JEE Challenger
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Analysis of Resistor Circuit with Load Resistance and Battery

For the circuit shown in the figure

Diagram 0

Options

A

the current II through the battery is 7.5 mA7.5\text{ mA}

Correct
B

the potential difference across RLR_{\text{L}} is 18 V18\text{ V}

C

ratio of powers dissipated in R1R_1 and R2R_2 is 33

D

if R1R_1 and R2R_2 are interchanged, magnitude of the power dissipated in RLR_{\text{L}} will decrease by a factor of 99

Correct

Step-by-Step Solution

To determine the correct statements, we analyze the circuit step-by-step:

1. Initial Circuit Analysis

From the circuit diagram:

  • The battery voltage is V=24 VV = 24\text{ V}.
  • The resistors are R1=2 kΩR_1 = 2\text{ k}\Omega, R2=6 kΩR_2 = 6\text{ k}\Omega, and the load resistor is RL=1.5 kΩR_{\text{L}} = 1.5\text{ k}\Omega.
  • The resistor R2R_2 is in parallel with the load resistor RLR_{\text{L}}, and this combination is connected in series with R1R_1.

The equivalent resistance of the parallel combination of R2R_2 and RLR_{\text{L}} is:
R2∥L=R2RLR2+RL=6×1.56+1.5=97.5=1.2 kΩR_{2\parallel\text{L}} = \frac{R_2 R_{\text{L}}}{R_2 + R_{\text{L}}} = \frac{6 \times 1.5}{6 + 1.5} = \frac{9}{7.5} = 1.2\text{ k}\Omega

The total equivalent resistance of the circuit connected to the battery is:
Req=R1+R2∥L=2 kΩ+1.2 kΩ=3.2 kΩR_{\text{eq}} = R_1 + R_{2\parallel\text{L}} = 2\text{ k}\Omega + 1.2\text{ k}\Omega = 3.2\text{ k}\Omega

Current through the battery:

I=VReq=24 V3.2 kΩ=7.5 mAI = \frac{V}{R_{\text{eq}}} = \frac{24\text{ V}}{3.2\text{ k}\Omega} = 7.5\text{ mA}
Hence, Option (A) is correct.

Potential difference across RLR_{\text{L}}:

The voltage across RLR_{\text{L}} is the same as the voltage across the parallel combination:
VL=I×R2∥L=7.5 mA×1.2 kΩ=9 VV_{\text{L}} = I \times R_{2\parallel\text{L}} = 7.5\text{ mA} \times 1.2\text{ k}\Omega = 9\text{ V}
Since VL=9 V≠18 VV_{\text{L}} = 9\text{ V} \neq 18\text{ V}, Option (B) is incorrect.

Ratio of powers dissipated in R1R_1 and R2R_2:

The voltage across R1R_1 is:
V1=V−VL=24 V−9 V=15 VV_1 = V - V_{\text{L}} = 24\text{ V} - 9\text{ V} = 15\text{ V}

The power dissipated in R1R_1 is:
P1=V12R1=1522 kΩ=2252 mW=112.5 mWP_1 = \frac{V_1^2}{R_1} = \frac{15^2}{2\text{ k}\Omega} = \frac{225}{2}\text{ mW} = 112.5\text{ mW}

The power dissipated in R2R_2 is:
P2=VL2R2=926 kΩ=816 mW=13.5 mWP_2 = \frac{V_{\text{L}}^2}{R_2} = \frac{9^2}{6\text{ k}\Omega} = \frac{81}{6}\text{ mW} = 13.5\text{ mW}

The ratio of powers is:
P1P2=112.513.5=253≠3\frac{P_1}{P_2} = \frac{112.5}{13.5} = \frac{25}{3} \neq 3
Hence, Option (C) is incorrect.


2. When R1R_1 and R2R_2 are Interchanged

Now, the values are:

  • R1′=6 kΩR_1' = 6\text{ k}\Omega
  • R2′=2 kΩR_2' = 2\text{ k}\Omega
  • RL=1.5 kΩR_{\text{L}} = 1.5\text{ k}\Omega

The new equivalent resistance of the parallel combination of R2′R_2' and RLR_{\text{L}} is:
R2′∥L=R2′RLR2′+RL=2×1.52+1.5=33.5=67 kΩR_{2'\parallel\text{L}} = \frac{R_2' R_{\text{L}}}{R_2' + R_{\text{L}}} = \frac{2 \times 1.5}{2 + 1.5} = \frac{3}{3.5} = \frac{6}{7}\text{ k}\Omega

The total equivalent resistance becomes:
Req′=R1′+R2′∥L=6+67=487 kΩR_{\text{eq}}' = R_1' + R_{2'\parallel\text{L}} = 6 + \frac{6}{7} = \frac{48}{7}\text{ k}\Omega

Using the voltage divider formula, the new potential difference across RLR_{\text{L}} is:
VL′=V×R2′∥LReq′=24 V×67487=24×648=3 VV_{\text{L}}' = V \times \frac{R_{2'\parallel\text{L}}}{R_{\text{eq}}'} = 24\text{ V} \times \frac{\frac{6}{7}}{\frac{48}{7}} = 24 \times \frac{6}{48} = 3\text{ V}

The ratio of the new power dissipated in RLR_{\text{L}} to the initial power is:
PL′PL=(VL′)2RLVL2RL=(VL′VL)2=(3 V9 V)2=(13)2=19\frac{P_{\text{L}}'}{P_{\text{L}}} = \frac{\frac{(V_{\text{L}}')^2}{R_{\text{L}}}}{\frac{V_{\text{L}}^2}{R_{\text{L}}}} = \left(\frac{V_{\text{L}}'}{V_{\text{L}}}\right)^2 = \left(\frac{3\text{ V}}{9\text{ V}}\right)^2 = \left(\frac{1}{3}\right)^2 = \frac{1}{9}

Thus, the power dissipated in RLR_{\text{L}} decreases by a factor of 99.
Hence, Option (D) is correct.


Conclusion

The correct options are A and D.