A small bulb has a resistance of 2Ω when cold. It takes up a current of 0.4 A from a source of 4V and then starts glowing. Calculate (i) the resistance of the bulb when it is glowing and(ii) Elaborate on the reason for the difference in resistance?
Question
A small bulb has a resistance of 2Ω when cold. It takes up a current of 0.4 A from a source of 4V and then starts glowing. Calculate (i) the resistance of the bulb when it is glowing and(ii) Elaborate on the reason for the difference in resistance?
Solution
To calculate the resistance of the bulb when it is glowing, we can use Ohm's Law, which states that resistance (R) is equal to voltage (V) divided by current (I).
(i) The resistance of the bulb when it is glowing can be calculated as follows: R = V / I R = 4V / 0.4A R = 10Ω
(ii) The reason for the difference in resistance between when the bulb is cold and when it is glowing is due to the phenomenon of thermal expansion. When the bulb is cold, the filament inside the bulb has a lower temperature and therefore a lower resistance. However, when the bulb is connected to the power source and starts glowing, the filament heats up and its temperature increases. This increase in temperature causes the atoms in the filament to vibrate more vigorously, leading to an increase in resistance. Therefore, the resistance of the bulb when it is glowing is higher than when it is cold.
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