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Quick Quick electrical current concept question

  1. May 29, 2012 #1
    Hello all

    I have a quick concept question concerning electrical current. For real life components, would they only draw the current needed......In other words, if the current of a voltage source of 12V happened to be 500A going through a resistor of 2 ohms, would the resistor pull only 6A or would it pull the 500A from the power supply?

    Thanks.
    Stephen
     
  2. jcsd
  3. May 29, 2012 #2
    I = V/R. With 12 volts and 2 ohms, the current is not 500 amps.
     
  4. May 29, 2012 #3

    gneill

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    Staff: Mentor

    Two essential types of sources you'll run into are voltage sources and current sources. In its ideal form a voltage source will produce a fixed voltage no matter what, and will generate ANY amount of current required to maintain that fixed voltage no matter what the source is connected to. An ideal current source, on the other hand, maintains a fixed current flow no matter what, and will generate ANY amount of voltage required to maintain that fixed current flow no matter what the source is connected to.

    A typical real-life voltage supply will be rated for a given fixed voltage which it can maintain up to some maximum current that it is capable of producing. It sounds like the source you described is of this variety (12V at a maximum of 500A). Use Ohm's law to determine the actual current it must produce for that fixed voltage when it's connected to a given resistor load.
     
  5. Apr 2, 2013 #4
    The current depends only on the resistance and voltage in your case it is 6A ,the current source can supply maximum current of 500A but the circuit draws only what it really want
     
  6. Apr 2, 2013 #5

    CWatters

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    Science Advisor
    Homework Helper

    It is better to write that as...

    "If a voltage source of 12V (capable of delivering 500A) is connected to a 2 Ohm resistor, would the resistor draw 6A or 500A?"

    The answer is 6A.

    Voltage sources deliver constant voltage. The current will depend on the load but there will normally be a maximium current limit. For example if you short circuit a voltage source the current is theoretically infinite which is impossible. In the real world the current will be limited by something. Lab bench supplies usually have a current limiter built in. They behave like voltage sources until the current gets too high then the voltage starts to fall to protect itself.

    Current sources deliver a constant current. The voltage will normally depend on the load but there will be a maximium voltage limit. For example if you open circuit a current source it's output voltage will soar as it tries to push the same current through the now open circuit. In the real world the maximium voltage is limited. An example would be a constant current charger for NiCad Batteries. These should be capable of charging any number of cells in series but there will be a cell limit dictated by maximium voltage the charger can deliver.
     
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