If there is no charge enclosed by a closed surface, then the amount of electric field flowing into it must exactly cancel with the electric field flowing out of it. (en)
Because the flow of magnetic field out of a closed surface must cancel with the flow into it, magnets must have both North and South poles which cannot be separated into monopoles. (en)
The right-hand grip rule for a straight wire and for a coiled wire . Electrical current passed through a wire coiled around an iron core can produce an electromagnet. (en)
The force exerted on a positive charge by an electric field and a magnetic field combine to give the Lorentz force. (en)