Question

Unit of emissive power of any body is _________
A. Unitless
B. $\dfrac{{J{s^{ - 1}}}}{{{m^2}}}$
C. Watt
D. None

Answer 1

The ability of a material's surface to emit energy as thermal radiation is measured by its emissivity. Thermal radiation is electromagnetic radiation that includes both visible (light) and infrared (infrared) wavelengths that are invisible to the naked eye. Thermal radiation from extremely hot objects (as seen in the photo) is clearly apparent to the naked eye.

Complete step by step solution:
The Stefan–Boltzmann equation defines emissivity as the ratio of thermal radiation from a surface to the radiation from an ideal black surface at the same temperature. The ratio ranges between 0 and 1. At ambient temperature (25 °C, 298.15 K), the surface of a perfect black body (with an emissivity of 1) emits roughly 448 watts per square metre of thermal radiation; all actual things have emissivities less than 1.0 and emit radiation at proportionally lower rates.
The quantity of radiant energy released by a body at a particular temperature per unit time per unit surface area of the body is known as its emissive power. If 'Q' is the quantity of radiant energy emitted, 'A' is the body's surface area, and 't' is the period of time the body radiates energy, the emissive power equals
$E = \dfrac{Q}{{At}}$
The total quantity of radiation released by a body per unit time and unit area is known as its emissive power. It's measured in W/m. Watt is measured in J/s, thus the total may be represented as $\dfrac{{J{s^{ - 1}}}}{{{m^2}}}$.
Hence option B is correct.

Note:
The total quantity of thermal energy released per unit area per unit time for all conceivable wavelengths is known as emittance (or emissive power). At a particular temperature, emissivity is defined as the ratio of a body's total emissive power to the total emissive power of a completely black body at that temperature. The total energy radiated grows with temperature, according to Planck's law, while the emission spectrum's peak changes to shorter wavelengths. Shorter wavelengths emit more energy, which increases more quickly as temperature rises.