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Positive and negative electrical terminals

  1. Feb 26, 2010 #1
    I am slightly confused as to why some circuits are drawn with a positive input terminal and a negative input terminal, whilst others are just drawn with a positive terminal and a ground (OV) terminal.

    What is the difference between ground and negative? Does negative indicate an excess of electrons (negative charge) and ground indicate a balance of positive and negative charge?

    Thanks

    Paul
     
  2. jcsd
  3. Feb 26, 2010 #2

    rcgldr

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    Ground implies a non-charged stage. Negative implies a negatively charged state. In the case of common circuitry, the ground reference is usually non-charged. I've seen some older circuitry that used +5 and -2 volts for the high and low states of the internal logic (HP 2100 series computers from the 1970's).

    Power supplies for most devices use the ground reference from a 3 prong AC plug. Batteries are self-relative. You could connnect either terminal of a battery to a ground reference without significant effect on the battery.
     
  4. Feb 26, 2010 #3
    Thanks for the reply.

    Does this mean that the negative / positive terminals of the battery are barely higher/lower in potential than ground. Otherwise surely just connecting the negative end of the battery to ground would cause current to flow?

    Also, why does connecting the positive terminal of one battery to the negative terminal of another identical battery not cause current to flow between the two?
     
  5. Feb 26, 2010 #4
    First and foremost it is good to fully understand terms you introduce.

    A circuit is called a circuit, not a wiring diagram or a configuration diagram etc, because it is just that.

    A loop or circuit can be traced, passing from and through one circuit element to the next until you return to your start point.

    The characteristic is an essential part of useful electrical/electronic theory. This is because without a complete circuit no current can flow and nothing useful occurs.

    So to answer the question in your second post, note that connecting one terminal of a battery to something does not make or complete a circuit. So no current flows. Current only flows when you connect the second terminal as well.
     
  6. Feb 26, 2010 #5

    sophiecentaur

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    paul_harris77
    Batteries have a Potential Difference between their terminals. That is, relative to each other. You can choose to call one terminal 'ground' and you may connect it to Earth or the case of a device or the chassis of a vehicle. That's an arbitrary choice - usually based on the biggest 'local' conducting object. If an aeroplane is flying through the air it may acquire a charge and have a potential that is above (or below) Earth but we would still call it ground as far as the on board equipment is concerned.

    With two, 12V batteries, you can get +12V and -12V supplies by connecting the negative terminal of the first battery and the positive terminal of the second to 'Ground'.

    These two batteries are connected as you say but how can current flow if none can flow out of the +12V terminal and into the -12V terminal? You need a complete circuit for that to happen. A small surplus of electrons will flow to the -12V terminal but only just enough to charge the minuscule capacity between the two terminals (microCoulombs) AND STOP.
    Connect a 24V bulb between the two 'extreme' terminals and current can then flow.
     
  7. Feb 26, 2010 #6

    Dale

    Staff: Mentor

  8. Mar 21, 2012 #7
    I have a question,
    When a car battery is dead, you jumpstart it by connecting the positive terminals together and the negative terminals together. Why do you connect the positive terminals to each other instead of connecting a positive terminal to a negative terminal
     
  9. Mar 21, 2012 #8

    sophiecentaur

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    Hi
    You want current to flow INTO the battery so that it will become charged. You also want current to flow the SAME WAY through the starter motor as it would from your battery (if it were working). SO you connect + to + and - to -. Simples
     
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