Negative Voltage!

Discussion in 'General Electronics Chat' started by nagle_doff, Sep 25, 2012.

  1. nagle_doff

    Thread Starter New Member

    Sep 25, 2012
    5
    0
    Hello,

    I'm a newbie in EE and just learning things. Now I'm reading about voltages and potential differences...but there's something confusing about it.

    Now a voltage source has two poles, + and -. Does this mean that each pole has a signed voltage? Does negative voltage exist? What does it mean?

    Second, in circuits, the negative voltage pole is earth-grounded, but how does this make it zero voltage? So the negative pole of the battery is not anymore negative? Am I misunderstanding something here?

    Thanks a lot!
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Voltage is a measure of potential difference.

    Voltage measured is always the difference between two points.

    When a voltage source such as a battery or DC power supply has markings + and -, it is only to show which is positive with respect to the other terminal.

    If you have a 9V battery, the terminals are marked + and -.

    Earth is considered so large that for all intents and purposes we use this as 0V reference. Anything connected to earth ground is considered 0V.

    Hence if you connect the -ve terminal of the 9V battery to earth ground, the +ve terminal will be at +9V.

    If you connect the +ve terminal to earth ground, the -ve terminal will be at -9V.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    a LARGE majority of equipment in the telecom world runs at -48VDC..

    The negative voltage is used to provide "cathodic protection" to increase equipment life..
     
  4. nagle_doff

    Thread Starter New Member

    Sep 25, 2012
    5
    0
    So they are just used as convention to determine the current/electrons flow?

    But is not the voltage measured between the two poles +ve and -ve is already 9 Voltas? How does connecting one pole to the ground make any difference? Does it zero the pole so that it becomes neutral?

    So how can I get a negative voltage out of a set of batteries, and how is it different from positive voltage?

    Sorry for my questions but I'm confused and cannot really find a clear answer in references.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    The only difference is which terminal you call the ground, i.e, to which one you measure the voltage difference.

    Note we use the term voltage difference, not just voltage. Just Voltage is used when the ground point is know or assumed. For example with batteries we usually call the negative the ground, even though you could call the positive terminal ground and then you would have all voltages in the battery circuit with negative numbers.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    take a battery... make the plus side your "ground point" and you are now working with negative voltages.. make the minus side your "ground point" and you are now working with positive voltages..

    Its still a 9V potential but the polarity is "with respect to ground"
     
  7. nagle_doff

    Thread Starter New Member

    Sep 25, 2012
    5
    0
    I see then it's merely a convention that determines the direction of current flow throughout an element. So when getting a negative voltage in analyzing a circuit we usually correct the voltage by reversing the assumed polarity across an element...like in the case of current, there's no negative current, both results + or - are current direction, and current is just a flow of electrons (negative charges) whether its result in the equations is + or -.
    Am I correct here?
     
  8. nagle_doff

    Thread Starter New Member

    Sep 25, 2012
    5
    0
    Another question :)

    I've never tried this myself but I would like to know if I connect a bulb's one end to a battery + pole and the other terminal to the ground, would there be any current flowing through the bulb? Since there's a potential difference, my guess current flows...or am I missing something? What about connecting to negative pole and ground? Same effect?

    Sorry after reading this:

    http://www.physicsclassroom.com/class/circuits/u9l2b.cfm

    It does not light! Because there's no closed circuit. But the potential difference between +ve and the ground (0) exists? Then a multimeter should read it, which in turns imply a current flow??

    Thanks!
     
    Last edited: Sep 25, 2012
  9. sbixby

    Active Member

    May 8, 2010
    57
    10
    What do you mean by ground (0) - are you speaking of a physical ground, like a copper rod driven into earth? Or are you speaking of the negative terminal of the battery?

    The voltage potential of the battery is between the battery's + & - terminals, not between the + terminal and earth/ground. If in turn you connect a wire between the battery's negative terminal and earth/ground, then measure between the battery's positive terminal and the earth ground, the meter will detect the battery's voltage because there is a common connection point between the meter's black probe and the battery's negative terminal - that wire from the terminal to earth/ground.
     
  10. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    In Electronics,we very loosely refer to "ground"or "earth",meaning a connection which may be to a common point,such as on a Printed Circuit Board,or the chassis/cabinet of a device,
    OR
    It may be to the actual "dirt" itself.

    If you were to take,say,a 9V battery,suspend it in the air,you may possibly see some voltage between either of the terminals & the "dirt"( let's assume you drove a huge "Earth" stake into the soil & bedrock,etc).

    This would have no relationship to the 9V across the battery terminals,& in fact,this miniscule voltage would probably be the same from each terminal of the battery.

    The reason there is 9V across the terminals of the battery,is because the chemical reaction inside it has caused there to be an excess of electrons at one terminal,& a deficit at the other.

    The voltage between those terminals is relative to those terminals--there is no absolute positive or negative voltage w.r.t the rest of the world.

    It is often convenient to connect one terminal of a battery,power supply,or other circuit to a common point.
    This saves the cost of extra wiring in many cases.

    If I went out & had a look at the battery on my car,I would find that the
    negative terminal was connected to the car body,& nothing else.

    If I now placed the DMM red probe on the positive battery terminal & the meter's common (black) lead on some bare metal part of the body or engine,I would read +12V,just as I would directly across the battery terminals.

    If I also had a 1950s British car,on that car,I would probably find that the positive terminal was connected to the body,
    Using the DMM black lead on the body or engine & the red lead on the negative battery terminal I would now read -12V.

    Note that in both cases,the car body is insulated from the the "Earth" that we walk around on by the rubber tyres.
    Nevertheless,If I was speaking to someone about the two cars,I would probably describe the modern car as negative earth,& the old Brit as positive earth.

    The actual Earth was used extensively as a return circuit for mains power systems & telegraph networks due to the savings in expensive cabling.
    Although the Earth's surface is a fairly poor conductor,this is made up for by the fact that it is huge.
    Any return current from say,a normally operating power line is spread over such an enormous area,that you can't get electrocuted by the current flowing in it.
    Some fault conditions can cause dangerous voltage differences between adjacent parts of the ground,as can lightning strikes,but these are rare occurrences.
     
  11. nagle_doff

    Thread Starter New Member

    Sep 25, 2012
    5
    0
    Thanks vk6zgo, very informative.
     
Loading...