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How I,V,R, and P relate and how Transformers Work?

  1. Jul 18, 2012 #1
    My first question is with the relationship between current and voltage. I understand how they are related mathematically but not conceptually. Why would increasing voltage decrease current if the resistance stays the same? Why couldn't the current increase if there is more "push."

    This leads to my next question. Who is the real "hero" for electrical power? Current is the flow of electrons, and voltage is the storage. However, you can generate the same power with different values. Is there a limit to how many amps you can have per volt or vise versa?

    My last question is with step up transformers. How can more voltage be created when the source is permanent? Seems counter intuitive to the conservation of energy... Does the secondary coil act as a capacitor and simple store more energy?

    I have read my text thoroughly and have searched online for these answers and cannot find them. It is not that I have not thought this through.

    Thanks for your time
  2. jcsd
  3. Jul 18, 2012 #2
    Current is directly proportional to voltage. If you double the voltage, the current will double. Power will quadruple so the energy used for a given length of time will quadruple.

    [itex]E=IR[/itex] and [itex]I=\frac{E}{R}[/itex]

    Voltage isn't really the 'storage'. Energy would be a more reasonable quantity for storage. As far as a limit on the amount of current that can be drawn, it depends on the source. A small dry cell will not be able to deliver much in the way of current in contrast to say a car battery which can deliver much more. I don't know off hand how to quantify this, however.

    In an ideal transformer, the power ratio between the primary and secondary coils is 1:1. If the voltage on the secondary is double the primary voltage, the current will be one-half, the power (energy/second) will be the same.

    Last edited: Jul 18, 2012
  4. Jul 18, 2012 #3
    What are you talking about? In what situation would this happen?
    Increasing the voltage across an element of constant resistance will increase the current through the element.
  5. Jul 18, 2012 #4
    Ok thanks this is really helping.

    So according to your derivation of P, I is the defining source of electrical power? I hope so because this makes sense.

    And great there is a trade of with a step up transformer.

    Now I am just curious as to how many amps can be created per volt. It seems evident that although amps are the source of power, it is of larger concern getting the amps to reach a location then having a significant amount of current.

    As a side note, I had a 150A fuse wired in series between my car battery and amplifier. I accidentally touched the positive and negative terminals when wiring. The fuse blew. This leads me to believe that a significant amount of current can be created with relatively low voltage (12V blew a 150A fuse).
  6. Jul 18, 2012 #5
    I was talking about in a transformer. Seeing the additional equations helped me to understand the trade off for a step up.
  7. Jul 18, 2012 #6
    In your original post, I assumed your transformer question was separate. The formula for power I gave is for DC circuits. Power in AC circuits is a bit more complex but the relationships between voltage, current, and power is the same.


    Generally, current- and power- depend on the source voltage and the load resistance.

    A typical car battery can deliver that much current. A quick Google search revealed one able to deliver 200A.

    The amount of current a battery can deliver depends on the internal resistance of the battery and that depends on the type of battery. See this for more info:

    Last edited: Jul 18, 2012
  8. Jul 18, 2012 #7


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    I think you should read this first to get a basic understanding of electricity, voltage, current and electrical energy/power. http://amasci.com/miscon/energ1.html
  9. Jul 18, 2012 #8


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    You can't say I or V is the thing that dictates power.

    In fact you could also say that it's the the load that determines the power. For example the power station does not determine how much power you use. That's down to how many devices you connect. This idea that a load determines the power is handy when it comes to understand some types of motor but that's for another day.

    However in any circuit some elements are constant and some are variable. So consider a 9V battery. The voltage is fixed at 9V and it just sits there doing nothing until you connect a load. So in that circuit it might be appropriate to consider that it's the load that determines the power.

    Consider the output of a HiFi. The load (the loudspeakers) typically have a fixed resistance, lets call it 8 Ohms. In this case it;s the output voltage that varies and controls the power.

    So although some feature of the circuit might suggest one (I,V or R) controls the power it's not really like that.

    You can write the equations for power several ways, for example..


    but since V=IR..


    are equally valid.
  10. Jul 18, 2012 #9
    What does it tell you about the resistance of the circuit you created on accident?
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