), current density allows you to find for instance the magnetic field inside a complicated material where only parts of the current have an effect. Another motivation for using densities is that the differential forms of equations, (e.g. Maxwell equations) are more mathematically convenient to work with, and they have to be expressed in terms of local properties.

Both current and current density are vectors i.e. they both have direction but currents are rarely written in vector form probably because they're written too often and it's assumed obvious

8. Jun 3, 2017

### Staff: Mentor

Current is in fact a scalar. The current through a surface S (e.g. a cross-section of a wire) is defined as $$I = \iint_S \vec J \cdot d\vec S$$ where $\vec J$ is the current density at each point on the surface and $d \vec S$ is the infinitesimal surface element at that point.

9. Jun 3, 2017

### marcusl

Perhaps an example would help: You can't make any progress on eddy currents without solving for the spatially varying current density.

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