What is real body?

Short Answer:

A real body is an object that does not absorb or emit all the radiation incident on it. Unlike a black body, a real body absorbs only a part of the radiation and reflects or transmits the rest. The radiation emitted by a real body is always less than that of a black body at the same temperature. The ratio of energy emitted by a real body to that by a black body at the same temperature is known as emissivity.

In practical situations, all materials and surfaces are real bodies since no object can perfectly absorb or emit radiation. Examples include metals, wood, water, and paints, which all have different emissivity values. The concept of a real body is important in thermal engineering for calculating heat transfer by radiation in real-life systems such as furnaces, heat exchangers, and engines.

Detailed Explanation :

Real Body

A real body is any physical object that emits, absorbs, and reflects thermal radiation but not perfectly. In other words, it behaves differently from an ideal black body, which absorbs all incident radiation. In real bodies, the absorption of radiation depends on the nature of the surface, the wavelength of the radiation, and the temperature of the surface. For example, a dull black surface absorbs more radiation than a shiny or polished surface.

When a real body is heated, it emits thermal radiation, but the amount of radiation is always less than that emitted by a black body at the same temperature. The energy radiated per unit area of a real body is expressed as:

where:

  •  = emissive power of the real body (W/m²)
  •  = emissivity of the body (a number between 0 and 1)
  •  = Stefan–Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴)
  •  = absolute temperature of the surface (K)

This equation is known as the Stefan–Boltzmann law for real bodies.

Characteristics of a Real Body

  1. Partial Absorption:
    A real body absorbs only a portion of the incident radiation. The remaining energy is reflected or transmitted.
  2. Emission Depends on Surface:
    The emissivity of a real body depends on the material and surface finish. Rough and dark surfaces generally have higher emissivity, while smooth and shiny surfaces have lower emissivity.
  3. Temperature Dependence:
    The emissive power of a real body increases with temperature. However, it is still less than that of a black body at the same temperature.
  4. Spectral Variation:
    A real body does not emit uniformly over all wavelengths. Its radiation varies with wavelength and temperature.
  5. Realistic Behavior:
    All natural and industrial materials behave as real bodies. Even highly polished metals are not perfect reflectors or absorbers.

Emissivity and Its Importance

The emissivity of a real body is a measure of how efficiently it emits thermal radiation compared to a black body. It is defined as:

where  is the emissive power of the real body and  is that of a black body at the same temperature.

  • For a black body, .
  • For a real body, .
  • For example, the emissivity of polished aluminum is around 0.05, while for matte black paint it is about 0.95.

The emissivity plays a key role in radiation heat transfer analysis. Engineers use it to calculate energy losses in furnaces, boilers, and spacecraft surfaces, where accurate heat emission data are crucial for performance and safety.

Comparison Between Black Body and Real Body

  • A black body absorbs all radiation and emits maximum possible energy.
  • A real body absorbs and emits only a part of the radiation.
  • Therefore, real bodies are practical representations of actual materials, while the black body serves as a theoretical standard for radiation calculations.

For example, a red-hot iron surface emits visible radiation but still reflects some of the incident energy, showing that it is not a perfect emitter. Similarly, the Earth’s surface behaves as a real body, absorbing solar energy and emitting infrared radiation.

Applications of Real Body Concept

  1. Thermal Engineering: Used in analyzing heat exchangers, furnaces, and engine cylinders.
  2. Space Technology: Helps design satellite surfaces with proper emissivity to control temperature.
  3. Infrared Devices: Understanding emissivity is essential for accurate temperature measurement using infrared thermometers.
  4. Building Design: Helps in determining the heat radiation characteristics of construction materials.
Conclusion

A real body is a practical and realistic concept that represents all physical materials which neither perfectly absorb nor perfectly emit thermal radiation. The amount of radiation emitted by a real body is always less than that from a black body at the same temperature. The behavior of a real body depends on its surface condition, material type, and temperature. Understanding real body properties and emissivity is essential for accurate thermal calculations and efficient heat transfer design in engineering applications.