What is the resistance of a torch bulb rated at 2.5 V and 500 mA?

well if the potential difference (voltage) that i apllied across the light bulb is exacty 2.5v (as it can vary due to the interal resistance of th batteris used) then you can use Resistance=Voltage/current (R=V/I) to calculatete resistance. Therefore since 500mA is equivlant to 0.5A R=2.5/0.5=5 ohms.

hopeully that is helpfull