Question

If two electric bulbs have 40 W and 60 W rating at 220 V, then the ratio of their resistances will be

• A. 3 : 2
• B. 3 : 8
• C. 4 : 3
• D. 9 : 4

Answer 1

Answer: (A)

Key Idea: The rate at which electrical energy is dissipated into other forms of energy is called electric power P. Electrical power $={\epsilon }_0{E}_{rms}^2$ (V = iR) $=8.85\times {10}^{-12}\times {(720)}^2$ $=4.58\times {10}^{-6}J/m^3$ where Vis potential difference, R is resistance. Given, ${10}^{-10}s$ $f=\frac{v}{v-v_s}f$ $v_s$ $v=320m/s,v_s=4m/s,f=240Hz$ $f=\frac{320}{320-4}\times 240$ $=243Hz$ Note: Higher the wattage of a bulb smaller is the resistance of its filament that is thicker is the filament.