Asmall bulb has a resistance of 2 ohm when cold it take up a current of 0.4 ampere from a source of 4v then start glowing? calculate resistance when it is glow and elaborate the reason of difference between resistance ?

The resistance of the bulb when it is cold is .

The resistance of the bulb when it is crossed by a current of I=0.4 A under a potential difference of  can be calculated by using Ohm's law:


The reason for this difference in the resistance is the following: when the current passes through the bulb, the bulb heats up, and an increase in temperature in the filament of the bulb causes an increase in resistance of the filament. In fact, the resistance of a resistor increases as the temperature increases, because the atoms of the lattice of the material vibrate more (due to their greater energy) and therefore the electrons of the current are more likely to collide with them. As a result, the resistance of the material increases.