caps

Basic electronics question...
« on: November 21, 2005, 12:56:53 AM »
I have been wondering, can current and voltage share the same path, in either the same direction or opposite directions? If so, consequences?

Eg, voltage input at one point, current output through the same point that the voltage was applied at ?

Basic I know sorry, cant find a answer though...


pstamler

Basic electronics question...
« Reply #1 on: November 21, 2005, 02:03:42 AM »
Voltage and current normally share the same path. They are two manifestations of the same thing, electron flow.

Look at it this way: when electrons flow [current] through a resistance, a voltage develops across that resistance. The relationship is summed up in Ohm's Law, which is described by:

V = I R

where V is is volts, I is in amperes and R is in ohms.

Suggestion: go to a library or to http://www.abebooks.com and find copies of the various volumes of Basic Electricity by Van Valkenburgh, Nooger and Neville; when you've worked your way through those, look up Basic Electronics by the same authors. The books are old (they were developed to train members of the U.S. Army) but extremely clear and extremely thorough.

Peace,
Paul

hg_man

Basic electronics question...
« Reply #2 on: November 29, 2005, 04:55:00 PM »
I'm going to pick a nit or two here and try to clarify the situation with voltage and current.  

Current is defined as the movement or flow of electrons.  No work is done unless electrons are moving.

Voltage is defined as the potential to do work.  It's the "pressure" that causes current to flow.  

There can be voltage (the potential) between two points without current flowing from one to the other.  For example, a battery has a potential difference, i.e. voltage between its terminals.  If the battery is just sitting there without being plugged into something, it still has voltage, but no current is flowing.

So you can have voltage without current, but not current without voltage.  

For a good explanation of this, check out CJ's meta thread on the basics of electronics.
"Beati illi qui in circulum circumeant, feant enim magnae rotae."

PRR

Basic electronics question...
« Reply #3 on: November 29, 2005, 11:08:31 PM »
> So you can have voltage without current, but not current without voltage.

Picking nits again: your first statement implies an Infinite resistance, so why deny the second which implies a Zero resistance?

Neither is possible in practice. If I leave a 9V battery on the bench, a few electrons flow through the air. If I get some electrons flowing in a wire, and kill the voltage, they soon stop.

But we can show near-zero/infinite resistances experimentally. In particular, a superconducting coil can continue to flow current for hours after the EMF is removed. Current without voltage.

Showing near-zero current, even in good vacuum, mires us in surface-leakage effects, though we can probably show no electrons for many minutes.

Asking if we can get closer to infinity than to zero is probably fruitless.

I'd put it: "we can have voltage without current, AND current without voltage, but only in our dreams, never on the workbench".

> can current and voltage share the same path

The question makes no sense.

Voltage only has meaning between two points. Current only has meaning through one point.

The "second point" in a voltage measurement may be "implied". Same thing with pressure. What is the pressure inside your water pipes? A gauge may show "30PSI", but compared to what? Normally, compared to atmospheric pressure, 15PSI. In some cases, compared to zero pressure (unlike voltage, pressure does have an absolute zero). In a special case, compared to some other pressure: if you are a deep sea diver working in 50PSI water and air, and we send you some "30PSI" water to drink, you would measure -20PSI and not be able to get any water.


 

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