Electric field from a rotating coaxial ring

[Spriteling]

Homework Statement

A wire with uniform charge density λ per unit length is bent into a ring of radius a and rotates with angular velocity ω about an axis through its centre and perpendicular to the plane of the ring. Find the magnetic field on the axis at a distance z from the ring.

Homework Equations

The magnetic field at a distance z from a ring with current I flowing through it is

B = \frac{\mu_0 I a^2}{2(a^2 + z^2)^{3/2}}\hat{z}

The Attempt at a Solution

I assumed that the magnetic field would be the same as in a stationary ring with current I, where in this case I would be given by λωa. However, the solution appears to be less than this by a factor of 1/2 and I am completely at a loss as to where this extra 1/2 comes from.

 

[Spriteling]

Anyone?

 

[Spriteling]

Ack, I just realized that the title should be magnetic field, not electric field.

 

[zorro]

I think you are right.