When heat is radiated from a hot source, the amount of heat received by a nearby body depends on its emissivity.

Question

BlackTom AI · Accepted Answer

Absorptivity (how much incident radiation a surface takes in) is the direct property that determines the heat a body receives from an external radiating source. In radiative heat transfer the absorbed power is
Q_abs = A ∫α(λ,θ) G(λ,θ) dλ dΩ,
where α(λ,θ) is spectral-directional absorptance, G is the incident spectral irradiance, A is area. Emissivity ε(λ,θ) appears in the emitted power from a body at its own temperature: E = ε(λ,θ)·σT^4 (spectrally and directionally resolved).

Kirchhoff’s law of thermal radiation states that, for a body in thermodynamic equilibrium, at each wavelength and direction α(λ,θ) = ε(λ,θ). That equality is a conditional, wavelength- and direction-specific identity derived from detailed balance — it does not mean “emissivity always controls absorption” in every practical situation. The heat actually absorbed from a nearby hot source depends on the absorptivity of the receiver at the source’s emission wavelengths, the incident flux (set by source temperature, distance, view factor and geometry), and any reflection or transmission — not directly on the receiver’s emission behaviour at its own temperature.

Step-by-step reasoning:
1. Incident radiative flux from the hot source is determined by source temperature, solid angle/view factor and distance.
2. The receiver absorbs a fraction α of that incident flux; Q_abs = α·(incident flux) integrated over wavelengths.
3. Emissivity ε describes how much radiation the receiver itself emits when hot; it only equals α under the specific conditions of Kirchhoff’s law for matching wavelength/direction in equilibrium.
4. Therefore saying “the amount of heat received depends on its emissivity” as a blanket statement is incorrect.

Why “True” is incorrect: it conflates emissivity with absorptivity without mentioning the spectral/directional conditions required for Kirchhoff’s law. Other factors (geometry, distance, spectral match) matter. Conclusion: the correct answer is False, which matches the provided answer.

When heat is radiated from a hot source, the amount of heat received by a nearby body depends on its emissivity.判断题

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