Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple question about current draw

  1. Aug 7, 2011 #1
    Hello,

    I've been working with some electrical stuff at my internship lately, but I don't have any real background in it, so I've been trying to learn what I can from the internet. One thing that I don't get involves Ohms law:

    If I have a circuit powered by 10V with just a 1 Ohm resistor, the current flow will be 10A (I think). If that resistor is replaced by a motor, which would have a much larger resistance, the current flow will drop according to I=V/R. But common sense tells me that a larger motor would need more current for it to run. I'm sure I'm confusing the concepts. Basically, I can't mesh the idea that current draw will be higher to power larger motors and the idea that I=V/R.

    I hope that makes sense, any response would really be appreciated, thanks!
     
  2. jcsd
  3. Aug 7, 2011 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    I guess I'm not really clear what your issue is. Ohms law is it.

    If you maintain a 10v supply (capable of supplying whatever current is needed) and you have a 1 ohm resister load then you get 1 amp of current and if you increase the resistance then you get less current and if you decrease the resistance you get more current.

    A 10V DC motor generating 100Watts would need 10 amps (and so by implication would have a 1/10 ohm resistance.

    It is possible to have a DC power supply that generates 1am with no problem but just can't hack 10amps, so with a given input voltage, stronger motors require a power supply that can drive more load.
     
  4. Aug 7, 2011 #3

    Zryn

    User Avatar
    Gold Member

    In a theoretical diagram this is the case.

    In a real world application a voltage source will have a current supply limit. If you find a lab power supply, most of them have the ability to be current limited, so if you set the limit to 1A and then tried to put 10V across a 1R resister, the supply would hit 1A and the voltage will drop back according to Ohms Law to 1V.
     
  5. Aug 10, 2011 #4
    Thanks a lot, that actually did answer my poorly phrased question. I had it in my head that a larger motor that had a higher rated power must have a higher resistance than a lower power motor, which was why I was confused because that wouldn't make sense with Ohm's Law. I didn't realize that a lower power motor would have a higher resistance. So a:

    10 V, 100 W motor needs 10A and has 1 Ohm of resistance

    while at 10 V, a 10 W motor needs 1 A and is 10 Ohms?
     
  6. Aug 10, 2011 #5

    phinds

    User Avatar
    Gold Member
    2016 Award

    you got it
     
  7. Aug 10, 2011 #6
    Ok, I'm glad you cleared that up, thanks a lot it was really buggin me.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Simple question about current draw
Loading...