Two identical conducting wires, A and B of different materials, are joined in series across a battery. If the number density of electrons in A is thrice that in B, then the ratio of drift velocity of electrons in the two wires is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1 : 3
The current in a conductor is \( I = n e A v_d \). Wires in series carry the same current, and identical wires have the same area of cross-section, so the drift velocity is inversely proportional to the electron number density.
\( \displaystyle v_d = \frac{I}{n e A} \;\Rightarrow\; v_d \propto \frac{1}{n} \)
\( \displaystyle \frac{v_A}{v_B} = \frac{n_B}{n_A} = \frac{n_B}{3n_B} = \frac{1}{3} \)
So the ratio is \( 1 : 3 \).