# I am pretty sure my answer is right!!

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

1. ### 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.

Thanks

2. ### davennModerator

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

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.

13,500
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Sep 5, 2009
Dorke

5. ### 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. ### davennModerator

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 7. ### 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. Integrator741 likes this.
8. ### 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

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Jun 20, 2015
"I see that I get my answer in grams, why is that?

davenn likes this.
10. ### 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. ### davennModerator

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

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. ### davennModerator

13,500
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Sep 5, 2009
DOH ... that will teach me for doing maths at 6 am !!!  