# Why is the magnetic field inside an ideal solenoid uniform

1. May 3, 2013

### jayman16

I understand that the magnetic field at points inside the solenoid is the vector sum of the B field due to each ring. How can the field inside be uniform then since if you consider just one loop in the solenoid the value of the B field is different at different points in that circle or loop.

2. May 3, 2013

### ModusPwnd

But a solenoid is not one loop right? Ideally its an infinitely long line of loops. At each point your B field is the sum of the contribution from each of the infinite number of loops.

3. May 3, 2013

### jayman16

ya but at different points in each of the loops let it be the center or a point that is off center, the B is different is it not?

4. May 3, 2013

### vanhees71

Just use Ampere's Law in integral form. Due to symmetry of a very long coil, $\vec{H}$ must be along the coil's axis, and you can assume it's 0 outside.

For the closed line in the integral take a rectangle with one side (length $l$) along the axis, somewhere well inside the coil and the parallel side outside. Let there be $\lambda$ windings per unit length. Then you have, according to Ampere's Law (I neglect the signs here; you easily find the direction of the field, using the right-hand rule):
$$\int_{\partial A} \mathrm{d} \vec{r} \cdot \vec{H}=\frac{\lambda l}{c} I,$$
where $I$ is the current through the coil. This gives
$$|\vec{H}|=\frac{\lambda}{c},$$
independent of where you locate the rectangle's side within the coil. That's why $\vec{H}$ is uniform.

You can also argue with the differential form of Ampere's Law,
$$\vec{\nabla} \times \vec{H}=\frac{1}{c} \vec{j}.$$
In cylindrical coordinates, with the $z$ axis along the solenoid's axis and with the ansatz due to the symmetry of the problem $\vec{H}=\vec{e}_z H(r)$ you find, using the formulas for the curl in cylindrical coordinates $\vec{\nabla} \times \vec{H}=-H'(r)$. Since inside the coil there is no current density you get $H(r)=\text{const}$.

Last edited: May 3, 2013