1. Sep 18, 2012 #1

   Yarnzorrr

   

   here is a screenshot from one of my lectures talking about how to find active power (P) and reactive power (Q)
   
   He calculates P using 100*10 Cos (30-(-30), which I understand, though the answer he gets is 200W, but whenever I calculate it I get 500 because cos60 = 0.5 and 0.5*1000 is 500.
   
   Just wanting to know what I'm doing wrong.
   Thanks.

    

   Attached Files:

   • 

     Screen Shot 2012-09-18 at 5.18.48 PM.png

     File size:

     40.6 KB

     Views:

     141

   Yarnzorrr, Sep 18, 2012
2. 
   Phys.org - latest science and technology news stories on Phys.org

   • • Hairy robot
   • • Researchers develop a method to detoxify water with chlorine and ultraviolet radiation
   • • Print and probe—how 3-D printers could improve access to technologies essential for scientific discovery
3. Sep 18, 2012 #2

   NascentOxygen

   

   Staff: Mentor

   It does look like it should be 500.

    

   NascentOxygen, Sep 18, 2012
4. Sep 18, 2012 #3

   I_am_learning

   

   Yes, its obviously 500.
   Also, VA = Sqrt(Watts^2+Var^2)
   1000 ~= sqrt(500^2+866^2)

    

   I_am_learning, Sep 18, 2012
5. Sep 18, 2012 #4

   yungman

   

   I think it's 500
   
   I use complex number. From the picture of the board:
   
   V=100∠+30 deg = 100(0.866+j0.5)= 86.6+50j
   I=10∠-30 deg = 10(0.866-j0.5)= 8.66-j5.
   
   Power =Re[IV]= Re[(86.6+j50)(8.66-j5)]= 749.956-250=500.

    

   yungman, Sep 18, 2012

• Log in with Facebook
• Log in with Twitter

Your name or email address:

Do you already have an account?

• No, create an account now.
• Yes, my password is:
• Forgot your password?

Stay logged in