Q.30 Physics ⭐ 360° Option Elimination Active

NEET 2023 Physics Question 30 Solution

Topic: Alternating Current · Official NTA Booklet Code G5

Question

A 12 V, 60 W lamp is connected to the secondary of a step down transformer, whose primary is connected to ac mains of 220 V. Assuming the transformer to be ideal, what is the current in the primary winding?

✓

Verified Against NTA Final Answer Key

Option (2) : 0.27 A

📚 Alternating Current Easy

📋 Given Parameters & Problem Setup

Secondary load rating 12 V, 60 W lamp

Secondary power output (P_s) 60 W

Primary AC mains voltage (V_p) 220 V

Transformer efficiency (η) 100% (Ideal transformer)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Conservation of Power in Ideal Transformer: P_p = P_s

⏱️ Save ~40 seconds

For ideal transformer, Input Power = Output Power ⟹ V_p × I_p = 60 W ⟹ I_p = 60 / 220 = 3 / 11 ≈ 0.27 A in 10 seconds!

1 An ideal transformer operates with 100% efficiency: Input Power P_p = Output Power P_s.

2 The secondary delivers power P_s = 60 W to the lamp.

3 Equate input power: V_p × I_p = 60 W.

4 Substitute primary voltage V_p = 220 V: 220 × I_p = 60.

5 Solve for primary current: I_p = 60 / 220 = 6 / 22 = 3 / 11 ≈ 0.2727 A ≈ 0.27 A.

6 Lock Option (2) 0.27 A immediately!

🛡️

Why You Can 100% Trust This Shortcut: Alternating Current · Ideal Transformer Relations (For an ideal transformer with zero core and copper losses, V_p · I_p = V_s · I_s).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Principle of Power Conservation for an Ideal Transformer

In an ideal transformer, energy losses due to flux leakage, eddy currents, hysteresis, and Joule heating in windings are assumed to be zero. Consequently, the input electrical power fed into the primary winding equals the output power delivered by the secondary winding:

P_{\text{primary}} = P_{\text{secondary}}

Step 2

Expression for Electrical Power in the Primary Circuit

The power delivered to the primary winding from the AC supply is given by the product of the primary RMS voltage and primary RMS current (assuming unity power factor for the resistive lamp load):

P_p = V_p \cdot I_p

Step 3

Substitution of Given Parameters

Substitute the primary mains voltage V_p = 220 V and the secondary load power P_s = 60 W:

220 \cdot I_p = 60

Step 4

Evaluation of Primary Current

Divide both sides by 220 to isolate I_p:

I_p = \frac{60}{220} = \frac{3}{11} \approx 0.2727\text{ A} \approx 0.27\text{ A}

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Arithmetic distraction.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate miscalculated the decimal value of 3/11.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate value: 60 W / 220 V = 3/11 ≈ 0.27 A.

💡

Why this is airtight: None. Candidate applied conservation of power directly.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Off by a factor of 10.

⚠️

Student Fallacy (Why candidates pick this trap): Decimal point misplaced during 60 / 220.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Severe arithmetic blunder.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate computed secondary current incorrectly (60 / 12 = 5 A) or inverted fractions.

⚠️

NTA Trap Alert: Calculating Secondary Current Unnecessarily and Wasting Time

Many candidates calculate I_s = 60 / 12 = 5 A, then turns ratio N_s / N_p = 12 / 220, and then I_p = I_s × (N_s / N_p). While valid, this takes 4× longer and introduces arithmetic slip opportunities.

🛡️ How to avoid the -1 mark: Use direct power conservation: P_in = P_out ⟹ V_p · I_p = P_s ⟹ I_p = P_s / V_p.

📖

Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Alternating Current Standard Benchmark

Core Scientific Principle Tested:

"For an ideal transformer, input power equals output power: V_p I_p = V_s I_s."

Syllabus Focus: Transformers and Power Efficiency

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Physics Question 30 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Conservation of Power in Ideal Transformer: P_p = P_s

Step-by-Step Rigorous Workflow & Mathematical Proof

Principle of Power Conservation for an Ideal Transformer

Expression for Electrical Power in the Primary Circuit

Substitution of Given Parameters

Evaluation of Primary Current

360° Option Elimination Engine Key Feature

NTA Trap Alert: Calculating Secondary Current Unnecessarily and Wasting Time

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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