Connect with us

I am pretty sure my answer is right!!

Discussion in 'Electronics Homework Help' started by Integrator741, Nov 6, 2015.

Scroll to continue with content
  1. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    Good evening,

    I am practising my ability to manipulate units and stuff, and I came across this question that doesn't sound that hard but I just can't get the right answer.

    "The electricity meter on a plant initially reads 6032 kWh. If the plant now runs for 73 minutes at a steady power consumption of 320 kW determine a) Energy consumed in MJ b) The final reading of the meter"

    Answers:. a) 1401.6 MJ; b) 6421.3 kWh

    My working out is:

    320 kWh/60min = 5.3 kW per min.
    5.3 kW per min * 73 min = 386.9 kWh
    so the final reading will be 6032 + 386.9 = 6419 kWh

    now to convert 386.9 to MJ:

    1kWh = 3.6 MJ

    386.9/3.6 = 107.472 MJ;

    I am close with a) but b)... no idea.

    Please help.

    Thanks
     
  2. davenn

    davenn Moderator

    13,500
    1,830
    Sep 5, 2009
    why did you divide by 3.6 ? it should be multiply

    1kWh = 3.6 ...... ( 3.6MJ for each kWh)
    2kWh = 7.2
    10kWh = 36

    etc

    386.9 x 3.6 = ..... ?


    you now work that out :)


    cheers
    Dave
     
    Integrator741 likes this.
  3. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Very simple:
    Do not round the results!

    a) 320*(73/60)*3.6= ??
    b) 320*(73/60)+6032 = ??
     
    Last edited by a moderator: Nov 6, 2015
    Arouse1973 and Integrator741 like this.
  4. davenn

    davenn Moderator

    13,500
    1,830
    Sep 5, 2009
    Dorke
    Please don't give direct answers in homework section
     
  5. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    Thanks, I didn't think that it would make such a significant difference. Again thanks. Maybe you'd be able to help me with another question, about some copper wire coiled around a tube.

    "Copper wire (density 8900) of cross-section 1.7mm^2 is coiled onto a long cylindrical former. The resulting coil has 820 turns, and diameter 0.13m. Find the mass of wire in KG."

    What I did:

    First I need to know the length of the catble - length of one turn is (pi)*Diameter of the cylinder. (pi)*0.13m = 0.408m;
    Total length of the cable is: 0.408m * 820 turns = 334.9m;

    then I need to calculate the volume of the cable in order to calculate its mass.

    To find volume of the cylinder - I need diameter.

    I know that area=(pi)*r^2 so 0.13=(pi)*r^2; (solve for r)
    r=(square root)0.13/(pi) = 0.2m - so diameter is 0.4m;

    Volume of the cylinder is: ((pi)*d^2*l)/4 = I got 42.08m^3, then to find mass I multiply my answer by the given density and I get: 374512kg and the actual answer is 5.0669 kg - what the hell?
     
  6. davenn

    davenn Moderator

    13,500
    1,830
    Sep 5, 2009
    I like Dorke's way, at least it gives the expected answer

    his gives 389.333, where as yours gives 386.9 and of course that made the answer
    wrong for you and me as I used your number :D
     
  7. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    I think they are talking about a coil raped around a cylinder.
    That means the cylinder is hollow:
    there is only copper wire on it's "outer- case" .

    So you have a copper wire with a cross-section of
    1.7mm^2 wrapped around the cylinder of diameter 0.13m

    1.You did o.k with calculating the wire length,
    to be more accurate you need to add the diameter of the wire to the diameter of the cylinder!)-again don't round numbers.
    2.You need to convert the cross-section to meters and multiply by the length to find the wires volume in meters^3
    3.Last multiply by the density (in meters^3) to get the result. cylinder.JPG

     
    Integrator741 likes this.
  8. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    Thank you - I got it now.
    One question though, I see that I get my answer in grams, why is that?
     
  9. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    "I see that I get my answer in grams, why is that?
    Please,show your calculation and we shall find out why.
     
    davenn likes this.
  10. Integrator741

    Integrator741

    125
    4
    Jun 16, 2013
    Cable length = turns*circumference = 820 * ((pi)*0.13) = 334.89
    Volume = (1.7*10^-3) * 334.89 = 0.56932 m^3

    Volume*density = 0.56932 * 8900 = 5066.9
     
  11. davenn

    davenn Moderator

    13,500
    1,830
    Sep 5, 2009
    because the 8900 is 8900 grams / m3 ( 8.9 g / cm3 )
    so you ended up with 5066.9 grams = 5.066kg

    also again you are still rounding off ... stop it !! ;)

    you should be using 8.96 g / cm3
     
  12. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Dave,
    You both got it wrong...

    The density of copper is indeed 8900Kg/m^3 not gr.
    A 1x1x1 meter cube of copper weighs about 9tons not 9Kg...;)

    The problem is in transferring the wire diameter from mm^2 to m^2
    the factor should be 1x 10^-6 and not 1x10^-3...;)
     
    davenn likes this.
  13. davenn

    davenn Moderator

    13,500
    1,830
    Sep 5, 2009
    DOH ... that will teach me for doing maths at 6 am !!!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-