MadSci Network: Physics
Query:

Re: Is electricity ever 'used'?

Date: Mon Jan 17 10:58:31 2005
Posted By: Lawrence Skarin, Rochester Museum and Science Center Technical Assistance Group
Area of science: Physics
ID: 1103418847.Ph
Message:

It's a pleasure to deal with questions so well-expressed; a credit to you 
and your teachers.

First, let's agree on what "electricity" is.  For our purposes, we are 
talking of moving charges.  This is called "current."

A little more than 100 years ago, the electron was discovered and, in 
metals, moving electrons make up current.  Benjamin Franklin happened to 
get positive and negative wrong when charge was thought of as a fluid, and 
we now know that metals have outer orbit electrons that are free to move 
under influence of an electric field.  Let's call these "free electrons."  
And "conventional current" moves in a direction opposite that of the 
electron flow.

Now moving electrons can do work -- that is, transfer energy.  When an 
electromotive force -- voltage difference -- is applied to the ends of a 
path filled with free electrons, they move.

Moving electrons can transfer energy through their magnetic field (as with 
relays and motors), or
through collisions with stationary atoms (as with electric heaters).  But 
the ultimate source of the
energy is the voltage source.  The moving electrons are the means for 
energy transfer.  If you pull on a rope over a ceiling-mounted pulley to 
lift a bucket of nails, you are doing the work and the rope is the means 
of energy transfer.

The means for creating a voltage difference, whether by an electrochemical 
cell or turning a DC
generator or alternator, is important for us only in the fact they convert 
one form of energy (in these cases, chemical and rotational-mechanical) to 
electric.  So let's just take our ability to create a voltage difference 
as a given.

I want to set up a thought experiment.  Let's take a 10 centimeter piece 
of resistance wire that measures 0.15 ohms and place it across our 1.5 
volt cell.  We notice the wire gets hot -- 10 amperes of current is 
flowing.  One model we can use for this is the circuit theory model.  We 
represent the cell as an ideal voltage source of 1.5 volts and the 
resistance wire as a 0.15 ohm resistance.  The lines we draw from the 
source to the resistance ARE NOT WIRES!  They merely indicate connections 
between two ideal entities.  This model allows us to calculate that 10 
amperes come from the source and that 15 watts are dissipated in the 
resistance.  This model says nothing about electrons or the mechanism the 
resistance uses to change electric power into heat; it just describes 
power (rate of energy production) out of the source and power (rate of 
energy dissipation) into the resistance.

But this first model does not satisfy you.  You want to talk about 
electrons.  OK -- we'll use a second model.  We draw a 1-quadrant graph 
with 0 to 10 centimeters on the horizontal scale, and 0 to 1.5 electron-
volts on the vertical scale.  Electron-volts (eV) are energy units.  We 
draw a straight line from (0,1.5) to (10, 0).  We can think of this graph 
as the energy each electron has as it flows through the resistance wire.  
As it "rolls down the potential hill," it loses energy.

This model ignores the atoms the electrons that bump into and says nothing 
about quantum states.  But what would be gained if we did include these?  
The rule is, we use as simple a model as possible to approximate the real 
world.  In the second model electrons give up their energy as they travel 
through the wire.

Electrons never get used up. But their ability to do work does get used 
up. And, in this thought experiment, second model, 6.25E19 electrons are 
passing through the resistance wire every second.  Think of the free 
electrons in a conductor as an incompressible fluid.

Try Bill Beaty's site:  http://www.eskimo.com/~billb/

Choose 'My "Electricity" Articles'.  Then choose 'What is "Electricity?" ' 
Bill has done a wonderful job on this (and other issues).  I think you'll 
want to bookmark his site.

Finally, your sentence mentioning "...'flow of electrons' just an analogy 
to make it easier for high school kids like me to understand its nature?" 
shows a sensitivity to the learning situation that is somewhat rare.  The 
analogy is for humans of all ages.  And I think you'd make a fine teacher.

Best of luck, and cheers.

Larry Skarin



Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@madsci.org
© 1995-2005. All rights reserved.