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B Proper usage of the ammeter

  1. Apr 3, 2016 #1
    How come when an ammeter is placed in parallel with a resistor that is connected to a battery, the circuit is considered incorrect + "safe," whereas an ammeter connected in parallel with the whole battery is considered incorrect + dangerous?

    My guess is that it has something to do with the ammeter having ideally no resistance, and will get damaged by all the charge flowing through?
     
  2. jcsd
  3. Apr 3, 2016 #2
    An ammeter should never be placed in parallel. It's not that one way is dangerous and the other is safe. It's just that both ways would lead to a blown fuse.
     
  4. Apr 4, 2016 #3
    How big was the battery, surprised the ammeter didn't at least blow a fuse.
     
  5. Apr 4, 2016 #4

    andrewkirk

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    Where did you read that the first case is safe? I would have thought it made a short circuit. Some real-world ammeters may perhaps have circuit breakers to prevent that, but an 'ideal' one would not.
     
  6. Apr 4, 2016 #5

    vanhees71

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    Well, it's "dangerous" for the ammeter if it doesn't have a fuse ;-).
     
  7. Apr 4, 2016 #6
    6v
     
  8. Apr 4, 2016 #7

    davenn

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    agreed
    or cook the meter movement in the case on no fuse ... as many plain ammeters (panel meters) don't have

    The pretty much only time a resistor is placed in parallel to the ammeter is when it is a shunt resistor and it will have a very small resistance value
    compared to that of the meter movement winding

    ammeter-and-shunt-img_23361.jpg

    Dave
     
  9. Apr 4, 2016 #8
    Ok, so it should never be placed in parallel. What happens actually/physically that makes it so dangerous?
     
  10. Apr 4, 2016 #9
    Because the low resistance of the ammeter drops no voltage ideally. Hence you can create a short circuit thru yr ammeter which can't happen if it is in series with an existing load. Compare a voltmeter which ideally has an infinite resistance and draws no current.....ideally.

    Think about what happens when you have a simple bulb in a circuit, connecting the ammeter in series is like doing nothing to the circuit. Now connect a wire in parallel with the bulb, no current flows thru the bulb it instead goes thru the wire and draws a huge current limited only by the internal resistance of the battery. That wire is like the ammeter in parrallel.
     
  11. Apr 4, 2016 #10
    Yea, that seems to be the case. How are short circuits bad? Say you connect the two ends of a battery together, what happens physically? They say it shortens the life of the battery dramatically, but I can visualize the charge going from one end and going back into the battery...
     
  12. Apr 4, 2016 #11
    Its bad because you use an ammeter to measure the current in a circuit, if you short out a component then you are not measuring the circuit, you are measuring g a different circuit. That's important if you want to know if a circuit is actually working.

    Short circuits are bad because the current is not going where it is supposed to to do useful work, like instead of going g into the engine of the washing machine it is going thru the user and killing them. Death is generally considered bad.
     
  13. Apr 4, 2016 #12

    ZapperZ

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    Since the topic of this thread that you had created is on the "proper" use of an ammeter, I am curious as to why you have the ammeter in parallel in the first place? Is this a question simply to know the consequences of what might happen if you make a mistake and end up doing this? Or is this done on purpose? If it is the latter, for what reason? Just for fun? Because the ammeter is useless when in such a configuration. You might as well put a conductor in parallel to whatever you had.

    So please, what is the rationale in all of this?

    Zz.
     
  14. Apr 4, 2016 #13
    Oh no I wouldn't dream of deliberately putting an ammeter in parallel. This question merely came from my professor's midterm.
     
  15. Apr 5, 2016 #14

    Drakkith

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    Consider a typical 1.5 volt AA battery with around 0.15 ohms of resistance at room temperature. Total resistance of 4 of these batteries in series (to give a voltage of 6 volts) is 0.6 ohms. Shorting the ends of the circuit together gives 6 volts across a resistance of 0.6 ohms. By ohms law, I=V/R, current through this circuit will be 10 amps. Power dissipated by this circuit is given by the equation, P=IV, or 60 watts. Since most of the resistance is inside the batteries, most of the power is dissipated there.

    Congratulations. Your batteries are now dissipating the power of a 60-watt incandescent light-bulb.

    But that's not all. The internal resistance of the battery falls as the temperature increases. At 40 degrees Celsius, your batteries now have an internal resistance of 0.1 ohms, current has increased to 12 amps, and power dissipated is now 72 watts. And this is only the beginning.

    Quite soon your batteries will reach such a high temperature that pressure builds up and the batteries will fail, possibly in an explosion.
     
  16. Apr 5, 2016 #15
    I get it, but I thought resistance increases as temperature increases?
     
  17. Apr 5, 2016 #16
    Depends o the material.
     
  18. Apr 5, 2016 #17
    So for some materials, resistance decreases as temp increases?
     
  19. Apr 5, 2016 #18

    Drakkith

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    Not for a battery. From this PDF by energizer: http://data.energizer.com/PDFs/BatteryIR.pdf

    Cold temperatures cause the electrochemical reactions that take place within the battery to slow down and will reduce the ion mobility in the electrolyte. Subsequently, internal resistance will rise as ambient temperatures drops.

    You can see a graph of internal resistance vs temperature in the PDF.
     
  20. Apr 5, 2016 #19

    Drakkith

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    Indeed. Electrochemical cells (batteries) and semiconductors are two examples of materials which have a decrease in resistance as temperature rises.
     
  21. Apr 5, 2016 #20
    Thanks Drakkith, the pdf and explanations were very helpful
     
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