|MadSci Network: Physics|
Hello Ben. Nice questions. Let's look at the wire and filaments in a two (incandescent) lamp series circuit. It is clear there is a metallic path from the energy source terminals. Metals have electrons free to move -- in fact outer orbit electrons are not even thought to be associated with any particular atom. They comprise a cloud that is free to move under influence of an electric field. When you close the switch, an electric field gets established throughout the path almost instantaneously. Remember, nothing happens faster than light speed would allow. So the mobile electrons move almost instantaneously. Almost all delay in filament lighting occurs because of time needed to heat them -- so-called thermal inertia. It's not unlike a garden hose filled with water that begins spurting as soon as you open the valve 50 feet away. The pressure from the valve arrives at the hose end very quickly while a cubic centimeter of water at the valve takes longer to reach the hose end. Pure circuit theory does not account for time delays. It doesn't know anything about electrons, either -- it just knows about charge and charge flow under influence of voltage difference. In fact, we consider the "wires" between circuit elements like cells and lamps to be of zero length. We just draw them with length because zero length wires on paper are hard to see. Here's another way of looking at these phenomena: Actions some distance away cause waves to travel (propagate). Sometimes the wave propagation is so fast, we ignore it and use the simplest theory possible to explain the overall behavior of the system. When you strike a nail with a hammer, the force wave at the head travels to the tip at the speed of sound in the steel. Yet we are happy to consider the head force and tip force as simultaneous. If distances are long enough, we do use a more sophisticated theory for system behavior explanations. The Telegraph Equations (consequence of James Clerk Maxwell's work) are an example of such. See: http://colos1.fri.uni-lj.si/ ~colos/COLOS/TUTORIALS/JAVA/CONDUCTIVITY/MathModel.html But I would not worry about this until you study transmission lines and distributed circuit parameters. Thanks for your questions. Larry Skarin
Try the links in the MadSci Library for more information on Physics.