direction of current flow

Discussion in 'General Electronics Chat' started by sharanbr123, Oct 22, 2014.

  1. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Hello All,

    I would like to know if current can ever flow into the positive terminal of a voltage supply.
    I am using an example shown in the circuit that is in the attached figure.

    The description below the circuit indicates that this is not possible but what to conclude from these derived values from KVL and KCL?

    Thanks in advance
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    in electron flow world view that is always the case. in conventional current view, only from a source more positve than the battery potential.
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    Are you asking if current will flow when, for example, a 12V battery is attached to the leads of a 9V power supply?

    Your attachment did not come through, so we only speculate.
     
  4. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    You may have forgotten to upload your attachment as it does not show up here.

    For a low impedance battery, current can flow in either direction depending one whether it is powering something or something is powering it.
    For example, when a battery powers a resistor it always flows from the positive terminal out using conventional current flow.
    But when we charge a battery we use a source voltage that is higher than the battery voltage so that we can force current to flow into the positive terminal.

    For a voltage regulator it is quite different because many voltage regulators only source current, they can not absorb power like a battery. If you try to push current into most voltage regulators they will simply go out of regulation and allow the output voltage to go to whatever voltage the external source wants to provide.
    Only a few special type regulators will absorb power, but most of the IC's you see around are only able to source current not sink it. To get a regular regulator to sink current you have to design it differently or at least provide a constant resistive load that can sink some current.

    In the above we used conventional current flow, but if you want to use electron current flow then reverse all the currents.
     
  5. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    I was referring more from conventional current flow. My question basically after seeing circuit that has multiple voltage sources.
    Anyway, I have attached the figure now.
     
  6. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Yes, something similar to that. What happens when similar leads of battery are connected together with one source of higher potential than the other.
     
  7. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    I have uploaded the figure now. Mainly, if a path would exist for current (convential flow) from positive lead of one source to positive lead of another, given one of them is at a higher potential.
     
  8. Johann

    Senior Member

    Nov 27, 2006
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    The higher source will charge the lower "source" if it is a battery.
     
  9. Jony130

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    Feb 17, 2009
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    The stronger battery will charge the weaker battery.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    But if instead of a battery, the the lower voltage source is a power supply, reverse current flow may damage the regulator if the supply was not designed for reverse current protection. Results are unpredictable without a schematic.
     
  11. Jony130

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    Feb 17, 2009
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    And If you do math you will find that the voltage at point when all resistor are connected is 8V. And this is why the current will flow from this point toward B2 battery.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You should note the following.

    1)If you parallel two ideal voltages sources across a load the calculation is impossible.

    2) All (realworld) voltage sources have an internal series resistance. Incorporating these makes the calculation then possible.
    Your diagram shows series resistances.

    3)The same is true of Alternating generators.

    4)As others have said there is also the question of the ability of a voltage source to act as a sink. Some will be damaged if you try to force this, some will not, but even those which can accept some sinking (charging) will be damaged by excessive chargine or sink current.
     
  13. GopherT

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    Nov 23, 2012
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    The figure in the attachment indicates electron flow (not conventional flow). Also, the text asks the question, does it ALWAYS happen that a higher voltage battery will push current backwards through a lower voltage battery, their answer is correctly NO, as they explain you must mathematically analyze each specific circuit and corresponding resistor values to determine current flow.
     
  14. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Ok great, we can see your attachment now, very good :)

    From the wording they want to use electron current flow. That means current flowing OUT of the positive terminal means it is getting charged.

    You can see that one battery is getting charged while the other is supplying power. If resistor R2 is made very low like 0.001 ohms then BOTH batteries will be supplying power not just one. As they clearly state, you have to do a complete analysis of the network to be sure what is happening.

    I will add that it is usually easier to use conventional current flow. The only time you really need to use electron flow is in physics problems that depend on the correct sign of the current to give the right physical results, or in cases like this where the author chose to use that instead.
     
  15. MikeML

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    Oct 2, 2009
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    Here is the Conventional Current world view:

    216.gif
     
  16. Kermit2

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    Feb 5, 2010
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    im from a very small world, hence the very narrow view.
     
  17. MikeML

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    Oct 2, 2009
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    The problem is that any student at a college or university is going to get the Conventional Current World view. This site, and others, add to student's confusion by perpetuating the "electron flow" nonsense. Even the LTSpice simulator example I posted above naturally presents current directions in terms of conventional current. The is no switch on it to make it conform to the very narrow view perpetuated on this web site!
     
  18. wayneh

    Expert

    Sep 9, 2010
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    Did the concept of electron flow have more value in the days of tubes? Unless you're actually beaming electrons around, I can't see how using electron flow adds any insight to understanding a circuit.
     
  19. MrAl

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    Jun 17, 2014
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    Hi,

    It's mainly needed in physics where the charge has to be negative to really show what is happening sometimes. So we are much more likely to see electron current flow being used in a physics book and conventional current flow in an electronics circuit book.
    .
     
  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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    It doesn't. The direction of conventional current is just that - a sign convention.

    The signs and directions in physics equations are all adjusted to fit it.

    For instance consider a Hall generator.
    Both electrons and holes are deflected in the same direction.
    So knowing the sign of the carrier charge will not help you.
     
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