Home Physics Kinetic Theory of Gases MHT CET Previous Years Question Two perfectly black spheres and having rad…
Physics Kinetic Theory of Gases MHT CET Previous Years Question Single Correct MCQ
Published on: September 12, 2026

Two perfectly black spheres and having radii and are maintained at temperatures and respectively. The ratio of the energy radiated by to that by is

A
B

C
D

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Text Solution

Verified by Experts
The correct answer is:
B
To calculate the ratio of energy radiated by two black bodies, we use the Stefan-Boltzmann Law, which states:
  • The energy radiated per unit area of a black body is given by E = \sigma T^4, where \sigma is the Stefan-Boltzmann constant and T is the absolute temperature of the body in Kelvin.
  • Assuming the spheres are black bodies, the total energy emitted by each sphere is given by Q = A E, where A is the surface area, which depends on the radius r as A = 4\pi r^2.
For Sphere 1 with radius \( r_1 \) and temperature \( T_1 \) and Sphere 2 with radius \( r_2 \) and temperature \( T_2 \):
  • Radius of Sphere 1 = 2 cm = 0.02 m
  • Radius of Sphere 2 = 8 cm = 0.08 m
  • Temperature of Sphere 1 = 127 °C = 400 K
  • Temperature of Sphere 2 = 527 °C = 800 K
Energy emitted by Sphere 1: $$Q_1 = (4\pi (0.02)^2) \sigma (400)^4$$ Energy emitted by Sphere 2: $$Q_2 = (4\pi (0.08)^2) \sigma (800)^4$$ Now, taking the ratio of the two: $$\frac{Q_1}{Q_2} = \frac{(0.02)^2 (400)^4}{(0.08)^2 (800)^4}$$ This simplifies to: $$\frac{Q_1}{Q_2} = \frac{(0.04) (6.4 \times 10^{10})}{(0.64) (4.096 \times 10^{10})} = \frac{1}{2}$$ Thus, the ratio of energy radiated by Sphere 1 to Sphere 2 is \(1:2\).
Therefore, the correct option is B.

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