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# University of Cambridge ENGAA Section 2 Question

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1. 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? "

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
2. (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? "

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.
3. 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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