- #1

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- Homework Statement
- Question: A sphere of radius a centered at origin is made of linear isotropic homogeneous conducting material. The potential on surface is maintained at values given in spherical coordinates by $$V=V_0cosθ$$ $V_0$= constant. Find the free current density $J_f$ everywhere inside.

- Relevant Equations
- 1. $$J_f = \sigma E$$

2. $$E = - \nabla V$$

Since sphere is made of l.i.h material, $$\vec{J_f}= \sigma \vec{E}$$

We compute electric field E using

$$\vec{E} = -\nabla V$$

$$= -\nabla \left(V_0cos\theta\right)$$

$$= -\frac{\hat\theta}{r}\frac{{\partial}}{{\partial\theta}}\left(V_0cos\theta\right)$$

$$\vec{E}= \frac{V_0sin\theta}{r}\hat\theta$$

This yields, $$J_f = \displaystyle\frac{\sigma V_0 sin\theta}{r}$$

I understand that this isn't the required answer as final answer should be a constant as there is no motion of free charge $$\left(\displaystyle\frac{\partial\rho_f}{\partial t} =0\right)$$ and continuity equation yields

$$\nabla . J_f + \frac{\partial\rho_f}{\partial t}=0$$ $$\nabla . J_f = 0$$ $$J_f = \mathrm{constant}$$

But I am stuck here. Furthermore, I don't understand the required final answer is pointing in direction of z axis.

We compute electric field E using

$$\vec{E} = -\nabla V$$

$$= -\nabla \left(V_0cos\theta\right)$$

$$= -\frac{\hat\theta}{r}\frac{{\partial}}{{\partial\theta}}\left(V_0cos\theta\right)$$

$$\vec{E}= \frac{V_0sin\theta}{r}\hat\theta$$

This yields, $$J_f = \displaystyle\frac{\sigma V_0 sin\theta}{r}$$

I understand that this isn't the required answer as final answer should be a constant as there is no motion of free charge $$\left(\displaystyle\frac{\partial\rho_f}{\partial t} =0\right)$$ and continuity equation yields

$$\nabla . J_f + \frac{\partial\rho_f}{\partial t}=0$$ $$\nabla . J_f = 0$$ $$J_f = \mathrm{constant}$$

But I am stuck here. Furthermore, I don't understand the required final answer is pointing in direction of z axis.