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CBSE Class 10 Science Question 13 of 18

Electricity — Question 15

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Question 15

Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?

Answer

As the two lamps are connected in parallel, the voltage across each of them will be the same.

Let I1 be the current drawn by 100 W bulb:

I1 = PV=100220=511 A\dfrac{\text{P}}{\text{V}} = \dfrac{100}{220} = \dfrac{5}{11} \text{ A}

Let I2 be the current drawn by 60 W bulb:

I2 = PV=60220=311 A\dfrac{\text{P}}{\text{V}} = \dfrac{60}{220} = \dfrac{3}{11} \text{ A}

∴ Current drawn from line = I1 + I2

=511+311=5+311=811 A=0.727272 A≈0.73 A= \dfrac{5}{11} + \dfrac{3}{11} \\[1em] = \dfrac{5 + 3}{11} \\[1em] = \dfrac{8}{11} \text{ A} \\[1em] = 0.727272 \text{ A} \approx 0.73 \text{ A}

Hence, 0.73 A current flows through the line.

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Science | Chapter 11: ElectricityWeb Content

Chapter 11: Electricity — Quick Revision Guide

Introduction

Electricity powers modern life. This chapter covers electric current, potential difference, Ohm's law, resistance, series and parallel circuits, and electrical power and energy calculations.

Key Points at a Glance

  1. Current I = Q/t (ampere); potential difference V = W/Q (volt); Ohm's law V = IR
  2. Resistance R = V/I (ohm); factors: R ∝ l, R ∝ 1/A, material (ρ), temperature; R = ρl/A
  3. Conductors: low ρ (~10−8 Ω·m); alloys: higher ρ, used in heating elements (nichrome)
  4. Series: Req = R1 + R2 + ...; same current; voltage divides; one failure breaks all
  5. Parallel: 1/Req = 1/R1 + 1/R2 + ...; same voltage; current divides; independent operation; used in homes
  6. Power: P = VI = I2R = V2/R (watt); Energy: E = Pt (joule); 1 kWh = 3.6 × 106 J
  7. Heating effect: H = I2Rt; applications: heater, iron, fuse (low m.p. alloy, series with live wire)
  8. Electric bill: units (kWh) = power(kW) × time(h); cost = units × rate

Real-World Connections

Household wiring in parallel allows independent appliance use; fuses and MCBs prevent fire; LED bulbs save energy (lower power for same brightness); electricity bill management.

Quick Self-Test (5 Questions)

  1. What is the most important concept you learned from this chapter?
  2. Can you write three key equations/formulae from this chapter from memory?
  3. Draw a labelled diagram relevant to this chapter without looking at your notes.
  4. Explain one real-world application of a concept from this chapter.
  5. What is one common mistake students make in this chapter, and how can you avoid it?

Further Study

  • NCERT Textbook Chapter 11
  • NCERT Exemplar Problems
  • Bright Tutorials Detailed Notes: ch11-electricity.html
  • Bright Tutorials Practice Questions: ch11-electricity.html
  • Previous Year CBSE Board Papers

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