# Useful Formulae

46 ELECTROMAGNETICFIELDS charge, flux and handedness There are strong parallels between gravitational and electro magnetic fields. Charge behaves in an electromagnetic field similarly to mass in a gravitational field. The force Fon a charge Qin an electric field of strength Eis given by F EQ. The force Fon a wire carrying a current I and with length l in a magnetic field is given by F BIl,where B is the magnetic flux density,measured in teslas. B,like F,has a direction as well as a magnitude at each point in the field. Other situations are shown opposite. Note that 0 is the permeability of free space,the magnetic constant see appendix. A point charge Qmoving with velocity vcreates a magnetic field. If point Phas distance r from Qand is at an angle to its direction of motion,then according to the BiotSavart Law B where Bs direction is as illustrated opposite top left. A magnetic field in motion produces an electric field and vice versa a point charge in motion generates a magnetic field. Using magnets and coils of wire carrying an electric current, electrical energy can be transformed into mechanical energy a motor and vice versa a dynamo. Flemings left and righthand rules apply respectively and are illustrated opposite. Qvsin r2 0 4 47