CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 12

(a) A lamp filament has to be heated by the current until it glows white-hot. Tungsten has a very high melting point ($3380^{\circ }$C), so it does not melt at this temperature, and its fairly high resistivity lets a thin coil of it get hot enough to glow; the bulb is filled with inactive nitrogen and argon to prolong the life of the filament. (b) In series the same current flows through all the appliances although they need different currents, the $220$ V is shared so no appliance gets its full voltage, and if one appliance is switched off or fails the circuit breaks and all the others stop. In parallel each appliance gets the full $220$ V and can be switched on or off independently. (c) Copper and aluminium have very low resistivity, so long transmission wires made of them have a small resistance and little electrical energy is wasted as heat ($H=I^{2}Rt$); they are also far cheaper than silver, and aluminium is light.