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

Homework Help: How much current does the motor draw?

  1. Mar 6, 2010 #1
    1. The problem statement, all variables and given/known data
    The starter motor in an automobile has a resistance of 0.37 Ω in its armature windings. The motor operates on 12 V and has a back emf of 10 V when running at normal operating speed.
    A)How much current does the motor draw at its operating speed?
    B)How much current does the motor draw when running at half its final rotational speed?

    2. Relevant equations

    3. The attempt at a solution
    I found part A) to be 5.4 A. Our professor told of to divide this by two to get part B. Mastering Physics tells me that 2.7 A is a wrong answer. Is this a MP glitch or do I have the wrong info?
  2. jcsd
  3. Mar 6, 2010 #2
    Oops, never mind. He meant take half of the back emf, not half the current.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook