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Homework Help: Quick Questions on Electric Circuits (Current directions and Kirchoff)

  1. Nov 11, 2013 #1
    Hi guys, i need some help here...

    1. I know that by convention, the positive charge of the voltage source is taken to be the direction of current, but I'm confused by this MIT lecture vid that drew the current as appearing to be flowing from the negative terminal. You can see the drawing in the video here: http://youtu.be/RsJ1eg7XNVs?t=10m5s and I have specified the timing in the link already. So why is this the case?

    Is that just a reference current? Meaning if we use kirchoff loop or junction rule, the direction actually doesn't matter as it's just your own personal preference. But while this is tempting to do, why is the current drawn as if the reference current is the real current?

    2. For Kirchoff's Loop rule, we let sum of voltages in a loop V1+V2+V3=0 right? V2 and V3 can be expressed as IR where I is the current flowing through it and R the resistance of the component. Let's say we start from a particular voltage source of V1. If that's the case, when we use the loop rule starting from this voltage source of V1, V1 must first be positive right?

    3. Lastly, what if there is two voltage sources in the circuit (DC for example), do we always take the highest voltage source and the smaller one as a component (like a resistor) only? Does the current actually "cancels" out each other if they flow in opposite direction? If these 2 sources have currents that "collide" at a junction, which is the direction of the current that actually flows?

    Thanks a lot for your help!
  2. jcsd
  3. Nov 11, 2013 #2


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    Staff: Mentor

    1. He is considering current source I and showing how the current from it splits to follow 2 paths. Maybe current is flowing into the top of the battery, as though charging it, we don't really care at this stage in the analysis. (More formally, he is summing all currents into the node marked as "e" volts and equating that sum to 0.)

    2. You assume a current direction around the loop, then the voltages are defined by that current. For each element, the current direction signifies that direction of a drop in voltage (that's the only way to get current to flow in an element). The polarity of a battery does not influence what you take to be the drop in potential in the analysis, you strictly observe the rule that current flows from a point of higher potential to one of lower potential, and leave the algebra to sort out the actual polarities.

    3. Voltages cancel, perhaps partially, and the current that flows is determined by that nett voltage. So, no, two opposing currents don't flow, only a nett current flows. (That is the simplest way to think about it in metals, anyway.)
    Last edited: Nov 11, 2013
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