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Homework Help: Current and Resistance

  1. Sep 21, 2009 #1
    Hi guys, I'm currently doing current electricity but I'm not rlly sure why current is always the same throughout a circuit, even with resistors. I've searched sites such as Wikipedia, which states that
    If this is so, shouldn't the current be lesser after it passes through a resistor? Any help is greatly appreciated. Thanks.
  2. jcsd
  3. Sep 21, 2009 #2
    It's because current is the rate at which a conserved quantity is flowing through a path. Suppose you connected several garden hoses together end-to-end. The water's flow rate in gallons per second would have to be the same for all of the hoses. A given amount of water per unit of time comes out of one hose and goes into the next one in sequence. Same idea, just change gallons of water to coulombs of charge. If electrical components are connected end-to-end, which is a series connection, the flow rate of charge per unit of time, which is called current, will be the same for all components in that branch.
    Last edited: Sep 21, 2009
  4. Sep 21, 2009 #3
    Thanks for the help, but if we use water as an analogy like you said, won't the speed of water decrease as it moves through a constriction like a resistor for electricity? Won't the speed of water be lesser after it moves throught the constriction apart from the pressure(voltage) decrease?
  5. Sep 21, 2009 #4
    True,in your analogy the water will be faster in the constriction,but current (Coulombs per second)is not analogous to speed(metres per second) but to volume of water per second(cubic metres per second).The faster speed is compensated for by the smaller cross sectional area.
  6. Sep 21, 2009 #5
    It can be reasoned simply from the fact that there is no ability to store anything. There is no secret hiding place for something to continuously go in but then not come back out at the same rate. A garden hose can't save up some water in an invisible tank attached alongside of it, and a resistor can't store away some charges for later. Flow rate in and flow rate out could be unequal for a brief instant, but not in a continuous process. And if that's true then you could have many of them connected in series, and the same amount per second would have to flow through all of them.
  7. Sep 22, 2009 #6
    Oh, I see so just to be clear, current is the amount of charge between a certain region per second. A resistor in a circuit will maintain the amount of charge/second throughout the circuit but the voltage will decrease as some work is done because of the resistor. Am I right?
    Last edited: Sep 22, 2009
  8. Sep 22, 2009 #7
    The phrase "between a certain region" is fuzzy. The right way to say it is: current is the rate at which charge flows past a given point per second.

    Imagine you're sitting on the side of the road at a certain point, and cars drive by you at a rate of three cars per second. This kind of measurement of charge per second going by a given point is constant at all points along a particular series branch.

    There seems to be a problem in that sentence after the word "but." The way you said that, it sounds as though you are picturing voltage as something that gets "used up" and therefore decreases gradually with time. That's wrong.

    Think of the voltage as forming fixed steps along a path from a point of maximum potential point to a point of minimum potential.

    Picture you're on top of a mountain at an altitude of 5000 meters above the base. You want to go down to the bottom. You can go down in one big hike of 5000 m. Or, if you want, you can go down by following several short trails that will take you down 2000 m, then another 500, then another 1500, then another 1000, and now you're at the bottom. There is a fixed total difference in altitude between two extreme end points, regardless of which path you take.

    Similarly, the voltage across a resistor in a circuit is how much of a drop there is in the electrical potential between two points. If there are several resistors in series, then their individual voltage drops will have to add up to be the total voltage drop for the whole trip, from the most positive point to the most negative point.
  9. Sep 23, 2009 #8
    Ok, so the voltage is something like the unit of measurement for potential so it can't be used up. I see. K, thanks for the all the help. It's really helped me understand electric current better. Thanks.
  10. Sep 23, 2009 #9
    A "potential" is at one point. A "potential difference" is between two points.

    The voltage across any two points is the same thing as the potential difference between those two points. These terms are synonymous.

    Every voltage is between two points. If someone speaks of the voltage at a single point, there must have been a previous understanding that such phrases will mean the voltage between a point and some agreed-upon reference point. For example, it's common to label one point in a circuit "ground", and then say that various points in the circuit have certain voltages with respect to ground.
  11. Oct 1, 2009 #10
    Oops, sorry for the late reply. Kinda got caught up in sme work. K, thnks for the help in understanding the definition of voltage and all.
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