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University of Cambridge ENGAA Section 2 Question Watch

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    The question comes from University of Cambridge's ENGAA Specimen Section 2 Paper. The question is replicated as follows:

    "A power station generates 1GW of electricity and supplies it to a city 100 km away over a pair of aluminium cables, each of diameter 30 mm. The voltage between the cables at the power station is 400 kV. The resistivity of aluminium is 2.6 * 10–8 Ωm.
    a. What is the approximate resistance of each of the cables that connect the power station to the city? "

    Answer choices are :
    A. 37 mΩ
    B. 37 μΩkm^(-1)
    C. 37 μΩ
    D. 37 mΩkm^(-1)
    E. 3.7Ωkm^(-1)

    According to the official answer key, the correct answer is D. Can anyone explain how to solve this problem? I'm slightly confused why the answer for resistance would incorporate a unit of distance.

    Thank you
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    (Original post by 17zhangw)
    The question comes from University of Cambridge's ENGAA Specimen Section 2 Paper. The question is replicated as follows:

    "A power station generates 1GW of electricity and supplies it to a city 100 km away over a pair of aluminium cables, each of diameter 30 mm. The voltage between the cables at the power station is 400 kV. The resistivity of aluminium is 2.6 * 10–8 Ωm.
    a. What is the approximate resistance of each of the cables that connect the power station to the city? "

    Answer choices are :
    A. 37 mΩ
    B. 37 μΩkm^(-1)
    C. 37 μΩ
    D. 37 mΩkm^(-1)
    E. 3.7Ωkm^(-1)

    According to the official answer key, the correct answer is D. Can anyone explain how to solve this problem? I'm slightly confused why the answer for resistance would incorporate a unit of distance.

    Thank you
    Let us begin by noting the formula R = resistivity * length / area. If we plug in the values straight away, we get R = 3.7 ohm which is not in the options. Now let us find R/length where length is in km, we find this is 37 milli-ohm per km, same as D. This means that the resistance is 37 milli-ohm for every one kilometer of the wire.
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    They're hoping you'll select the answer that's true / most accurate... sometimes you see that in multi choice, it makes you work a little bit harder.
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    (Original post by 17zhangw)
    The question comes from University of Cambridge's ENGAA Specimen Section 2 Paper. The question is replicated as follows:

    "A power station generates 1GW of electricity and supplies it to a city 100 km away over a pair of aluminium cables, each of diameter 30 mm. The voltage between the cables at the power station is 400 kV. The resistivity of aluminium is 2.6 * 10–8 Ωm.
    a. What is the approximate resistance of each of the cables that connect the power station to the city? "

    Answer choices are :
    A. 37 mΩ
    B. 37 μΩkm^(-1)
    C. 37 μΩ
    D. 37 mΩkm^(-1)
    E. 3.7Ωkm^(-1)

    According to the official answer key, the correct answer is D. Can anyone explain how to solve this problem? I'm slightly confused why the answer for resistance would incorporate a unit of distance.

    Thank you
    we had 100 km of wires so you could simplify it by saying 3.7Ω ,its the same thing


    How did you find the pre-interveiw assesment tho?
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    (Original post by davozgenius)
    we had 100 km of wires so you could simplify it by saying 3.7Ω ,its the same thing


    How did you find the pre-interveiw assesment tho?
    Mixed-feeling about the pre-interview assessment.
    I felt the math-type questions were much harder than the actual physics.
    Your opinion?
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    It was alright but I couldnt finish, how did you find the time allocated
 
 
 
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