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Question on current

  1. Jul 16, 2015 #1
    I was told the current remains the same throughout a circuit. That's why l1=l2+l3. However,won't resistors decrease the current? The formula seem to contradict with the fact that resistors will decrease current. Can someone explain to me please? Thanks
     
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  3. Jul 16, 2015 #2

    A.T.

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    Yes, throughout the circuit.
     
  4. Jul 16, 2015 #3
    No Resistors dont decrease current in a circuit .

    Resistors simply cause a potential drop when current passes through them .

    What are l1,l2,l3 ?
     
  5. Jul 16, 2015 #4
    A resistor will decrease the voltage (ie the electromotive force of the current), but the current itself will be unchanged. If you want to decrease the current through a particular load you need to provide a parallel pathway to it. Also you will certainly effect the current of the circuit as a whole by adding a resistor (assuming the voltage remains the same, the resistance and current are inversely proportional to each other; Ohm's Law), but the current is still the same at DC+ as it is at DC- and every series point in between. The only time it is different is, as I noted previously, in the case of parallel loads in which case the voltage is equal across them and you can calculate the individual currents from there.

    When you refer to L1/L2/L3 are you instead referring to the AC output from a 3-phase generator?
     
  6. Jul 16, 2015 #5
    I thought EMF causes current ?
     
  7. Jul 16, 2015 #6

    ZapperZ

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    You really should sit back and look at what you have written. You seem to think that we get to see what is in your head, such that we should know what I1, I2, and I3 are.

    Assuming that I2 and I3 are parallel arms of a circuit and they all add up to I1, then we'll continue. However, do not make a habit out of this. This forum is a great place to learn not only physics, but how to communicate effectively, something that you will need to do sooner or later, no matter what profession you enter.

    Go back to the definition of what "current" is. It is the rate of charge flow per second.

    While this is a flawed analogy, the picture of water flow in pipes is often used. No matter how many branches you make the water flow into, the sum total of it will remain the same if there are no pile-up or leaks. Same with current. Unless there is a charge build-up somewhere, the total current in a series remains the same.

    Zz.
     
  8. Jul 16, 2015 #7
    It does, once it has a path to complete the circuit. Let's take the example of a 12V battery. At rest the battery shows 12V but there is no current since there is no circuit. If you wire a 10 ohm resistor between + and - the difference at the battery will cause a 1.2 amp current. Since there is only one resistor, the voltage drop across it will equal the DC voltage of the battery (12V, excluding negligible drop from the wires themselves), but an ammeter will measure 1.2A regardless of which side of the circuit you wire it into relative to the resistor.
     
  9. Jul 16, 2015 #8
    My point was that the difference in potential of the battery electrodes would cause a flow of current .

    The point that resistor does not cause change in current was always clear .
     
  10. Jul 16, 2015 #9
    The analogy of water flowing through a pipe can be really helpful here. Current can be analogized to rate of water flow at a given point in a loop of the pipe system (the circuit) and a resistor can be analogized to a particularly narrow part of the pipe, like a constriction. Assuming water has been flowing continuously for some time, the constriction will cause the rate of flow anywhere in the pipe system to decrease, and, similarly a smaller constriction causes less "resistance" and water flow, current, will increase.

    Edit: This is not contradictory to ZapperZ's analogy. I just wanted to break it down further.

    I think part of the misunderstanding is seeing current as it's own entity and not as a property of the whole loop within a circuit.

    According to Ohm's Law, I = V/R, given an independent voltage source, reducing the total resistance "facing " that source will raise current flowing through that source.
     
  11. Jul 16, 2015 #10
    Yes. To use an analogy, if you take two containers under different pressure and connect them to each other, air will flow from one to the other until the pressure equalizes. Likewise, if you provide a path from the positive to the negative of a battery, current will flow until the potential of the two equalizes (zero voltage). And just like you can use a variety of apparatuses to control the airflow from the higher-pressure to lower-pressure container, you do the same by adding resistors/loads/other devices in the circuit.
     
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