The Big Ideas
The name electric current is given to the phenomenon that occurs when an electric field moves down a wire at close to the speed of light. Voltage is the electrical energy density (energy divided by charge) and differences in this density (voltage) cause electric current. Resistance is the amount a device in the wire resists the flow of current by converting electrical energy into other forms of energy. The device, called the resistor, could be a light bulb transferring electrical energy into heat and light, or an electric motor that converts electric energy into mechanical energy. The difference in energy density across a resistor or other electrical device is called voltage drop.
In electric circuits (closed loops of wire with resistors and constant voltage sources) energy must be conserved. It follows that changes in energy density, the algebraic sum of voltage drops and voltage sources, around any closed loop will equal zero.
In an electric junction there is more than one possible path for current to flow. For charge to be conserved at a junction the current into the junction must equal the current out of the junction.