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# University of Cambridge ENGAA Section 2 Question Watch

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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.
4. (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
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?
5. (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?
I felt the math-type questions were much harder than the actual physics.
6. It was alright but I couldnt finish, how did you find the time allocated

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