Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

High voltage but low current how?

  1. Nov 28, 2011 #1
    high voltage but low current...how??

    we know that the potential difference between two points is the driving force for current , so in this sense more the potential difference or voltage between two points more is the current...

    but a transformer produces either very high voltage but low current OR very low voltage but high currents, doesnt this go against the initial theory of how current is produced...?? i.e more the voltage stronger the current.

    the expression P = I x V says that if the power is kept constant I is inversely proportional to V...but this contradicts the ohm's law that V= I x R

    i've never been able to grasp this concept any help is appreciated...thank you.
     
  2. jcsd
  3. Nov 28, 2011 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    Re: high voltage but low current...how??

    Transformers don't PRODUCE anything at all, they just transform electricity from one set of conditions to another, and they have physical limits on how much power they can transfer [that is, any given physical transformer does --- ideal ones can do anything]

    Transformers do NOT necessarily output low voltage / high current or low current / high voltage, but because they are limited in the power they can output, and because of the characteristics of the winding, this is often the case.

    You are confusing POTENTIAL current for a given voltage with actual current produced by a transformer. If a transformer input is 120 volts and for a given load on the transformer, lets say this particular transformer it draws 3 amps and the output is, say 60 volts, then the POSSIBLE current on the output is 6 amps (assuming no loss for the sake of simplicity) Thus power in = 360 watts , power out = 360 watts. Where's the problem?
     
  4. Nov 28, 2011 #3
    Re: high voltage but low current...how??

    stark, think of a cell phone charger. The plug that goes into the wall socket is a transformer. It switches AC to DC power. It does not alter the power. It enables the cellphone for instance to draw a charge at its specified voltage and current.
     
  5. Nov 28, 2011 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: high voltage but low current...how??

    Follow this step by step ---
    All the transformer does is to change the Voltage (up or down, according to the ratio of the numbers of turns in the primary and the secondary). It is the voltage across the secondary (output winding) that is applied to the load. The current that flows int the load depends upon the secondary volts:
    Isecondary = Vsecondary/ Rload

    Now, this current, that flows in the secondary, produces a varying magnetic field in the core, which has the effect of governing the current that flows in the primary (input) winding. The result, not surprisingly, is that the Primary Current times the Primary Voltage equals the Secondary Current times the Secondary Voltage. Anything else would violate the conservation of energy principle - which applies everywhere.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook