Electric Current & Resistance
Electric Current Flow
Electric current flow can be defined as the flow of electrons through a conductor (wire) or circuit. This passage of electrons is often described as being analogous to the flow of water in a pipe or hose. For example, water flows through a pipe or hose because it is under pressure. Similarly, electric current surges through a wire because it is under pressure. Earlier, voltage was defined as the pressure, or moving (electromotive) force, that causes current (electrons) to flow in an electrical circuit. Furthermore, just as the size of a hose or pipe can affect the degree of water pressure, the size of an electrical wire can affect the flow of current passing through it. The maximum current-carrying capacity of a particular-size wire is called its ampacity.
As electric current passes through your electrical system, it reaches your receptacles and switches where, again like water, it becomes available for use, provided that you flip the switch on your wall or appliance just as you would turn the faucet on at your sink. And, like the water, once the electric current is used, it exits the system. Instead of exiting through a drainpipe, the current exits (or returns to the utility) by means of a grounded conductor.
The passage of electric current through a wire is not only restricted by the size of the wire and the amount of voltage pressure but also by the material of which it is made. Some materials resist the flow of electricity more than others because of their chemical composition. Imagine water trying to flow along an incline; if the incline is downward, the flow will be unrestricted; if the incline is upward, the flow will be resisted. Whether the incline is sharp or shallow will affect the speed of the water flow, and if the pressure is not sufficient or the upward incline is too great, then the flow may be stopped altogether. Further, if the incline is strewn with obstacles, like the bed of a stream is strewn with boulders and stones, then the flow will be slowed in comparison with that on a smooth incline. It is the chemical composition of a given material that determines whether it is "rock strewn" or "smooth."
Materials that allow electric current to pass through them fairly easily are electrical conductors, while materials that prevent the passage of electric current are insulators. Common conductors include copper and aluminum, which are used in the manufacture of electrical wiring. Most metals are good electrical conductors, yet even these offer some resistance to the flow of electric current. This property can be measured in units of resistance called ohms. Materials commonly used as insulators include glass, various plastics, and rubber.