Newbie Questions About Electric Potential In Circuits

Discussion in 'General Electronics Chat' started by mkevil, Feb 14, 2014.

  1. mkevil

    Thread Starter New Member

    Feb 14, 2014
    2
    0
    Hello. I'am a real newbie in electronics world. Just found this site while I was trying to learn. Some of my questions may sound stupid or funny, you may even laugh hard but I will still ask :)

    I can solve some problems just by memorizing Ohm's law and some simple rules but in reality I understood nothing about electricity. Particularly I'm having hard time understanding electric potential energy.

    [​IMG]

    I found this example on the Internet. In this, we spent 30 joules of energy against the force exerted by the big negative charge for pulling +3 coulombs of charges apart from that negative charge. Now they have a potential energy of 30 joules for 3C (means that they now have potential to do 30 joules of free work when we leave them free because the big negative charge attracts them, it's just like gravitational potential energy). And this is 10 joules/coulomb so it's said the potential difference between two points is 10 Volts. This makes perfect sense for me. But the problem is I can't imagine the same situation in a real circuit. For example we say its the resistance what uses the voltage thus reason of voltage drops. But in this example there is no resistor or any kind of resistance. Then how does the voltage get used up just by moving? (see question 1 below).

    I also have few more questions:

    1- Is resistance the only thing that uses potential energy in a circuit? If yes I can't explain the situation described in the above image. If no, why do we neglect the energy loss caused by moving electrons in real circuits and only distribute voltage drops among resistors.

    2- Imagine I just put the wires around the battery and think about the moment when it took 1 electron from high potential to low potential. That electron now has a potential energy equal to the energy spent when moving the charge from high to low potential. But since a battery doesn't generate electrons (and just providing force to move them) and all electrons were already there in the copper wire, how did other electrons get their potential energy? The battery hasn't done any work for them (or did it?). I know they should have potential energy because the negative charges repel and and positive charges attract thus they are biased toward the positive end but I can't explain this in terms of work and energy. To simplify: Why do electrons flowing through the wire has potential energy. Is it because the battery spent energy taking them from low potential to high potential or is it because of repel/attaction caused by charges in terminals or both(need explanation)?

    Probably stupid questions and I think something(if not everything) wrongly, I've always been very bad in Physics :(.
     
  2. bertz

    Member

    Nov 11, 2013
    238
    31
    The reason you cant grasp the concept of potential energy in an actual circuit is that it doesn't exist. There is a concept of potential difference which allows current to flow but that is a different issue. Think of it this way. A rock sitting at the top of a cliff has potential energy much the same as a battery sitting by itself. Once the rock starts falling it now has kinetic energy, the same as connecting a battery to a circuit.

    Actually, electrons do not "flow" in a circuit the way fluids flow in a pipe, even though that analogy is often used. What happens when a potential difference exists is that the electrons in the outer shell of the copper (or any other metal) become excited and can result in an exchange in electrons between adjacent atoms. So what happens is that when the circuit is disconnected the conductor has essentially the same electrons it started with.

    I think you are getting confused between potential energy and potential difference in a circuit. What resistance does is change the potential difference between two points in a circuit. If the resistance increases, then the potential difference decreases, thus electrical current in the circuit is reduced. Current is the rate change of the electrical charge and is directly proportional to the potential difference. Current is expressed as one coulomb per second or 1 ampere.
     
  3. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    Stupid questions are good. They make us look smart.:p

    Please try to start out simple. 1 or 2 questions at a time. All that reading makes my head start buzzing with different ways to guess which aspect is the most important one and then I don't know where to start the answer.

    We still need you to define the problem fairly well. Don't skip that part. Just remember that you are starting a conversation that can go back and forth dozens of times. It's the back and forth that gets us communicating on the right level and the right part of the problem. We're really very helpful types, but we deal with every level of education here, and we need to get a feel for how much detail is the right amount for you.

    Welcome to AAC...for as long as you want.
     
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