MadSci Network: Engineering
Query:

Re: What are amps in AC since electrons don't move?

Date: Thu Jun 29 07:07:05 2006
Posted By: Gene L. Ewald, Secondary School Teacher, Retired, Amer. Assoc. of Physics Teachers
Area of science: Engineering
ID: 1150812980.Eg
Message:

Dear Chris,

I will try to answer your question by trying to clear up a few misconceptions you have about 
currents … AC & DC.

     If amps are slow electron flow in DC, what are they in AC? …
You’re correct that electrons move slowly compared to the “speed of light” that many believe. 
However, like water in a pipe, the drift speed depends on the current (or flow rate) and the cross-
section of the conductors involved. Except in extreme cases, this is true for all currents and is 
usually a speed that you could follow with your finger if you could see a charge to follow. The 
difference between AC/DC currents is that DC is normally constant over observable time periods 
while AC not only changes direction at the quoted frequency but speed on a continual basis. If 
water models work for you, think of DC being the water in your cold water pipes and AC being 
the salt water moving in and out of a marsh. If air models work for you (actually a better model 
since air and charges can be compressed more than water), think of the natural gas coming into 
your home versus the air going in and out of your lungs.

     If you create more resistance (turn on more lights) …
Turning on more lights in a parallel circuit (home or auto) actually reduces the resistance. By 
offering more options for charge to move, more charge will move. This is a greater current, and 
will result in more light, more power, and usually more cost. Resistance is measured as pressure 
needed per resulting flow. If lights were in series, you would have needed them all on to work at 
all.

   how does the electric company give you more amps since all the electrons do in AC is vibrate? 
What the electric company provides for us is a pressure in the lines. If we have appliances shut 
off, we don’t allow that pressure to be applied to our stuff. Again, think water company or gas 
company. However, when we do hook up, this allows charge to flow into and out of the 
appliances. Less resistance allows more electrons to move farther along the wire before it’s time 
to reverse direction. Your use of “vibrate” implies that there is no lateral displacement during a 
cycle. Actually, the electrons have a finite length of time to move along the wire before reversing. 
In the case of a light bulb, electrons must enter one end of the filament and exit the other in 
order to create the friction for lighting. Consider sandpaper. It affects wood only if the grains can 
cut the little wood fibers. Whether a belt sander (DC) or a vibrating sander (AC), there must be 
motion relative to the wood surface. 

On an atomic scale, even at a speed of millimeters per second, a 1/120th of a second is a long 
string of atoms to pass the electrons and generate heat, light or magnetism. So to finally answer 
your question, they are the same thing except that DC tends to be steady while AC is very much 
changing in both direction and intensity … depending on the waveform of the AC source and the 
different components in the circuit. 

I hope this helps,
Gene



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