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3 Beginner Questions

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  1. Jan 18, 2015 #1
    Hi guys, complete beginner here. I've been reading about electricity the past couple of days and I am confused about some of the most basic concepts. I would greatly appreciate it if you guys could answer a few questions in a dumbed-down manner. I've been learning with kid-friendly analogies, so I'm modeling my questions in regard to them.

    Why do we need to create a circuit from a battery’s positive to negative terminal for current to move? If electricity just wants to flow from a higher voltage to a lower voltage, why can’t we connect the positive terminal to a different source of lower voltage, like a metal chair? In other words, why does the lower voltage source need to be the negative terminal of a battery?

    When I connect a wire to just the positive terminal of a battery, will that wire immediately "fill" with electrons, even though I am not completing any circuit? Based upon the higher voltage => lower voltage principle, I assume that some of the electrons from the battery will immediately move into the "empty space" of the wire right when I connect it to the positive terminal.

    Why are batteries described only in voltage? Doesn't the amperage matter too? I assume that electronics are very picky about the amount of amperage received. It's confusing to me how voltage for some electronics seem to be precisely defined, but there is no mention of amperage. For example, I always read about making sure you have 1.5V, 9V, etc batteries, but no mention about amperage.

    Thanks!
     
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  3. Jan 18, 2015 #2

    Averagesupernova

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    Electrons have a negative charge so the current actually flows negative to positive. So, to tweak your question a bit about connecting a wire to the NEGATIVE terminal of a battery will not do anything as far as electrons filling in empty space. A copper wire has a fixed number of electrons and that is it. You cannot shove any more in or out. Only through.
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    As to the question about voltage and current on batteries. Yes, current is important. However, if you study up on ohms law, you will find that amperes are drawn only by the amount the circuit connected dictates. So amperes is important in that the battery is capable of supplying the demand. But just because a battery can supply many amps does not mean that is going to.
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    There are many sites on the web that cover the basics. Do a search.
     
  4. Jan 18, 2015 #3
    Thanks. Your comment regarding the copper wire actually helps a ton.

    Gotcha. Thanks for explaining that.

    I've been reading many articles explaining these basic ideas, but I've been failing to fully understand the concepts, no matter how many times my eyes look at the words/visualizations. I looked around for a science forum welcoming the most basic of questions, but couldn't find an active one. But I guess your point is that this forum is reserved for more advanced discussion. Point taken!
     
  5. Jan 18, 2015 #4

    Averagesupernova

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    You asked some fairly specific questions and that is ok. But it seems we often get very general questions that indicate to person wants to understand something without going to the trouble of learning it. That is not looked upon very highly here. I think if you are pointed in the right direction you will do ok. Take a look at ohms law. It will answer many questions for you as well as prompt some new ones. But it's ok, stick around. :)
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    Oh, and welcome aboard. There is a wavy welcome flag thing that some members post but I have never stopped to figure out how to use it. So just imagine it's flying.
     
  6. Jan 19, 2015 #5
    Thanks for the welcome! I'm going back and rereading about ohm's law. I personally feel like basic physics is deceptively hard to learn. So much knowledge and meaning can be packed into a tiny equation! Anyways, I'll put in my dues and be more patient when struggling to learn basic concepts, rather than giving up early and posting for help. Though admittedly I'm a slow learner so that may take days/weeks!
     
  7. Jan 19, 2015 #6

    Averagesupernova

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    There have been other beginners questions about ohms law here on PF. If you do a search for those you may find that some of your same questions were addressed. Chances are that others share some of your confusion.
     
  8. Jan 19, 2015 #7
    You ask good questions.
    In electrical circuits, voltages are almost always discussed in relative terms. So, the +ve terminal of a 12V battery is +12V with respect to the negative terminal of the battery. Reciprocally, the -ve terminal is -12V with respect to the positive terminal. The metal chair is nothing voltage with respect to the positive terminal. But, as soon as you connect a wire from the chair to the +ve terminal, the chair will have the same voltage as the +ve terminal of the battery. For current to flow, there must be some voltage ACCROSS a load, which only happens if you place something between the +ve and -ve terminals.

    For this reason, there won't be any current flow if you place a resistor between +ve terminal of one battery and -ve terminal of some other battery. Because the +ve terminal as some +ve voltage only with respect to its own -ve terminal.

    (This is a bit dummed down explanation and there are a lot of caveats)

    Actually, there are two Ameperage to be considered:
    1. The current that will flow out of the battery:
    This will depend upon what load (resistance) connected. Unlike the voltage which is fixed (like say 12V battery), the current that will flow out of the battery is given by I = V/R (ohms law). Remember a battery is a voltage source. So, there is no point (and no way) to specify this ampere in the battery since it depends upon the load. But, the electronics could, and I would prefer that they do, specify the ampere it will draw. But since most electronics draw variable ampere depending upon use (like the alarm clock drawing higher power when rining), maybe they don't bother.

    2. The maximum current the battery can allow to flow out without getting damaged (exploding)
    Although, the current depends upon the load as I = V/R, put if you use too small a resistance, then the high current will damage the battery. This maximum safe current might, although useful, isn't always specified, because most of the practical loads designed to be run on battery never excede the limit.

    There is also another related thing, called the Energy Capacity or Ampere-hour capacity. It means for how long (hours?) can the battery sustain a given current. This is specified in the battery but isn't of concern for the electronics.
    So, a 1800mAh AAA battery can sustain 1.8A for one hour*.
     
  9. Jan 19, 2015 #8

    sophiecentaur

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    There is an 'order' to put things in, if you want to get on with Electricity.
    1 A battery supplies a certain Voltage (say 9V)
    2. When 9V connected to a certain Resistance, say 10Ω, it will pass 9/10 A = 0.9 A (I = V/R)
    3. If the battery is designed right (physically big enough, mainly) it will supply that current happily and maintain its 9V output. A tiny calculator battery will not be able to supply 0.9A and its output volts may drop to less than 1V and you will knacker the battery very soon. But that is not the 'ideal' case.

    A battery can supply a limited charge (Current times running time). LED bulb for 20 hours but powerful motor for 30 minutes.

    Small, domestic batteries are rated at a certain current output and a certain capacity but it's not usually printed on the side. This is because manufacturers would rather sell their batteries with adverts of happy bunnies running longer than other toys than actually commit themselves to the actual charge in them (ampere hours). Big batteries (for cars etc) are always rated also in terms of their Ampere Hours capacity (it's written on the side) and so are most rechargeables (cordless drills etc).
     
  10. Jan 20, 2015 #9
    There are MANY topics / sub-topics in electricity, so be careful about getting off track. Perhaps start a notebook, so when you have an off topic question you can write it down for exploration later.
    For example - looking at Ohm's Law.... you can use it as is, accept it and learn about circuits, design and build them, and not need to know "why" is Ohm's law the way it is.

    When studying electricity - and learning I believe it is critical to think about how you learn. You can ask questions with words, ok for concepts sometimes but not so effective , you can study the mathematics, better, you can study and draw schematics, better still, you can build the circuits physically, etc.... to become reasonably proficient - you should be able to describe circuits many different ways.

    Have you tried a circuit simulator.... I just found this one (DC/AC Virtual Lab), seems to work pretty well to build basic circuits.
     
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