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Stupid question about voltage

  1. Apr 20, 2008 #1
    I think I am a bit confused here.
    How does higher current output power supply differ from high voltage supply?
    Is it just the internal resistances of the supply?
    For example a Lithium Ion battery from an Alkaline battery.

    I am asking coz, I was reading about static. Your body can charge upto 15000V which results in a spark when you get near a grounded conductor.
    so what makes high voltage high, is the number of charges with respect to ground?
  2. jcsd
  3. Apr 20, 2008 #2
    The voltage in a battery is a function of the chemistry within it. In a wet cell, for instance, the electrolytes used (say, lead and sulfuric acid) determine the voltage produced (as characterized by the "standard electrode potential"). The output current is a function of the internal resistance and the voltage, as set by the chemistry. Take a look at Wikipedia's electrochemical cell and electrode potential pages if you still have questions.

    Hope that helps,
  4. Apr 20, 2008 #3


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    Staff: Mentor

    Batteries use a chemical reaction to establish a potential which could be 3, 6, 9, 12 V. Batteries are generally low voltage, and small batteries generally put out a low current. Batteries for automobilies can put out a relatively high current at low volage, but for a short period of time, otherwise the battery would overheat.

    AC current in the house is ~110/220V, which is stepped down from 4 kV local distribution voltage. Depending on the application, the low voltage out of a distribution substation could be 4kV, 12.5kV, 13.2kV, 13.8kV, or 14.4kV (distribution voltage), and the high voltage into the substation could be 230kV, 115kV, or 69kV (transmission voltage).


    The AC power is provided from a synchronous generator at a power (generation) station. The voltage and current are transformed by transformers with a given power rating. If the voltage is adjusted up, the current decreases, and vice versa. Voltage in a generator is established by a time varying magnetic field by a process called induction.

    High voltages can be developed by special electrostatic generators such as a Van der Graff geneator - http://www.wright.edu/academics/physics/reu/Little.pdf - or Cockroft-Walton accelerator - http://www.lbl.gov/abc/wallchart/chapters/11/1.html

    See also -


    The basic concept for electrostatic high voltage source is to maintain an accumulation of charge, or really a separation of charge, because on one electrode there is a collection of electrons, while on another electrode, there is a lack of electrons or surplus of positive charge.

    As for high current - automobile batteries can supply high current at low voltage, which has to do with the size and number of plates (electrodes) in the battery.


    Homopolar generators however can supply very large currents (kA) at low voltage (<100 V). Basically, the HPG consists of a rotating conducting disc in a static magnetic field.
  5. Apr 21, 2008 #4
    a high voltage supply is when you have a charge potential between two points because of the accumulation of lots of charge. this potential is a potential to do work and the higher the potential, the greater the work you can accomplish.
    a high current supply is one where you have lots of charge stored that can flow easily. the flow of the current is actually the work being done as a result of the potential.

    Now ideally, your HV supply should have a very high resistance to prevent the flow of charge in order to maintain its potential and you high current supply should have a low resistance to allow the charge to flow freely.
    However, in real life it doesnt work that way as the loops have to be connected in order to do the work.
    Therefore you need a source of charge stored/generated for both cases so that your system will work at the level that you require.

    Only noted difference between these types is that the Lithium types hold and release more charge over a shorter period and so are good for high current devices like laptops and so on. Alkalines will deliver a lower current but over a longer time.

    Building up static from walking on a carpet is common....very easy to pick up lots of charge and you are generally isolated by (high resistance) air so charge potential remains until you get near a grounded (low resistance) point that will drain the charge off your body.
    As you only have the charge that you have stored on your clothes and hair etc, you couldnt be regarded as a high current source, just a HV one or a high current source that lasts for an extremely short period of time.
  6. Apr 23, 2008 #5
    Thanks for the reply all.
    Deakie, the part that confused me was both high voltage and high current source need lots of electrons.
    Any analogy ?
  7. Apr 25, 2008 #6

    Think of it this way....

    The high voltage bit...
    You have two cylinders. one is short and one tall. Both have the same diameter.
    you fill both with water and there is a small hole at the bottom of each cylinder.
    Now you know that the tall cylinder will spew the water further out than the short one yes?...why? The answer is the analogy of a high voltage system to that of a low voltage one. Its a matter of 'potential to do work'. We could neglect the effects of the additional water and go straight for the effects caused by gravity as the height of the water in the tall cyclinder offers a longer drop = more force.
    So if we want to move our water over a further distance, we put it higher....

    now for the High Current bit...
    we need to shift a lot of water at the same time due to how we use it....so how do we do this?
    we make the cylinders wider.....so that for any given height, we have more water and so our flow will last longer.
    The difference here is not how fast it flows as above but how much longer it will flow....

    Was that good enough?:smile:
  8. Apr 25, 2008 #7


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    Staff: Mentor

    As a matter of fact, many of the common ones are just plain 1.5 (or something rather close). If you crack open a 9v battery, for example, you find 6 1.5V cells (AAA perhaps) in series with each other. A 6V lantern battery is just a bunch of 1.5V AA's in series and parallel.
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