[PLUG-TALK] Volts, Amps, Coulombs (was ... help Acer Laptop)

Keith Lofstrom keithl at kl-ic.com
Thu Aug 3 13:19:00 UTC 2006


On Thu, 3 Aug 2006, Keith Lofstrom wrote:

> The difference between current and voltage is like the difference between
> salary and education.  One is a flow, the other a potential.

On Thu, Aug 03, 2006 at 12:06:51PM -0700, Rich Shepard wrote:
> The way I learned it was that current is a quantity and voltage is a rate
> at which that quantity of electrons flows. Too simple?

No, just wrong.  Charge is a static quantity, measured in coulombs.  A
bucket full of 16.021 quadrillion electrons (1.6021e19) is a coulomb.
One colomb per second is an ampere, a flow rate.  Voltage is a potential
difference, the force available to push the electrons.

To put it in environmental terms, voltage is like the head of water 
above a dam.  Current in amperes is like water flow in cubic feet 
per minute.  Charge is like cubic feet of water.  Turbines and
generators convert between mechanical power ( head * flow ) and
electrical power ( voltage * current ), and pumps go the other way.
Both are measured in watts, after some conversion factors for the
moving water.  

Where they differ some is that electrons don't have much inertia,
and don't move very fast down wires (though the "pressure waves"
move at the speed of light, just like pressure waves in water move
at the hydraulic speed of sound, 1500 meters per second).  Moving
water packs quite a wallop, because water current moves much faster
and weighs a lot.

Keith

P.S.:  Electric charge moves slowly?  A meter of #12 copper wire
weighs 29.4 grams, and contains 123,000 coulombs of free electrons,
with a mass of 1.8 micrograms.  If you put 10 amps through the wire,
then the average electron flows at 29 centimeters per hour.  Again,
it is the voltage waves that move at the speed of light.  So the
electricity flowing into your house at 60Hz is actually just sloshing
in and out by about 1 micron, though the pressure waves travel
through the house in microseconds.  Hard to imagine, huh? 
Physics is fun...

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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