Bob Reference Frame: A charged particle moves through a uniform magnetic field with velocity V. No electric field is present and gravitational forces may be neglected. The charge thus experiences a force due to this magnetic field equal to F = qV x B. B is the strength of the magnetic field. SALLY RF: Sally runs at velocity V (V remember is the velocity of the charged particle as well). Because the charged particle is at rest in Sally's RF, there is no magnetic force present. The charged particle must experience the same force though in all RF's so the force calculated in Bob's RF (due to the magetic field in Bob's RF) must be somewhere else in Sally's RF. It is found that in Sally's RF, there is an electric field equal to F' = qE'. F must equal F' so qE' = qV x B. CONCLUSION / INQUIRY The charged particle, according to Bob, travels through a magnetic field. The same charged particle, according to Sally, travels through an electric field. Question: Is Bob incorrect if he said the charged particle traveled through a magnetic field only? Is Sally incorrect if she said the charged particle traveled through an electric field only? Am I incorrect if I say the particle traveled through an electromagnetic field only?