Q127 / 150 · MHT CET 2021 (22 Sep Shift 2) · Physics

A step-down transformer has turns ratio 20: 1 . If $8 \mathrm{~V}$ is applied across 0.4 ohm secondary coil, then the primary current will

  • A
    2 A
  • Correct answer
    B
    1 A
  • C
    0.5 A
  • D
    4 A

Solution

$\begin{aligned} & \mathrm{V}_{\mathrm{s}}=8 \mathrm{~V}, \mathrm{R}_{\mathrm{s}}=0.4 \Omega \\ & \mathrm{I}_{\mathrm{s}}=\frac{\mathrm{V}_{\mathrm{s}}}{\mathrm{R}_{\mathrm{s}}}=\frac{8}{0.4}=20 \mathrm{~A} \\ & \frac{\mathrm{I}_{\mathrm{p}}}{\mathrm{I}_{\mathrm{s}}}=\frac{\mathrm{N}_{\mathrm{s}}}{\mathrm{N}_{\mathrm{p}}}=\frac{1}{20} \\ & \therefore \mathrm{I}_{\mathrm{p}}=\frac{\mathrm{I}_{\mathrm{s}}}{20}=\frac{20}{20}=1 \mathrm{~A}\end{aligned}$

Hence, the correct answer is option: 'B'.

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