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Negative Voltage

  1. Aug 21, 2009 #1
    I've been getting confusing explanation on what negative voltage represents. In electrolytic cells my teacher explains that negative voltage means that voltage needs to be added for current to flow but in AC circuits I get the feel that negative voltage is applied in opposite direction than positive voltage and in DC circuits I was told that current still flows with a negative voltage (when I use opposite leads on a voltmeter to measure the voltage on a battery). Can anyone explain this or clarify any misconceptions I have?

    Thanks for any help that you can provide
     
  2. jcsd
  3. Aug 21, 2009 #2

    negitron

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    Negative voltage simply means the voltage level is lower than the ground reference. Voltage is always a relative measurement; it must be made with respect to another point. Conventionally, this is either Earth ground or a point designated to represent zero volts--usually the negative terminal of the power supply.
     
  4. Aug 21, 2009 #3
    But then how come, in an electrolytic cell, negative voltage means that voltage must be added for the cell to work
     
  5. Aug 21, 2009 #4

    negitron

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    What do you mean?
     
  6. Aug 21, 2009 #5
    I was taught in school that in an electrolytic cell, the initial voltage is always negative because the reaction is non spontaneous, and that you need to add voltage to make the reaction go; so basically a negative voltage in an electrolytic cell means current doesn't exist which doesn't seem to be the case in an actual electric circuit. Is this true?
     
  7. Aug 21, 2009 #6

    negitron

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    I have never heard of this. Now, it is possible with some rechargeable chemistries to either recharge them improperly or mix charged and discharged cells in a device and push the voltage negative in one or more cells, but this basically kills the cell and renders it unusable.
     
  8. Aug 22, 2009 #7

    Dale

    Staff: Mentor

    This is correct. If a circuit is linear (i.e. resistors, capacitors, inductors, but not diodes or transistors) then a negative voltage will simply mean the same current flow but in the opposite direction.

    The chemical reaction in a battery goes one way (oxidation on one end reduction on the other) so a negative voltage corresponds to switching the anode and the cathode, not to trying to run the reaction in reverse which would require energy input.
     
  9. Aug 22, 2009 #8
    Thanks for the reply

    I have some follow up questions on this topic

    When I switch the leads of the voltmeter to measure voltage on a battery (so the voltage reads -1.5V instead of 1.5), is there still current going through the voltmeter? What is the difference between the above case and putting a battery backwards in, for example, a flashlight (putting the positive end on the negative end of the casing and vice versa) where no current flows? Do both result in negative voltages?

    In an electrolytic cell, I was taught that the magnitude of the negative voltage of the redox reaction is the amount of positive voltage that needs to be added for the reaction to go. I never really understood this because, from a physics perspective, adding the positive voltage which is equal in magnitude to the negative voltage of the reaction results in 0V; how does electrons move if it has 0 potential difference from anode to cathode?
     
  10. Aug 22, 2009 #9
    Current does flow in the flashlight. If you put batteries into a flashlight backwards, it will only fail to work in these cases:

    -- if it's an LED flashlight

    -- if the batteries aren't making contact, because the metal parts inside the flashlight were made in a certain shape to touch the bump that sticks out of the positive side of a battery and the flat surface of the negative side of a battery.
     
  11. Aug 22, 2009 #10

    Dale

    Staff: Mentor

    Yes, but a good voltmeter has a very high impedance so there is very little current.

    I don't know what you are actually describing here. When an oxidation reaction is spontaneous that means that the oxidized state is more stable than the reduced state. That means that in the oxidation reaction work is done on the electrons as they leave. You can make the reaction go in the opposite direction (reduction), but you have to do work on the electrons in order to move them from the more stable to the less stable state.
     
  12. Aug 23, 2009 #11
    " In an electrolytic cell, I was taught that the magnitude of the negative voltage of the redox reaction is the amount of positive voltage that needs to be added..."

    this does not describe a battery.


    Seems like there may be confusion here between electrolytic cell and galvanic (battery)cell.

    a galvanic cell is a unit/component of a battery and produces electricity, like an automobile battery.

    an electrolytic cell CONSUMES electricity and might be used used for plating things, for example, like chrome on bronze.

    Try wikipedia for a bit more of an explanation:
    http://en.wikipedia.org/wiki/Electrolytic_cell#Galvanic_cells_compared_to_electrolytic_cells
     
  13. Aug 23, 2009 #12
    Thanks for the replies

    My confusion is basically how the negative voltage of an electrolytic cell is determined, and why adding enough voltage to increase the negative voltage of the electrolytic cell to 0 would move the electrons.
     
  14. Aug 23, 2009 #13
    So switching the leads on a voltmeter (which results in a negative voltage) is not the same as putting a battery the opposite way on a battery casing inside of a flashlight because in one current flows and in the other current does not?
     
  15. Aug 23, 2009 #14

    Redbelly98

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    Current would flow even if the battery is switched the opposite way, as long as:

    1. the battery terminals still make contact with the flashlight wiring

    and

    2. it is a standard bulb (i.e not an LED).

    That was mikelepore's point earlier.
     
  16. Aug 24, 2009 #15

    Dale

    Staff: Mentor

    Remember what voltage is, it is an amount of energy or work per unit charge. One Volt is one Joule/Coulomb, or equivalently one ElectronVolt (a unit of energy) per ElementaryCharge (the amount of charge on a proton or electron). So, if you have a 1.5 V battery that means that on a molecular level the redox reaction does 1.5 ElectronVolts of work on each electron that flows. If the reaction is reversible then you can do 1.5 ElectronVolts of external work on each electron and run the reaction in reverse.
     
  17. Aug 24, 2009 #16
    So the casing on a flashlight is designed so that there is only one way to positive the batteries so that the terminals make contact with the wiring. But if, say for a casing (assuming the terminals make contact with the wiring) or for a voltmeter, we switch the terminals to the opposite way, does the direction of the current change as you implied with your second statement? How does that work because I thought that current MUST flow from positive to negative or did I misunderstand your statement. Thanks
     
  18. Aug 24, 2009 #17
    Oh so it's just applying work against the potential of the electrolytic cell.

    Thanks for answering my question!
     
  19. Aug 25, 2009 #18
    Yes, if you reverse the batteries in a flashlight, you will reverse the direction of the current through the bulb. If you reverse the wires to any meter, you will reverse the direction of the current through the meter.

    People usually visualize voltage as as having the same role that the sense of up and down has for gravity problems. Imagine if you were to flip a mountain upside down so that it's peak is down and its base is up, then you could drop a ball on it and it would begin to move toward the peak instead of toward the base. There is still a spontanoeus direction for motion even though you have reversed the path. A battery is a device that puts charge in a location where it will spontaneously move to another location.
     
  20. Aug 25, 2009 #19
    I have some trouble adjusting to this concept. Because if we're using a voltmeter, and we switch the leads, the electrons will go from positive terminal to the negative terminal and not the other way around?
     
    Last edited: Aug 25, 2009
  21. Aug 25, 2009 #20

    Dale

    Staff: Mentor

    Yes, but it doesn't matter which direction the current goes through a lightbulb.
     
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