### 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
Message:
```
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

distributed circuit parameters.

Larry Skarin

```

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