A boy has two wires of iron and copper of equal length and diameter. He first joins the two wires in series and passes electric current through this combination which increases gradually with tiem. After that he joins them in parallel and repeat the process of passing the current in this arrangement also. Which wire will glow first in each case and why ?

Explanation

We know that resistivity of iron is more than that of copper.
In series combination, the same current `I` flows through iron and copper wires.
Heat produced, `H = I^(2) Rt = I^(2) (rho l)/(A)` i.e., `H prop rho`
As `rho_("iron") gt rho_("Cu")` so `H_("iron") gt H_("cu")`. Therefore, iron will start glowing first in series combination. In parallel combination, the voltage V is same across iron and copper wires, So
Heat produced, `H = V^(2)/R t = (V^(2)t)/(rhol//A)` i.e., `H prop 1/rho`
As `rho_("iron") gt rho_("cu")` so `H_("iron") lt H_("cu")`
Therefore, copper will start glowing first in parallel combination of wires.