NCERT Class X Science Solutions: Chapter 12 – Electricity Part 6

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Question 16:

Which uses more energy, a set in 1 hr., or a 1200 W toaster in 10 minutes?

Answer:

Energy consumed by an electrical appliance is given by

For the TV set:

So, energy consumed

For the toaster:

So, energy consumed

Hence, TV set uses more energy than toaster.

Question 17:

An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.

Answer:

Heat developed in the heater is given by

Where,

The rate at which heat is developed is given by

Question 18:

Explain the following.

  1. Why is the tungsten used almost exclusively for filament of electric lamps?

  2. Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?

  3. Why is the series arrangement not used for domestic circuits?

  4. How does the resistance of a wire vary with its area of cross-section?

  5. Why copper and aluminium wires are usually employed for electricity transmission?

Answer:

  1. The melting point and resistivity of tungsten are very high. It does not burn readily at a high temperature. The electric lamps glow at very high temperatures. Hence, tungsten is mainly used as heating element of electric bulbs.

  2. The conductors of electric heating devices such as bread toasters and electric irons are made of alloy because resistivity of an alloy is more than that of metals. It produces large amount of heat and do not burn easily.

  3. There is voltage division in series circuits. Each component of a series circuit receives a small voltage for a large supply voltage. As a result, the amount of current decreases and the device becomes hot. Hence, series arrangement is not used in domestic circuits.

  4. Resistance (R) of a wire is inversely proportional to its area of cross-section

(A):

  1. Copper and aluminium wires have low resistivity. They are good conductors of electricity. Hence, they are usually employed for electricity transmission.

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Electricity Transmission

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