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Homework Help: Electric Circuit Misconceptions

  1. Oct 22, 2006 #1
    There are a couple of things that just make no sense to me regarding circuits.

    Let me start off with my train of thought and how it leads into my questions, so someone can tell me where I go wrong.

    Question 1 (comes directly from a picture attached):

    A potential difference is setup between two points, perhaps the terminals in a battery, and this causes charges to move. However, the potential difference is of course created by an electric field.


    Every potential difference, or voltage, MUST be created by some electric field.

    Now the electric field of course produces forces on the charges, which in turn causes them to move, creating a current. Shouldn't the electric field be weakening as the inverse square of distance?

    Why then is the electric field taken to be constant?

    Current is how much charge per unit time is passing through a certain point, so the current should be different at every single point since the force which moves the current is different at every point and the mass of the charge of course doesn't change.

    So why is it said that the current will be the same through every point in a resistor in series combination?

    This question originates from the picture I have attached...Quick Quiz 28.2...I thought the answer would be a) smaller...but apparently the answer is the same.

    Why would the charges still be moving at the same speed when not only should the field be changing, but their potential has dropped.

    If anybody could explain the error in my thinking here, I'd be very thankful.

    Attached Files:

  2. jcsd
  3. Oct 22, 2006 #2
    Question 2:

    Later on in my book, the author is VERY clear to point out that current DOES not take the path of least resistance.

    Then how in the world, in Quiz 28.4 (above pic attached), does the current in the closed switch scenario not even go through R2?

    And an even more interesting question...how the hell do electrons or any type of mobile charges know or see that a resistor would be 'up ahead' and would cause them more resistance?

    If I make a small analogy to traffic, my question might become a bit clearer. I would naturally always want to take the road with less traffic so I can get home quicker.

    Now how in the world would I know that there is a traffic jam (electrical resistance) on a certain expressway if I don't go check it out first? Listening to the radio?

    Do electrons have radios?

    Pretty lost on this one.
  4. Oct 22, 2006 #3
    Another question

    Question 3

    Now...this is another strange property of circuits.

    Let me show my thought process so someone can correct me.

    A current as we know is the rate of charge passing through a certain point per unit time...so the speed of their movement has much to do with the magnitude of a current.

    Now...why is the current in a circuit constant throughout, and it's dependent on the resistance/resistors present? (Of course I'm talking about my introductory simple circuits here)

    It would seem that that, somehow, once again, electrons have radios or something that allows them to see up ahead and notice resistance.

    Here is my problem with this elucidated with an analogy:

    Let's say I'm driving around a nice little city (my circuit) and there is absolutely not a car on the road for about two blocks. However, after a turn on 2nd street, a HUGE traffic jam due to free hamburgers (I encounter resistance), so I slow down to .5 mph, just fast enough to get a burger myself. Same as the charge slowing down in the face of resistance...the current slowing.

    Now what the charges are doing, is analogous to me driving around at .5mph EVERYWHERE! Even before I turn on 2nd street, even if there are no cars on the road...I'm just going crazy slow for no reason at all, not even a hamburger.

    So why in the world does this happen? By the way...pardon the hamburger interjections...I'm starving.
  5. Oct 23, 2006 #4


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    No work is required to move an electron on a metalllic surface. So if you apply a potential to a circuit the electrons drift through the wires without any energy expenditure. I interpret this to mean that the electic field is zero in the wires - no charge buildup is sustained. Therefore (virtually) no work is done by the battery to move the electrons through the wires. This strange phenomena is demonstrated in superconducting materials (with virtually no resistance watsoever). Once the current in a super conductor is started with a power supply it can be disconnected from it and the current will continue flowing indefinitely.
  6. Oct 23, 2006 #5


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    Current flow behaves very much like water flow - you cannot cause a localized flow without causing all of the electrons to move (drift) in the circuit. It behaves like water in a pipe - all or nothing moves in unison.

    For QQ 28.2 the diagram on the right with the switch open should be considered. That is in a series circuit the current is the same through all of the components. The electrons push each other to drift at the same speed through all of the circuit from the low potential to the high potential terminal. Any pileup (large resistor, capacitor) will impede the flow throught the whole of the circuit.
  7. Oct 24, 2006 #6


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    You need to remember that each electron communicates its motion to its neightbours (via its electric field) and in this way the flow of current in a circuit tends to acts as a unit being driven by the power supply. You should not think of single cars moving but rather a stream of cars moving along the highway (the sea of free electrons in the material). If the lanes are reduced (higher resitance somewhere in the circuit) the flow of traffic will be reduced all the way down along the highway.
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