Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Introductory Physics Homework (http://www.physicsforums.com/forumdisplay.php?f=153)
-   -   Is current used up in a resistor or the voltage is used up? (http://www.physicsforums.com/showthread.php?t=68595)

F|234K Mar24-05 07:03 PM

is current used up in a resistor or the voltage is used up?
 
is current used up in a resistor or the voltage is used up?

Integral Mar24-05 07:25 PM

Neither.

What do you mean by "used up" that is not a term I commonly see used in this contex.

F|234K Mar24-05 07:30 PM

lol, that is EXACT wording of the question in my book.

"is current used up in a resistor"

Integral Mar24-05 07:31 PM

What do you know about current in a resistor?

chroot Mar24-05 07:31 PM

Throw your book out the window, because it is poorly written.

Resistors follow Ohm's law: V = IR. That's the most precise, and most correct statement you can make about them.

- Warren

F|234K Mar24-05 07:35 PM

well, current decreases if there is a resistor present, and i think voltage decreases at the same time.....then it's both decreases....but i remember the teacher said something about voltage decreases. therefor, i don't really know.

Data Mar24-05 07:37 PM

What is current?

chroot Mar24-05 07:39 PM

It's really just a horribly-worded question with no clear answer.

If you add a resistor in series with another, the total resistance increases, and the current drops (as Ohm's law indicates). Each resistor also has a voltage drop across (again, as Ohm's law indicates).

On the other hand, if you place a resistor in parallel with another resistor, the total resistances goes down, and more current flows, while the voltage on the first resistor isn't changed at all!

So write that down.

- Warren

F|234K Mar24-05 07:39 PM

well.....the charge flow through something for a specific period of time...

chroot Mar24-05 07:39 PM

Quote:

Quote by Data
What is current?

The flow of charge; i.e. the amount of charge that passes a fixed point in the circuit per unit time.

- Warren

vincentchan Mar24-05 07:40 PM

current = flow of electric charge

all power line, wire and cable carry current...

F|234K Mar24-05 07:41 PM

chroot, thanks a lot.

Data Mar24-05 07:56 PM

I was asking the poster :bugeye:

In RC circuits, current is always conserved, since otherwise you'd have to get a charge buildup at some point, which would quickly stop the current altogether.

chroot Mar24-05 08:19 PM

Quote:

Quote by Data
you'd have to get a charge buildup at some point, which would quickly stop the current altogether.

...which is exactly what a capacitor is.

You're talking generally about the continuity equation for current, which holds in a purely resistive circuit. The same continuity equation certainly does not apply for a capacitive circuit, however -- there can be a current into the capacitor's leads, but no current flows between its plates.

- Warren

xanthym Mar24-05 08:31 PM

Quote:

Quote by F|234K
is current used up in a resistor or the voltage is used up?

For Resistors (to which this question refers) in any circuit, the Current entering a Resistor EQUALS the Current exiting the Resistor (otherwise, like Data points out, electrons would accumulate inside the Resistor). Thus, there is "no loss of current" inside a Resistor.

However, when this Current "I" flows thru a Resistor "R", a VOLTAGE DROP "V" of magnitude {V = I*R} occurs across the Resistor. (The measured Voltage on one side of the Resistor will be less than that on the other when referenced to the same "zero point" in the circuit). Thus, in some sense, the situation is like "losing voltage". (<--- bad term, we're just humoring the textbook)

It's better to think in terms of water pressure: Higher pressure (i.e., higher Voltage) is required on one side of the Resistor than the other in order to supply power for "pushing electrons" thru the Resistor (which presents "resistance" to electron flow). ((This process represents electrical power "W" of magnitude W = I*V = V2/R = I2*R being converted to heat.)) In any case, the Resistor's Voltage Drop {V = I*R} is used extensively in circuit theory (e.g., Kirchoff's Laws, etc.).


~~

F|234K Mar24-05 08:58 PM

thanks, xanthym. i was doing the problems and i found, just like you said, the voltage "drops", and the value is equal to V=IR


All times are GMT -5. The time now is 04:52 PM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums