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Simple question about current draw

  1. Aug 7, 2011 #1

    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


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    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


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    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


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    you got it
  7. Aug 10, 2011 #6
    Ok, I'm glad you cleared that up, thanks a lot it was really buggin me.
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