# Magnetic cone

**Electricity and Magnetism**Level 3

A hollow cone (like a party hat) has vertex angle \(2\theta\), slant height \(L\), and surface charge density \(\sigma\). It spins around its symmetry axis with angular frequency \(w\). What is the magnetic field at the tip.

Use \(\theta= 30^\circ , L = 8\text{ m}, \sigma = \dfrac2{\mu_0} , w= 18\).