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Ratio of Electric Flux Through Concentric Spheres

Two point charges 8 μC8\text{ }\mu\text{C} and 2 μC-2\text{ }\mu\text{C} are located at x=2 cmx = 2\text{ cm} and x=4 cmx = 4\text{ cm}, respectively on the xx-axis. The ratio of electric flux due to these charges through two spheres of radii 3 cm3\text{ cm} and 5 cm5\text{ cm} with their centers at the origin is _____.

Options

A

4:14 : 1

B

3:43 : 4

C

4:34 : 3

Correct
D

4:54 : 5

Step-by-Step Solution

To determine the ratio of electric flux through the two concentric spheres, we apply Gauss's Law, Φ=Qencε0\Phi = \frac{Q_{\text{enc}}}{\varepsilon_0}, where QencQ_{\text{enc}} is the total charge enclosed within a sphere centered at the origin.

  1. For the first sphere of radius R1=3 cmR_1 = 3\text{ cm}, only the charge q1=8 μCq_1 = 8\text{ }\mu\text{C} at x=2 cmx = 2\text{ cm} lies inside the sphere, so Qenc,1=8 μCQ_{\text{enc}, 1} = 8\text{ }\mu\text{C}.
  2. For the second sphere of radius R2=5 cmR_2 = 5\text{ cm}, both charges q1=8 μCq_1 = 8\text{ }\mu\text{C} at x=2 cmx = 2\text{ cm} and q2=2 μCq_2 = -2\text{ }\mu\text{C} at x=4 cmx = 4\text{ cm} lie inside the sphere, yielding Qenc,2=8 μC+(2 μC)=6 μCQ_{\text{enc}, 2} = 8\text{ }\mu\text{C} + (-2\text{ }\mu\text{C}) = 6\text{ }\mu\text{C}.

Taking the ratio of the enclosed charges gives: Φ1Φ2=Qenc,1Qenc,2=86=43\frac{\Phi_1}{\Phi_2} = \frac{Q_{\text{enc}, 1}}{Q_{\text{enc}, 2}} = \frac{8}{6} = \frac{4}{3}

Thus, the ratio of electric flux through the two spheres is 4:34 : 3, which corresponds to Option C.

Ratio of Electric Flux Through Concentric Spheres | Physics PYQ Solution - JEE Challenger