Hi, I'm a newbie here. I've been reading the ebook on this site.

I read that there's 0v across "electrically common" points. If the voltage across electrically common points is 0 then doesn't that make current 0 according to ohm's law I = E/R? If current is 0 then doesn't that contradict the statement that current is constant in a series circuit? I'm a bit confused here. Can someone point me in the right direction?

 

I'm not sure what you are reading but here goes:

A voltage can only exist between TWO points. Saying there is a group of "electrically common points" is saying this is assumes to be one common piece, say a good hunk of metal that several items are connected to. There can be currents flowing into and out of this hunk while the voltage is still zero as it is one point, not two points.

To continue on with ohm's law use it in the form V = I * R, and when R is zero then V is also zero even if there is a current.

 

The problem you have created is to make up a 'voltage' for the common point.

If you have ZERO volts then you don't have a voltage therefore you have no current.

The ohms law you quoted is not applicable for ZERO volts.

When you DO have current flow, then SOMEWHERE there is a voltage that is not ZERO.

 

oh i see, it's because even though i picked two points that were electrically common that was still basically only one point. That makes so much sense I didn't see that. Thanks for the swift reply!

 

The problem you have created is to make up a 'voltage' for the common point.

If you have ZERO volts then you don't have a voltage therefore you have no current.

The ohms law you quoted is not applicable for ZERO volts.

When you DO have current flow, then SOMEWHERE there is a voltage that is not ZERO.

Unless it's a superconductor.

 

Superconductors aside, even very good conductors (like copper and silver) have SOME resistance. Even then, it can take a rather large flow of current through the conductor before you can measure a voltage between two points with a typical meter.

 

What is the minimum voltage to make current flow in a block of copper? It can't be zero because then copper would be a superconductor.

 

There is no minimum voltage. Current will flow as long as there is a difference in potential.
In reality, if you wish to get into the physics, free electrons in copper are in constant motion.
What we measure as current is the net movement of electrons in the bulk medium such as a block of copper.
Ohm's Law still applies. I = V/R
As long as V is greater than zero there will be finite I.

 

But isn't there a minimum voltage or quantum level that is required to get an electron to move? My understanding was that the valence band overlapped the conduction band in metals so electrons continuously exchanged but that a voltage must be high enough to keep one electrons worth of energy in the moving current forming state.

 

No. That is why copper is a conductor. There are free electrons constantly in motion.

 

Electrons don't carry energy, only charge. Photons are the energy carriers. Mass is not needed for energy transfer but the properties of mass affects the wave impedance of space when energy is transferred and alters the relationship of energy in the electric or magnetic fields.

http://en.wikipedia.org/wiki/Wave_impedance

Wave impedance of copper: http://books.google.com/books?id=fUBPN8T9bwUC&pg=PA183&lpg=PA182&dq=Wave+impedance+of+copper&source=bl&ots=_XOBRqrwxX&sig=mOT6lHWefG5TFZ_0ku2hKXPSOdU&hl=en&ei=epYQTpunI4LEsAO9ndmaDg&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDwQ6AEwBQ#v=onepage&q=Wave impedance of copper&f=false

Electron = domino.
http://youtu.be/TYfyAhL5Mhw

 

As usual, the hydraulics analogy is helpful here, as it is many places.

If you think of a piece of wire as a piece of hose that's full of semi-liquid Jello; if you lay this piece of hose flat on the ground so that both ends are exactly level with each other, the Jello stays in the hose.

If you lift one end of the hose 2 feet off the ground and leave the other end on the ground, there is now a "potential" difference between the higher end and the lower end, and Jello starts to flow out of the lower end of the hose with a force that's created by the difference in level between the two ends.

The Jello still exists in the hose when it is not flowing; in other words, the potential for current is there without any voltage. When the Jello starts to flow out the end, the pressure it comes out with is the voltage (controlled by the height difference between the low an high ends), and the amount that comes out is the current.

 

Nothing personal (I use it sometimes) , the dreaded drain-pipe analogy while cute, has sent so many people on the wrong track when thinking about a circuit problem it pains to think about it.

 

To obtain a voltage without current
This is the easy one.
Take a battery, but don't connect the terminal to anything.
Voila you have a voltage but no current.

To obtain a current with no voltage
This is much more tricky, but it can be done.
Take a vacuum tube, preferably a 'soft filled' one.
Connect a sensitive microammeter between anode and cathode.
Do not bias the electrodes, but heat the cathode in the normal manner.
Voila you will measure a current, but there will be no voltage.
It will not be much current, certainly nothing like the current you would obtain with proper biasing.
But it will be there.

 

Alpha and beta emitters produce a current even in the absence of a voltage difference.
The photo-electric effect also produces current without the need for voltage.

Now that I come to think of it, an electric generator produces current without requiring a potential difference.
When you move a length of copper wire in a magnetic field, the electrons experience an EMF (electro-motive force), not a voltage difference, that cause them to move in one direction.