BrainJuice
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#1
Report Thread starter 4 years ago
#1
I don't know what they do in the first step of question 2 (last part of it). To be honest I don't know why they use 2000 either.

Can you please help. Thank you for reading and in advance if you can and do help.

Paper: http://pmt.physicsandmathstutor.com/...%20A-level.pdf

MarkScheme: http://pmt.physicsandmathstutor.com/...%20A-level.pdf
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Callicious
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#2
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#2
When we increase the area of something, we reduce resistance. The strands are all arranged like this for contortion purposes and not to allow individual transmission of electricity.

Thus, the area of the total strand is approx 38 times the area of each strand, and hence since resistivity is the same the total resistance per km will be 1/38th the resistance of each strand.

Hope I helped!
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BrainJuice
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#3
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(Original post by Callicious)
When we increase the area of something, we reduce resistance. The strands are all arranged like this for contortion purposes and not to allow individual transmission of electricity.

Thus, the area of the total strand is approx 38 times the area of each strand, and hence since resistivity is the same the total resistance per km will be 1/38th the resistance of each strand.

Hope I helped!
Thank you for your help, sorry though because I mean the last part of this question. The power one.
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britishtf2
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#4
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(Original post by BrainJuice)
Thank you for your help, sorry though because I mean the last part of this question. The power one.
I am assuming you are comfortable with up to part (c)(iii).
In part (ii) you worked out the number of cables needed [12 cables].
In part (iii) you worked out the energy lost per cable per kilometre [10kW].

You can use this to work out how much energy is lost in total through all of the cable:
Energy Lost = (Energy lost per kilometre of cable)*(Length of cable [in km])
= 10kW*(12*100km)
= (10*10^3)*(12)*(100)
= 12*10^6 W

The energy input to the cable is said to be 2000MW = 2000*10^6 W
Efficiency = (Useful energy out)/(Total energy In)
= 100%*((2000-12)*10^6)/(2000*10^6)
= 99.4%, as given.
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