It's not an exception - it's the application of the rules!
Please read through the whole of this post very carefully. I have written it in the fullest way possible. All of the information you need to understand the concept of resistivity is in here.
The materials property of resistivity is used to calculate the actual total resistance of any given volume of that material:
Resistivity is an intrinsic property of the material to inhibit current flow. It relates the electrical properties of a material to it's physical dimensions of cross sectional area and length.
The unit of resistivity is
ohm metres=Ωmρ=LRAequating units:
LRA=mΩ x m2=ΩmResistance on its own is purely a ratio between voltage and current and no information can be deduced about the physical dimensions of the material giving rise to that resistance.
Ohms law:
R=IVThe unit of resistance is
ohms=ΩThe analogy to resistivity would be the density of a material for instance. i.e 20 kg/m
3 is not the same as saying 20 kg. The latter of which says nothing about the dimensions of that 20kg whereas the former can be applied to the physical dimensions in order to calculate the total mass. But even this is not accurate enough because the direct equivalent for resistance and resistivity would be mass and densitivity respectively with densitivity units of kg metres!
Going back to the original question
Resistance:
The question tells is the resistance of the cable is 2 ohms per km. That is, the total resistance of the cable is increasing by 2 ohms for every 1km length.
i.e. more information than just pure resistance is given, which therefore relates the length of the cable to resistance. But, it's still only in one dimension (length). It's still not resistivity.
Thus if the cable was 10 km long, then its total resistance would be
R=2 x 1km10km=20ΩIf the cable was 400m long, then it's total resistance would be
R=2 x 1km0.4km=0.8ΩIn our question, we are also told the length if the cable is 1km, therefore the total resistance for this 1km length of cable is:
R=2 x 1km1km=2ΩIf the question stated the resistance of the cable was 27 ohms per 624 metres and the length was 2km then the total resistance would be calculated as:
R=27 x 0.624km2km=86.54ΩOhms/m or ohms/km is a standard unit of measure relating a cables resistance to a standard unit of length.
Resistivity:
The question asks for the cross sectional area and gives us the resistivity of the cable material as
ρ=2.6x10−8ΩmResistivity is defined as
ρ=LRAtherefore
A=RρLwhere
R=2Ω (previously calculated)
L=1kmρ=2.6x10−8Ωmsubstituting values:
A=RρL=22.6x10−8 x 1x103=1.3x10−5m2