CBSE Class 8 Annual Assessment

Chapter 4: Electricity: Magnetic and Heating Effects

Summary

This chapter shows that an electric current does more than light a lamp. When current flows through a wire, it produces a magnetic field around it, deflecting a nearby compass needle; this is the magnetic effect of electric current, first discovered by Hans Christian Oersted. A coil of wire carrying current behaves like a magnet, and inserting an iron core makes a much stronger electromagnet with two poles. The strength of an electromagnet can be increased by using more cells (more current) or more turns of the coil, and its poles reverse if the current direction reverses. Lifting electromagnets on cranes use this on-off control to move heavy iron and steel. The chapter also studies the heating effect: a current-carrying conductor faces resistance, converting some electrical energy into heat. Nichrome wire has high resistance and is used in heaters, irons, kettles, and immersion rods. The heat generated depends on the current, and on the material, thickness, length, and time. Finally, the chapter explains how cells and batteries generate electricity through chemical reactions. It describes the Voltaic (Galvanic) cell with electrodes and an electrolyte, a homemade lemon cell, dry cells with a moist paste electrolyte, and modern rechargeable batteries, including a note on lithium-ion and the value of safe e-waste recycling.

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