From: Ronald Wong (ronwong@inreach.com)
Date: Thu Jun 26 2003 - 15:47:46 PDT
Message-Id: <l03102800bb1fe52806e0@[209.209.19.71]> Date: Thu, 26 Jun 2003 15:47:46 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Current
Jhumki said/asked:
>...
>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?
There are two parts to this question that need to be addressed.
The first has to do with the term "electricity".
Gravity, magnetism, and electricity are forms of energy that are associated
with fields. Without these fields, there is no energy.
When you connect one end of a circuit to one terminal of a battery and the
other end of the circuit to the other terminal, an electric field is
propagated throughout the circuit and the cells of the battery.
As Paul pointed out earlier when he responded to your question, this
doesn't take very long. The speed of propagation is only a little less than
the speed of light in a vacuum. If the wiring in the circuit adds up to a
meter in length, it will take about 5 nanoseconds to establish the field
throughout the circuit. If the wiring in the circuit is as long as the
earth is round (about 4 X 10^7 m) it will take a little longer - about 1/5
of a second.
The second part has to do with expression "charge flow".
When a rock at rest suddenly finds itself in a gravitational field, it
immediately experiences a force acting on it pushing it in a particular
direction. If there is nothing to prevent it from moving, it will convert
it's energy of position (potential energy) into energy of motion (kinetic
energy) as it moves from the region of higher gravitational potential to
lower gravitational potential. That's the way it is when rocks fall to the
earth.
The same thing happens to an electric charge in a conductor.
Until the circuit is connected to the battery, there is no electric field
in the circuit and therefore no net force acting on the charges. Once the
connection is made, an electric field "suddenly" appears in the circuit.
The electric charge now feels a force acting on it and, since there is
initially no impediment to it's motion, it moves from the region of higher
electric potential to one of lower electric potential - converting it's
electric potential energy into kinetic energy in the process.
Unfortunately, there are a lot of atoms in the way so the charges are
constantly running into them and, as a result of the collision,
transferring their energy to the atoms. In turn, the atoms give this energy
off in the form of heat and, in the case of the filament of your lamps,
light as well. Despite these collisions - and the loss of energy that they
entail - the charges continue to move through the circuit after the
collisions. This is because they are still in the presence of the electric
field in the circuit and, as a result, they continue to move from the
region of higher electric potential to lower electric potential converting
their electric potential energy into...
The actual path and progress of a charge through the circuit is not unlike
that of an individual trying to navigate across a very crowded dance floor
full of people who are jumping around all over the place. It's overall
progress is very slow due to the erratic path taken as it bounces off of
one vibrating atom (with great speed by the way) and then another.
When a current of 2 amperes flows through a copper wire 1 mm in diameter,
the charges that make up this current will typically be moving at a speed
of 0.1 mm/s in the direction of the current (a snail can do better than
that).
So it all comes down to this:
You connect the wire to the battery (or close the switch) and:
1. suddenly, in less than the blink of an eye, an electric field appears
everywhere in the circuit and
2. immediately an overwhelming number of charges throughout the electric
circuit simultaneously find themselves in an electric field and
begin to erratically drift slowly through the circuit en masse from
the region of higher....
This is why, when you throw the light switch on, all the lights in the room
go on at once. All the charges in the entire circuit are, for all
practical purposes, set in motion at the same time so all the lights light
up together (okay, okay - if the light bulbs are one meter apart, one will
light up about 5 nanoseconds before the other. But let's be reasonable. Do
YOU have the technology to measure time intervals as small as this?).
Cheers
ron
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