From: pauld@exploratorium.edu
Date: Tue Jun 24 2003 - 21:19:50 PDT
Message-Id: <200306250419.h5P4Jkr22258@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole Current Date: Tue, 24 Jun 2003 21:19:46 US/Pacific
Hi Jhumki
Consider 10 glass marbles falling through a vertical clear plastic tube of olive
oil each marble's center is 5 cm from that of its nearest neighbors.
They fall at terminal velocity.
the spacing remains 5 cm between marble centers.
The marble current above the center 10 cm of the tube is the same as the marble
current below the center 10 cm. (They have the same spacing and the same speed)
Yet the fluid in the middle 10 cm is warmer after the marble have passed.
The marbles have converted gravitational potential energy to the thermal energy
of the oil.
The electrons do the same thing they convert electrical potentail energy (charge
times voltage) intpo thermal energy in the filament. yet they have the same speed
and spacing before and after the filament... the same current. It's just that
after the filament they are at a lower electrical potential energy. (i.e. at a
different volatge.)
The electrons start moving at different places along the wire with a time delay
that determined by the distance the electrons are from the switch and the speed
of electric field propagation in a wire which is close to the speed of light. The
time delay is too fast for you to see, separate the bulbs by a foot and it takes
a few nanoseconds for the one farther from the switch to start to come on.
Paul D
> I've been taking a class where we've been tracking the current in wires in a
> circuit by observing compass deflection. It's the kind of class in which you
> don't think/believe anything unless you observe it directly or indirectly,
> and we can't really ask questions about things other than what is directly
> in front of us. So that's why I'm asking pinhole!
>
> We've observed that the compass deflects approximately the same amount on
> both sides of a bulb, suggesting that the charge flow rate is the same on
> both sides of the wire connected to a battery.
>
> Here's what I don't understand:
> Electrons are moving through the wire and then supposedly "bump into" atoms
> in the tungsten filament raising the tungsten electrons to higher energy
> levels, which then "fall back" down, producing light. So don't the original
> electrons in the wire lose some energy (i.e. the kinetic energy they lost
> from transferring energy to the tungsten electrons)? Does this loss of
> energy not show up in a slight reduction in current?
>
> Maybe the loss shows up as a reduction in the moving back-and-forth
> (vibrational) energy that all electrons possess. If this is true, why did
> the electron have to be flowing in the first place to light the bulb? Isn't
> the virbrational energy available without a current?
>
> Second, my teacher says that since two light bulbs in a series circuit seem
> to light at the same time, we should assume, from our observations, that
> electricity travels instantaneously. But I can imagine that the electricity
> travels too fast for us to observe its movement through the circuit. If the
> bulbs are lighting instantaneously, then how can there be a directional
> charge flow? Doesn't flow imply that time is passing?
>
> Thanks, Jhumki
>
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