A research laboratory’s low-voltage power supply feeds the circuit shown below
PHYSICS · Multi-part · CBSE · Physics · Current Electricity · Current Electricity
Problem
A research laboratory’s low-voltage power supply feeds the circuit shown below. The internal resistance of the supply is small but not negligible. A single indicator lamp (bulb L) of resistance RL is connected in series with a variable-resistance sensor RS that can be removed and replaced by the researcher.
The researcher considers two separate modifications aimed at increasing the brightness of bulb L:
Modification I: Place a resistor RP in parallel with the sensor RS.
Modification II: Remove a separate resistor RX that was originally in series with both RL and RS elsewhere in the circuit.
Assume RP and RX are both comparable in magnitude to RS and RL. Neglect any temperature effects on resistance.
(a) For each modification, describe qualitatively how the total external resistance seen by the power supply would change. Provide clear reasoning that refers to how the resistors are connected.
(b) For each modification, predict the qualitative change (increase, decrease, or no change) in the current passing through bulb L. Justify your predictions in terms of the changes identified in part (a).
(c) Using the relationships among current, potential difference, and power dissipation, argue which modification—if implemented separately—would lead to the greater increase in brightness of bulb L. State any additional assumptions you must make and defend their reasonableness.
(d) Discuss how the non-negligible internal resistance of the power supply influences your comparison in part (c), and explain whether it could alter which modification is preferable.