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No current?

  1. Apr 10, 2005 #1
    Hi! I connect 9 volts to a 1 ohm resistor and thought I would get I = U/R = 9 A. But when I connect the ampmeter, I cannot read anything! Where is the problem?
     
  2. jcsd
  3. Apr 10, 2005 #2
    What 9 V voltage source are you using? Is it a dry cell or a lab power source?
    How do you connect the ammeter to test the current?
     
  4. Apr 10, 2005 #3

    chroot

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    You're probably just connecting the ammeter incorrectly. Besides, a 9-volt dry cell will not be able to produce 9 amperes of current.

    - Warren
     
  5. Apr 10, 2005 #4

    Gokul43201

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    A standard resistor is designed to handle anywhere from 0.25W to about 1 watt. Unless you used a power resistor, you've toasted your resistor (at full current, you're trying to make 81 watts !!). You can check if the resistor is still any good by using your multimeter in the resistance mode. This would be the mode of failure if you used a DC power supply. If you used a dry cell (9V battery), I'm not sure which you would have destroyed first, the battery or the resistor. Check the open circuit voltage of the battery to see if it's still any good.

    Lessons you have learnt :

    1. Batteries can drive only so much current. You can kill a bettery by shorting it with a low resistance. Look up the specs for the battery to see what it's rated to deliver, before using it under doubtful conditions.

    2. Resistors can handle only so much power. Again, make sure you are not going to exceed design constraints before hooking up a circuit.
     
  6. Apr 13, 2005 #5
    But how can I determ from the battery how many amps it will deliver?
     
  7. Apr 13, 2005 #6
    BTW, I connect like this. Battery plus -> Resistor -> Ampmeter plus -> Ampmeter minus -> Battery minus
     
  8. Apr 13, 2005 #7

    chroot

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    You can look up the Equivalent Series Resistance of the battery. A battery can be considered an ideal voltage source (which could provide an infinite amount of current) in series with a resistor. This series resistor limits the total current the battery can deliver. If you look up the ESR, you can calculate the maximum possible current.

    - Warren
     
  9. Apr 13, 2005 #8
    I also assume you are using a small rectangular 9 volt battery but cannot be sure since you have not said. So what's the story? Is it or isn't it? If not, what is it?
     
  10. Apr 14, 2005 #9

    Ouabache

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    Since you did not specify what type of voltage source you are using, we are only guessing that it is Vdc. (It could be a Vac source, where you may also be learning about RMS, peak and peak-to-peak voltages).

    Once you have confirmed that your voltage source is still working, (by testing it with your voltmeter and making sure you also select the appropriate Vdc or Vac range depending on your source, here is a new experiment to try;

    using the same voltage source, replace you [tex] 1\Omega [/tex] resistor with a [tex] 500\Omega [/tex] or [tex] 100\Omega [/tex] resistor and measure current.

    (hint: you might calculate ahead of time what current you expect, so you will know what range to set your ammeter to. If you are still uncertain and you want to protect your ammeter from unexpected currents, start with the largest range and work your way down to smaller ranges. Open your circuit between switching to a new range)
     
  11. Apr 16, 2005 #10
    Now it seems to work. Thanks
     
  12. Apr 18, 2005 #11

    Ouabache

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    Glad we could steer you in the right direction. Though I don't quite understand what you mean by "seems to work" (My circuits either work or don't work). Do you mean you are fuzzy about how your circuit presently works? :confused:
     
  13. Apr 19, 2005 #12
    It's working perfectly. I just expressed myself a little sloppy.
     
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