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Watts To Joules In One Minute?

  1. Mar 17, 2008 #1
    1. The problem statement, all variables and given/known data

    A steam iron draws 8.0 A when connected to a potential difference of 120 V.
    (a) What is the power rating of this iron? 960 W <<<<correct

    (b) How many joules of energy are produced in 20.0 min?
    320 J <<<incorrect why???

    (c) How much does it cost to run the iron for 20.0 min at $0.010/kW·h?
    .0032 $ <<<<correct

    2. Relevant equations

    for part b: 960*(20/60) = 320

    whys it wrong?!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 18, 2008 #2


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    Homework Helper

    Look at your units: 960 W is 960 Joules per second. How many seconds are there is 20 minutes? This is the number of seconds you would multiply 960 W by (it is a rather larger number than 20/60).

    If part (c) is correct, you managed to make a self-canceling error...
  4. Mar 18, 2008 #3
    too late now.

    thanks anyway.
  5. Mar 18, 2008 #4
    About the step for calculating joules... Perhaps, early in the course, the teacher mentioned a topic called dimensional analysis. When you're solving a problem, you write units next to your numbers. That includes conversion factors, such as 1 minute/60 seconds, or, if necessary to flip that upside down, that might be 60 seconds/1 minute. You see what units will cancel out. After you cancel out whatever you can, take a look: do you end up with joules in the numerator and nothing in the demonimator, as you were hoping for? If not, then you made an algebra mistake. That's what dimensional analysis is, more than anything else - it's a way to receive warnings about our algebra mistakes.
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