1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is my calculation correct?

  1. Oct 5, 2009 #1

    dvn

    User Avatar

    Hello,

    I have been researching on electrical output and it power and working out how much power I use at home and have been doing a project to make power.

    I wanted to make sure if this calculation is correct...

    I use a 12V DC motor with an output of 70mA which creates 0.84 Watt according to PIE method. 12 X 0.07 = 0.84W

    So I use 20 x dc motors and combined of 20 motors will make 16.8 Watt?
    = 20 X 0.84 = 16.8W

    If I get the motors running for an hour then it will create 1.008 kWh??
    = 16.8 x 60 = 1008W

    Please correct me if I am wrong.

    Thank you
     
  2. jcsd
  3. Oct 5, 2009 #2

    MATLABdude

    User Avatar
    Science Advisor

    Everything's correct, except for the last line. You'd use up 1008 J, but only 16.8 Watt*hour. Why? You're multiplying 16.8 watts by, well, 1 hour!

    It won't actually produce 16.8 Watt*hour of energy, since a portion of the input power goes to heat.
     
  4. Oct 5, 2009 #3

    dvn

    User Avatar

    I multipled 60 because of 60 minutes in an hour. So you are saying that the energy 16.8W is applied as an hour?
     
  5. Oct 5, 2009 #4
    One watt is one joule per second.
     
  6. Oct 5, 2009 #5

    dvn

    User Avatar

    So if I get the dc motors running for 2 hours then it will produce 33.6W?

    = 16.8 x 2 = 33.6W

    So how do I work out the watt for minutes and seconds?
     
  7. Oct 5, 2009 #6

    russ_watters

    User Avatar

    Staff: Mentor

    A watt-hour is a watt for an hour, so 16.8 watts for an hour is 16.8 watt-hours.
    No, 16.8 Watts x 2 hours = 33.6 Watt-hours. Make sure you follow through with your units!
    Watts are a unit of power - they are already in terms of seconds (as already stated: joules per second). Energy is power times time. Watt-hours.
     
  8. Oct 5, 2009 #7

    dvn

    User Avatar

    If I want to measure the power output for 1 minute.

    Do I divide 16.8 by 60 = 0.28 joules?
     
  9. Oct 5, 2009 #8

    russ_watters

    User Avatar

    Staff: Mentor

    No, watts is power: 16.8 watts. It doesn't matter if you measure it for a second, an hour, or a day, 16.8 watts of power is 16.8 watts of power. And a watt is a joule per second, so you can express it as 16.8 joules per second.

    If you take a book that weighs 1 N and pick it up off the floor and put it on a 1m table, that's 1 N-m -- 1 Joule of energy. If you do it in 1 second, that's still 1 joule of energy, but expended at a rate of 1 joule per second or 1 watt.

    Take your car: if your car is moving at 60 miles per hour, does measuring it for 2 hours mean you were driving at 120 miles per hour? No, it is still 60 miles per hour.
     
  10. Oct 5, 2009 #9

    dvn

    User Avatar

    Now I understand what you mean. Sorry I was thinking about recharging theory.

    Yes you are right about the constant energy output that the dc motor make. What I have forgotten to tell is that if I recharge the battery or secondary battery or so, so if I use 16.8W and recharge the battery in 2 hours then it will recharge as 33.6w or something?
     
  11. Oct 5, 2009 #10

    russ_watters

    User Avatar

    Staff: Mentor

    No, still 16.8W. Watts is power. Power is energy per unit time. If you want to know how much energy it takes, that's watt-hours.

    This could all be cleared-up if you write down your math and include the units when doing the math. You'll see that multiplying a watt by an hour can't yield a watt, it must yield a watt-hour!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook