Four wires of the same diameter are connected in turn between two… - Vidyadip

MCQ

Four wires of the same diameter are connected in turn between two points, maintained at a constant potential difference. Their resistivities are; $\rho $ and $L$ (wire $1$ )., $1.2\,\rho $ and $1.2\,L$ (wire $2$ ), $0.9\,\rho $ and $0.9\,L$ (wire $3$ ) and $\rho $ and $1.5\,L$ (wire $4$ ). Rank the wires according to the rates at which energy is dissipated as heat, greatest first

A
$4 > 3 > 1 > 2$
B
$4 > 2 > 1 > 3$
C
$1 > 2 > 3 > 4$
✓
$3 > 1 > 2 > 4$

✓

Answer

Correct option: D.

$3 > 1 > 2 > 4$

d
$\mathrm{R}_{1}=\frac{\rho \mathrm{L}}{\mathrm{A}}$

$\mathrm{R}_{2}=\frac{(1.2 \rho)(1.2 \mathrm{L})}{\mathrm{A}}=\frac{1.44 \rho \mathrm{L}}{\mathrm{A}}$

$\mathrm{R}_{3}=\frac{(0.9 \mathrm{\rho })(0.9 \mathrm{L})}{\mathrm{A}}=\frac{0.81 \rho \mathrm{L}}{\mathrm{A}}$

$\mathrm{R}_{4}=\frac{\rho(1.5 \mathrm{L})}{\mathrm{A}}=\frac{1.5 \mathrm{\rho L}}{\mathrm{A}}$

$\therefore $ $\mathrm{R}_{3}<\mathrm{R}_{1}<\mathrm{R}_{2}<\mathrm{R}_{4}$

For constant potential, $\mathrm{P} \propto \frac{1}{\mathrm{R}}$

$\therefore P_{3} < P_{1} < P_{2} < P_{4}$

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