MadSci Network: Physics

Re: Why is the second bulb in a series circuit not dimmer than the first?

Date: Fri Jan 23 07:54:28 2004
Posted By: Lawrence Skarin, Rochester Museum and Science Center Technical Assistance Group
Area of science: Physics
ID: 1074612653.Ph

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 

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.  


But I would not worry about this until you study transmission lines and 
distributed circuit parameters.

Thanks for your questions.

Larry Skarin

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