Question 18
Explain the following.
(a) Why is the tungsten used almost exclusively for filament of electric lamps?
(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?
(c) Why is the series arrangement not used for domestic circuits?
(d) How does the resistance of a wire vary with its area of cross-section?
(e) Why are copper and aluminium wires usually employed for electricity transmission?
(a) The resistivity and melting point of tungsten is very high hence it doesn't burn readily when heated. Therefore, tungsten is used almost exclusively for filament of electric lamps.
(b) Resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidize (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices like electric iron, toasters etc.
(c) The series arrangement is not used for domestic circuits due to the following reasons:
- All the connected appliances cannot be operated independently. If one device is defective, then the entire circuit will not function.
- The overall voltage gets distributed in a series circuit. As a result, electric appliances may not get the rated power for their operation.
- The total resistance becomes large, hence, the current is reduced.
(d) Resistance (R) of a wire is inversely proportional to its area of cross-section (A) i.e., when the area of cross section increases the resistance decreases and vice versa.
R = ρ
(e) Copper and aluminium both are good conductors of electricity and have low resistivity. Hence, they can efficiently carry electrical current with minimal resistance, which reduces energy losses as heat during transmission.
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
- Current I = Q/t (ampere); potential difference V = W/Q (volt); Ohm's law V = IR
- Resistance R = V/I (ohm); factors: R ∝ l, R ∝ 1/A, material (ρ), temperature; R = ρl/A
- Conductors: low ρ (~10−8 Ω·m); alloys: higher ρ, used in heating elements (nichrome)
- Series: Req = R1 + R2 + ...; same current; voltage divides; one failure breaks all
- Parallel: 1/Req = 1/R1 + 1/R2 + ...; same voltage; current divides; independent operation; used in homes
- Power: P = VI = I2R = V2/R (watt); Energy: E = Pt (joule); 1 kWh = 3.6 × 106 J
- Heating effect: H = I2Rt; applications: heater, iron, fuse (low m.p. alloy, series with live wire)
- 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)
- What is the most important concept you learned from this chapter?
- Can you write three key equations/formulae from this chapter from memory?
- Draw a labelled diagram relevant to this chapter without looking at your notes.
- Explain one real-world application of a concept from this chapter.
- 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