Electricity and Resistance

CHAPTER - 12 ELECTRICITY HOTS Questions for Practice 1. Name a substance whose resistance almost remains unchanged by increase of temperature. 2. Name two special characteristics of heater coil. 3. A wire of resistance 4 ohms is bent to form a circle. What is the resistance between two diametrically opposite ends? 4. How does the resistance of a conductor change if its temperature is increased? 5. A current of 4A flows in a wire of resistance 60 ohms. Calculate electrical energy consumed in 2 minutes. 6. V-I graph for two resistors is given. Which of the two has minimum resistance? 7. Alloys are used in electrical heating devices rather than pure metals. Give one reason. 8. An electric geyser has the ratings 2000W, 220V marked on it. What should be the minimum rating, in whole number of a fuse wire that may be required for safe use with this geyser? 9. The electrical resistivity of few materials is given below in ohm-meter. Which of these materials can be used for making element of a heating device? A. 6.84 x 10-8 B. 1.60 x 10-8 C. 1.00 x 10-4 D. 2.50 x 1012 E. 4.40 x 10-5 F. 2.30 x 1017

10. Where do we connect a fuse: with live wire or with neutral wire? 11. What is the resistance of an air gap? 12. Name two safety measures commonly used in electric circuits and appliances. 13. Two metallic wires A and B are connected in parallel. Wire A has length l and radius r, wire B has a length 2l and radius 2r. Compute the ratio of the total resistance of parallel combination and the resistance of wire A. 14. What is the meaning of the term ‘frequency’ of an alternating current? What is its value in India? Why is an alternating current considered to be advantageous over direct current for long-range transmission of electric energy? 16. An electric wire is stretched to increase its length by 25%.By what % will the resistance be increased and what will be increase in its resistivity?

17. An electric iron of resistance 20

takes a current of 5 A. Calculate the heat