Two wires off equal lengths,one of copper and other of magnesium have the same thickness .which one can be used for (i)electrical transmission lines (ii)electrical heating device .why ?

To make electrical transmission lines, we want to use a material which will give lower resistivity, so that there is smaller loss of energy during transmission, while for the electrical heating device, we need to use a material which has a high resistance so that a greater heat energy is released. 

 

Heat energy = V^2t/R and R is directly proportional to resistivity as the length and the thickness is constant. 

 

Copper has smaller resistivity (17.2n ohm-m) as compared to magnesium (43.9 n ohm-m) and hence would produce higher heat energy. So, copper can be used to make electrical heating device while magnesium can be used to make electrical transmission lines.