My teacher gave us this question that she meticulously designed: A high voltage cable is made up of six strands of aluminium and one of steel. The resistivity of steel is given and the resistivity of aluminium too. If the cross sectional are A is also given, and we know that by including the steel strand we reduce the the resistance of a length l of cable by 1.4*10^-4 ohms, we should be able to calculate the length l of cable. Any slight hint would be appreciated. I am unable to provide working, but what I did is I first stated that the resistivity of the cable without steel would be the same as the resistivity of aluminium, so rho_al=R*6A/l and with steel rho_eq=(R-1.4*10^-4 ohms)*7A/l then I equated the resistance R, but I do not know the equivalent resistivity. So I think there might be 1. An assumption I am missing or 2. A different path to tackle the question.
Original post by Javier García
My teacher gave us this question that she meticulously designed: A high voltage cable is made up of six strands of aluminium and one of steel. The resistivity of steel is given and the resistivity of aluminium too. If the cross sectional are A is also given, and we know that by including the steel strand we reduce the the resistance of a length l of cable by 1.4*10^-4 ohms, we should be able to calculate the length l of cable. Any slight hint would be appreciated. I am unable to provide working, but what I did is I first stated that the resistivity of the cable without steel would be the same as the resistivity of aluminium, so rho_al=R*6A/l and with steel rho_eq=(R-1.4*10^-4 ohms)*7A/l then I equated the resistance R, but I do not know the equivalent resistivity. So I think there might be 1. An assumption I am missing or 2. A different path to tackle the question.

it's a parallel resistors question... let me get the question right... at first there is 6 strands of aluinimum and then a 7th strand of steel is added which causes the total resistance to decrease by 1.4*10^-4 ohms??
Original post by Mr Naga Physics
it's a parallel resistors question... let me get the question right... at first there is 6 strands of aluinimum and then a 7th strand of steel is added which causes the total resistance to decrease by 1.4*10^-4 ohms??

if so... let Total resistance at start = resistance of wire aluminium strand / 6 ( i.e parallel so total resistance is small) and then Total resistance at end = 1/ [ 1/ resistance at start) + 1/resistance of steel] and finally total resistance at end = total resistance at start - 1.4*10^-4
Original post by Mr Naga Physics
if so... let Total resistance at start = resistance of wire aluminium strand / 6 ( i.e parallel so total resistance is small) and then Total resistance at end = 1/ [ 1/ resistance at start) + 1/resistance of steel] and finally total resistance at end = total resistance at start - 1.4*10^-4

you're right... it is a very nice question.
Original post by Mr Naga Physics
you're right... it is a very nice question.
Thank you, the thing was thinking about it as parallel resistors, you were right. I will see you in another one.