What is the branch of applied physics called which deals with the control of motion of electrons using different devices for various useful purposes? Charge is mobile and can flow freely in certain materials, called? Air, glass, most plastics, and rubber are? Potential barrier in a diode helps in moving? When p-type, of a P-N junction, is connected to positive and N-type to the negative terminal of battery the P-N junction is said to behave as?