Chapter 12 · Magnetic Effects of Electric Current

NCERT Class 10 Science Chapter 12 Intext Questions (Page 205)

All 3 questions of NCERT Class 10 Science Chapter 12 Intext Questions (Page 205) — Magnetic Effects of Electric Current — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Name ​two ​safety ​measures commonly used ​in electric ​circuits and ​appliances.
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   Two safety measures are a fuse of proper rating in the live wire and earthing of metal-bodied appliances.

   Two safety measures are: (i) An electric fuse of a proper rating, connected in series with the live wire of each circuit. When the current exceeds the safe value because of overloading or a short circuit, the Joule heating in the fuse melts it and breaks the circuit, protecting the wiring and the appliances. (ii) Earthing of appliances that have a metallic body: the body is joined by the green earth wire to a metal plate buried deep in the earth, which provides a low-resistance path for any leakage current and keeps the body at earth potential, so that the user does not receive a severe electric shock.

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2. Q2
   An electric oven of 2 ​kW ​power rating is operated in a domestic ​electric circuit ($220$ ​V) that has ​a current rating ​of $5$ ​A. What result do you expect? Explain.
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   The oven would draw about 9.1 A, more than the 5 A rating, so the fuse melts and the oven does not work.

   The oven would draw a current of $I=P/V=2000/220\approx 9.1$ A, which is far more than the 5 A the circuit is rated for. The circuit is therefore overloaded. The fuse in its live wire, chosen for the 5 A rating, is heated by this large current, melts and breaks the circuit, so the supply to the oven is cut off and the oven does not work. This is the fuse doing its job: without it the wiring, designed for 5 A, would overheat and could cause a fire. The oven should be run instead on the 15 A circuit provided for high-power appliances.

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3. Q3
   What ​precaution ​should be taken to avoid ​the overloading ​of ​domestic ​electric circuits?
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   Avoid too many appliances on one circuit, use circuits and fuses of correct rating, and keep the wiring and its insulation sound.

   To avoid overloading of domestic circuits: (i) Do not connect too many appliances to a single socket or to one circuit, since the currents they draw add up. (ii) Use separate circuits of suitable rating and put each appliance on the right one — high-power appliances such as geysers, air coolers and ovens on the 15 A circuit, and bulbs and fans on the 5 A circuit; never run an appliance on a circuit whose current rating is less than the current it draws. (iii) Fit a fuse or a miniature circuit breaker of the correct rating in series with the live wire of every circuit, and never replace it with one of a higher rating or with a piece of ordinary wire. (iv) Use good-quality wires with proper insulation and adequate current-carrying capacity, and repair damaged insulation and faulty appliances promptly, since the live and neutral wires touching cause a short circuit and a sudden rise of current. (v) Use a voltage stabiliser for sensitive appliances to guard against an accidental hike in the supply voltage, and switch off appliances that are not in use.

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