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Electron/conventional theory - which way does electrons actually flow in a car batter

  1. May 30, 2009 #1


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    electron/conventional theory - which way does electrons actually flow in a car battery?

    Ok, I'm confused about what I keep being told when I ask car mechanics about the jumpering of car batteries. I am hoping someone can help me with understanding which way electrons are actually going to flow from one terminal of the car battery to the other.

    Now if I remember correctly from each electricity book I've ever read. The positive side of the battery has less electrons and wants to receive electrons. This is the electron theory correct? That the flow of electrons will travel from the negative side to the positive side. The conventional, I believe, was the other way around; that that's how engineers were drawing up their circuits showing diodes pointing the opposite direction or some such reasons.

    Okies, what I would think is happening with the car battery would be like any other battery. The positive would still be wanting electrons to balance out the battery. The electrons are going to jump along any jumper attached between the two terminals of the battery, then the solution within the battery will pull electrons back over to the negative side creating an overload on the negative and a shortage on the positive thus repeating the cycle.
    Ok, I know that's not all of what's going on but I am trying to avoid writing a book here. Sorry, so long as it is...

    Every mechanic I talk to keeps telling me no. That electricity is being pushed out of the positive terminal of the good battery to the positive of the dead battery. Then it travels along the frame of the car's body until it gets to the negative jumper that's attached to the car's metal frame. It then jumpers from that through that battery cable jumper to the negative of the good battery.

    This is how I actually jump cars, I attach both positives of the batteries but I don't attach the negatives to each other. I attach the good battery's negative(via a jumper cable) to the metal framework of the car.

    I would have guessed before that the negative of the good battery would push the electrons out and find a path to the positive. But the confusion lies in that the positives are connected to each other. How/Why do electrons travel from one positive terminal to another? I can see how electrons will travel through the jumper, then through the framework to find a connection to the positive. But then the positive to positive has me flustered. I know I've hooked up a negative terminal in the past to another negative terminal and had that work to jump a car, well, until I found out it was very bad practice to do it that way because the spark that occurs from the jumper touching the negative terminal could cause battery leakage to ignite.

    I'm hoping someone can help me think this one through.

    Thanks in advance for your time.

    See this picture someone posted for me on another forum. Why can a positive side of a battery directly to a negative side not cause a direct short and damage the batteries or cause an explosion?
    Last edited: May 30, 2009
  2. jcsd
  3. May 30, 2009 #2
    Re: electron/conventional theory - which way does electrons actually flow in a car ba

    You are correct in thinking that electrons travel from negative to positive. But in most applications it makes no difference if you think of it the other way around.
    Electrons travel from the weak batteries positive terminal to the good batteries positive terminal because the weak batteries positive terminal is less positive than the good batteries positive terminal. And since being less positive is the same as being more negative you should be able to see that the current is still flowing from negative to positive. If both batteries had the exact same state of charge then no current and no spark would be produced when you connected them. That's because both negatives would be equally negative and both positives would be equally positive. Just because both terminals are marked + does not mean that one cannot be less positive or more negative than the other one.
    There can be no current and no spark until the circuit is complete. There has to be a circuit or loop before current can flow. So simply connecting the positive terminal on one battery to the negative terminal of another can cause no harm because there is no circuit. If the last connection is made to the load then the spark would be at a safe distance from the batteries. If however you make the last connection to one of the batteries then you would have a dangerous situation.
  4. May 30, 2009 #3
    Re: electron/conventional theory - which way does electrons actually flow in a car ba

    your mechanic is thinking in engineer's, or conventional current. you are correct that conventional current is just the opposite of electron flow.

    as for the dead battery, you shouldn't think of it as a battery anymore. it's now a load, just like the lights. when it gets recharged and you use it to restart the car, it will be acting like a battery then.

    and yes, the reason the last connection is made to the frame is that car batteries can leak hydrogen.
  5. May 30, 2009 #4


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    Re: electron/conventional theory - which way does electrons actually flow in a car ba

    The electrons flow out of the negative terminal of a battery and into the positive terminal.
    This is the same with all batteries when they are being used as a power source.

    When the battery is being charged (or jump started), this is reversed.

    Before anyone knew about electrons they had to make a 50-50 guess about which way current flowed. They got it wrong, and we have been stuck with conventional current ever since.
    Even the symbols for transistors indicate conventional current for the emitter.

    Yes, a battery will be short circuited if you join the positive and negative terminals of the same battery together.
    But nothing will happen if you touch the positive of one battery to the negative of another.
    You do this when you put batteries in a torch. Nothing happens until you turn the switch on.
  6. Mar 17, 2010 #5
    Re: electron/conventional theory - which way does electrons actually flow in a car ba

    I signed up for this forum just to thank you for that reply, (I got here after a lot of googling,) it got me through a mental block which prevented me from understanding diagrams because I was looking at all the ends of components as equal. Your answer cleared it up.
  7. May 4, 2010 #6
    Re: electron/conventional theory - which way does electrons actually flow in a car ba

    Sorry to intrude, but i have a question on the subject. In the Automotive field they go by the electron theory (negative to positive).
    My question is how is the battery charged ? It would make sense that the alternator or external charger forces electron back to the negative side in some way.
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