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Several questions about DC electricity

  1. Jul 14, 2013 #1

    I have few questions about batteries and AC->DC adapters

    1- in AC-DC converter:
    if I have a device required 12v and 1amp .. will it hurt the circuit if I connect 12v but more amps ? I heard it draw only required currents .. is that true ?

    2- in batteries:
    why when we buy batteries .. some of them last more than other .. some of them make the device runs faster (e.g. razor, rc car ... ) you really feel that it runs faster.
    -what does make devices runs faster/stronger .. battery with high voltage, amp or discharge rate (C)?
    - what makes battery last longer ? higher V or higher Amp ?
    - when battery is being used .. what is decreasing in it .. is it V or A ?

  2. jcsd
  3. Jul 14, 2013 #2
    OK, I'll give it a try....

    1) AC-DC converter: The voltage rating of any power supply says how many volts it supplies, and is often qualified with the maximum amperage that can be drawn while maintaining that voltage. Most "wall-wart" type converters are not voltage regulated, so the voltage will be higher if you are not drawing the max current. But in general a load like a lamp will draw a fixed number of amps based on the voltage applied. This is called Ohms's Law. So it's OK to use a 12v 100A supply to run your tiny flashlight battery.

    2) Batteries: Don't "convert" electricity but rather store it as "charge" usually using a chemical reaction that provides a specific voltage (which can vary somewhat but not so dramatically as the un-regulated wall-wart thing). For the most part more chemical means more charge (different chemistries have different capabilities as well, vis lead-acid, NIMH, LIPo, yadayada). But the manufacturing quality may have the most effect on longevity...here's some interesting graphs: http://www.batteryshowdown.com/results-hi.html
    Why some identical voltage-speced batteries may seem to run your car faster is a mystery to me, however their charge capacity (in Amp-Hours, how many amps you can get at the specd voltage usually spread over 20 hours)
    tells you how long they will last.
  4. Jul 15, 2013 #3
    Hello Schip666!

    Thank you very much for taking time and answering on my questions.
    1- it is very clear now .. this is why when I checked cheap 12v DC converters, most of them were 14+v !!

    2- I am searching more on it .. you helped me

  5. Jul 15, 2013 #4


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    Staff: Mentor

    The chemical activity is decreasing, chemicals are being used up. This reduces the ability of the cell to sustain its voltage while delivering amperes.

    So, yes, voltage is decreasing, and current is decreasing along with it. You know this, because the speed of your model car decreases as the battery gets used. But these are consequences of the slowing chemical reaction inside the cell.

    And welcome to Physics Forums! :smile:
  6. Jul 15, 2013 #5
    In devices that use a lot of power, e.g. rc cars, it can make a big difference what type of battery you use. Cheap zinc carbon batteries can not supply much current. When you use alkaline batteries instead the rc car will run faster because the battery can deliver more current. NiMH gives you even more current. However there is a maximum amount of current that a device will draw (at a given voltage). In a pocket radio for example the only difference you will notice between zinc carbon, alkaline and NiMH is the runtime. Everything else - volume, reception, etc. is the same since the maximum current drawn by the radio is small, so small even a zinc carbon battery can easily supply it.
  7. Jul 15, 2013 #6
    Back to the some batteries run faster question...

    It occurred to me that it might be the difference in voltage between, say, zinc and rechargeable (1.5 -> 1.2v).
    Also as DrZoid points out, the car may be drawing more current than the battery can provide (c.f. internal resistance...) and thus the available voltage is lower.
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