All the optical behaviors in electron microscopes (EMs) are described based on the theory of the interaction between moving electrons and electro-static and magnetic fields. The interaction can be described by Newton-Lorentz equation 
m -- Mass of an electron
-e -- Charge of an electron
-- Velocity of an electron
– Electro-static field
-- Magnetic field
The origins of and are different. The electrostatic field is generated by a voltage between electrodes and is described as a scalar potential Φ. The origin of magnetic field is an electrical current and is described by a vector potential .
 Wollnik H (1987) Optics of Charged Particles, (Academic Press, Orlando, FL).
 Hawkes P and Kasper E (1990-1996) Principle of Electron Optics, Vols 1-3. (Academic Press, London).