The ratio of the number of turns of the ...

CBSE Class 12 Physics 2025 All Sets Solved Paper

Question : 4 of 16

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The ratio of the number of turns of the primary to the secondary coils in an ideal transformer is 20 :1. If 240 V ac is applied from a source to the primary coil of transformer and a 6.0 Ω resistor is connected across the output terminals, then current drawn by the transformer from the source will be.

4.0 A
3.8 A
0.97 A
0.10 A

Solution:

Using the transformer equation

\;\frac{N_P}{N_S} = \;\frac{V_P}{V_S}

Given:

NP : N_S = 20 : 1 ,\; V_P = 240\,\text{V}

V_S = \;\frac{V_P}{20} = \;\frac{240}{20} = 12\,\text{V}

Using Ohm’s Law,

I_S = \;\frac{V_S}{R} = \;\frac{12\,\text{V}}{6\,\Omega} = 2\,\text{A}

From the transformer current relation:

I_P = I_S \times \;\frac{N_S}{N_P}

I_P = 2\,\text{A} \times \;\frac{1}{20} = 0.10\,\text{A}

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