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Basic question about electricity

  1. Feb 10, 2006 #1
    I'm a high school kid who's just gone back to school after a busy holiday, and we're reviewing electricity in school. I'm having trouble understanding basic concepts about electricity. I'd normally ask my science teacher, but it's the weekend and I just don't have the patience to wait a few days.

    My science teacher said that electricity is formed by moving electrons. Without moving electrons, there is no electricity. Okay so far. I look at this circuit diagram:


    I asked what would happen if the person in the picture was not holding down the switch. Would the lightbulb still glow?

    The teacher said it wouldn't, because the electricity wouldn't be flowing in a circuit.

    Now this is what I don't understand. Why do you need a circuit for electricity or electrons to flow? Couldn't it be just like this:


    The electrical current leaves the battery, flows through the lightbulb and gets to the switch, where it is stopped. While it is flowing through the lightbulb, why doesn't it light up? Isn't that a movement of electrons?

    Or is there something in the battery that stops the electrons inside from flowing out unless a complete circuit is made?

    What I guess I'm asking(in relation to this circuit diagram) is this. Why/how does a complete circuit make the lightbulb light up? What makes it do that?

    I realise these sound like stupid questions but if you would humour a kid...
  2. jcsd
  3. Feb 10, 2006 #2
    A simple answer to your question is: electricity not travelling in a circuit (IOW, travelling "freely") is referred to as "static". Have you ever been shocked by someone, just by touching them? That's static electricity. Now, imagine if the electricity were to travel as you have it travelling in your second diagram...it would flash out uncontrolled.
  4. Feb 10, 2006 #3


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    Staff: Mentor

    How can it flow through the light bulb if other electrons are in the way?

    It isn't a one-way trip: the electrons don't get absorbed by the light bulb, they really do have to flow through it.
    Pretty much the opposite of the above. A battery requires electrons to flow through it in order to produce electricity. But a bit more involved...

    Chemically, a battery involves a reaction where chemicals trade electrons. The start of the reaction pushes electrons out through the circuit and pushes ions through some sort of barrier (oversimplification) from one side of the battery to the other. When the electrons come back to the other side of the battery, they need to join their new chemical to complete the reaction.

    If you had a large source of free electrons and a large sink for them, it would be possible for electricity to flow in one direction: from the source to the sink (ie, lightning to the ground).
    Last edited: Feb 10, 2006
  5. Feb 10, 2006 #4


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    Staff: Mentor

    Your questions are not stupid at all, nooj. Keep on asking questions -- it's the way we learn about stuff. I'm not sure I understand what Mentat was trying to say, but the answer to your question is that electrons definitely flow around the complete electrical circuit shown in your diagram. The battery creates an electrical "potential" (voltage), and when the circuit is completed by external wires and lightbulbs and such, the electrons can flow externally. The battery is like a pump, and it pumps electrons up to a higher potential energy. Completing the external circuit is like opening a valve for a pipe that lets the pump push water out of its outlet, around a pipe circuit and back to its inlet.

    Another way of thinking of it is with balls and a ramp. The battery is pushing balls up from ground level up to some higher platform. The platform has a gate, and on the other side of the gate is a ramp for the balls to roll back down to ground. When the gate is closed, the balls just get stacked up on the top platform, but can't go anywhere. All potential energy and no kinetic energy in the balls. When you open the gate (like closing the contact on the electrical switch), that provides a path for the balls to roll down the ramp to ground. In a continuous situation, the ball pickup mechanism keeps pushing the balls from ground up to the platform (that's the batter function), and the balls roll through the gate and down the ramp.

    One subtlety with electrons and electrical circuits is that electrons carry a negative charge. They are the mobile thing in electric circuits, so they flow "backwards" compared to the "positive current" that we usually refer to. So in the battery and light bulb example, the battery is pushing electrons from its + terminal to its - terminal, and when an external conduction path is completed (like the switch and lightbulb), the electrons flow out of the battery - terminal, through the external circuit, and back to the battery's + terminal. The battery is supplying the potential energy to the electrons that are getting pushed out the - terminal, and that energy is getting delivered to the light bulb in the form of the kinetic energy of the electrons. That kinetic energy is transformed into heat at the lightbulb filament by collisions in the resistive filament material. Also keep in mind that there are so MANY electrons per cubic millimeter of metal (wire, etc) that they end up moving very slowly while they flow around the circuit. I haven't calculated it in a while, but even at moderately high currents, the electrons are moving less than a millimeter per second (if I remember it right -- plus I'm too lazy to re-calculate it at the moment ;)
  6. Feb 11, 2006 #5
    Thank you all for your great responses! Lets see if I got this right...

