# Find the Electric Field strength

1. Apr 3, 2010

### Zorba

1. The problem statement, all variables and given/known data
"A copper cable carrying 50A current has radius 0.003m. What is the electric field if copper has a resistivity of 1.7x10^-8 Ohm/m?"

2. Relevant equations
Formula for current density etc.

3. The attempt at a solution

So I write:
$$\vec{\mathbf{J}} = n|q|v_d = \frac{I}{A}$$

And then:
$$\vec{\mathbf{J}} \cdot \rho = \frac{I}{A} \cdot \frac{RA}{L} = \frac{V}{L}$$

And now do I just apply the work energy theorem?:
$$\int \vec{\mathbf{E}} \cdot d\vec{\mathbf{l}} = \Delta V_{ab}$$

It kind of feels as though I'm completely off tbh...

2. Apr 3, 2010

### Cyosis

Use Ohm's law $\mathbf{J}=\sigma \mathbf{E}$.