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If a Constant Potential Difference is Applied Across a Bulb, the Current Slightly Decreases as Time Passes and Then Becomes Constant. Explain. - Physics

Short Note

If a constant potential difference is applied across a bulb, the current slightly decreases as time passes and then becomes constant. Explain.

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As a constant potential difference is applied across a bulb, due to Joule's heating effect, the temperature of the bulb increases. As the temperature of the bulb filament increases, its resistance also increases, as resistance R is the function of temperature T. It is given by R = R0(1+αT). With an increase in the value of resistance, the value of current decreases as \[i = \frac{V}{R}.\] Now, the heat generated by the resistance is constantly radiated to the surroundings. Thus, the value of its temperature is maintained and hence its resistance. As a result, current through the bulb filament becomes constant.

Concept: Potential Difference and Emf of a Cell
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HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 11 Thermal and Chemical Effects of Current
Short Answers | Q 1 | Page 217
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