Question

A bulb is marked 100W – 220V and an electric heater is marked 1000W – 220V Answer the following questions :

(a) What is the ratio of resistance of the filament of the bulb to the element of the heater? 

(b) How does power-voltage rating of an electric appliance help us to decide the type of connecting wires (leads) to be used for it? 

(c) In the above mentioned devices in 2(a) which of the two devices needs a thicker wire. 

Answer 1

(b) The rate of heat produced in a conductor is given by the expression I² – R Copper wires have low resistance and the heat produced in them is so small that it gets radiated out without damaging the insulation. If current increases the heat produced burns the insulation and bare-copper wires comes in contact with each other and will cause electric fire. Hence power-voltage rating helps to decide the type of connecting wires (leads) to be used for it so that it can tolerate the current flowing through it. 

(c) In case of heater the lead should be thicker to offer less resistance.