    The battery is like a pump.
    A chemical reaction forces electrons out of one end of the battery, where they travel around the circuit to come back into the + terminal. And presumably the whole reaction starts again so more electrons get moving.

    So the battery 'pumps' out electrons using chemical reactions.

    I can't have a scenario like this:


    because when electrons are pushed by the battery, other electrons are already in the way. Normally, if I had a complete circuit, the other electrons would be pushed by the ones behind it and they'd be forced to travel through the lightbulb and circuit.


    But because the circuit is not complete and the switch is not being held down, the other electrons gets pushed but they have no where to go and just get into a traffic jam at the end of the switch.

    This doesn't count as movement of electrons and hence electricity because:

    So to produce electricity, electrons must go through one end and in the other. The above scenario doesn't let that happen.
    But Mentat, why would it flash out uncontrolled? My science teacher tells me this and that but he doesn't actually explain much.
    Apologies, but are you referring to the first or second one?

    So if I understand this all correctly, electrons just won't flow if a complete circuit isn't created. The part I have trouble grasping is...why? Why won't electrons flow if a circuit isn't made? Is it because electrons must reach the other end of the battery for it all to actually work?

    Sorry about the horrific drawings I made. I just find diagrams (however bad I make them) let me understand things better.
  7. Feb 11, 2006 #6


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    Staff: Mentor

    The electrons pass along a conductor, e.g. metal like copper, which allows electrons to easily flow. You'll learn about why in physics.

    The battery provides an electrochemical potential (which you will learn in chemistry some day) which provides a voltage or electromotive force which causes the electrons to move.

    The battery is part of the circuit, and it acts like a pump.
  8. Feb 12, 2006 #7
    Nooj, you need to think in terms of electric potentials and differences thereof. The purpose of the battery (or any other voltage source) is to provide a potential difference between two points in a circuit. When such a potential difference is established, current will flow from one point to the other in an attempt to bring both points to the same potential.

    A good analogy here would be heat diffusion along a length of wire. If a wire is heated at one end there will be a (thermal) potential difference between one end of the wire and the other. Thus, heat will flow from the hot end to the cold end. After a while the two ends will be of equal temperature and the heat will no longer flow. The negative terminal of a voltage source could be regarded as "hot" and the positive terminal "cold". As long as this potential difference is maintained, current will flow. If an equilibrium is attained, current ceases. So the voltage source maintains a potential difference and permits current to flow.

    I don't think you will have covered RC transients yet, but if you have you could model your open circuit as being a series RC circuit. The lightbulb may be modelled as a resistor (possibly with an inductive component?) while the open switch is equivalent to a very small capacitance. Because the capacitance is so small the time constant ( = RC) is also very small and so the current flows only very briefly (too quickly to perceive).
  9. Feb 24, 2006 #8
    In the second figure (circuit2.jpg), the electrons spread out in the wire connected to the lightbulb equally in all directions, trying to get away from one another. There's no preferred direction in the lightbulb. There's no electrostatic potential difference in the wire attached to the lightbulb. So there's no current.

    Just as air molecules only flow when there's a difference in air pressure between two points, electrons flow only when there's a conducting path AND there's an difference in electrostatic potential between two points.

    The lightbulb only illuminates when there is current flowing.

    Does this make sense?
  10. May 24, 2006 #9
    Absolutely. I want to thank all of you for the help in understanding currents and potential differences. I'm happy to say that a couple of weeks ago, we had a test about electricity. I passed with flying colours, and on a few questions, your answers made a big difference. :!!) :!!) :biggrin:

    And even better, the knowledge has stuck in my brain!
    Last edited: May 24, 2006
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