Thermal Radiation Quiz 1
Test your understanding of thermal radiation, the transfer of energy through electromagnetic waves. This medium-level quiz covers emission, absorption, blackbody behavior, radiation laws, and practical applications.
Choose the best answer for each question.
Q1. Which statement best distinguishes thermal radiation from conduction and convection?
Thermal radiation is electromagnetic energy emitted by matter and can travel through a vacuum, unlike conduction and convection.
Q2. A hot metal object appears red because it emits a significant amount of radiation in which region of the electromagnetic spectrum?
As the temperature of a body becomes sufficiently high, part of its thermal emission enters the visible region, producing a red or orange glow.
Q3. According to the Stefan–Boltzmann law, the total radiant power emitted per unit area by an ideal blackbody is proportional to:
The Stefan–Boltzmann law states that blackbody emissive power is proportional to the fourth power of absolute temperature, ##E = sigma T^4##.
Q4. If the absolute temperature of a blackbody is doubled, its emitted power per unit area becomes:
Because emission varies as ##T^4##, doubling the absolute temperature increases the emitted power by ##2^4 = 16## times.
Q5. Which surface generally has the greatest emissivity for thermal radiation?
Dull black surfaces are usually strong absorbers and strong emitters, giving them high emissivity compared with polished metallic surfaces.
Q6. A surface has an absorptivity of 0.75 at a particular wavelength and temperature. Under thermal equilibrium conditions, its emissivity at that wavelength is:
Kirchhoff's law states that, at thermal equilibrium, emissivity equals absorptivity for the same wavelength and temperature.
Q7. What does Wien's displacement law predict when the temperature of a blackbody increases?
Wien's law gives ##lambda_max T = constant##, so a higher temperature shifts the peak emission toward shorter wavelengths.
Q8. The Sun's radiation reaches Earth mainly by which mode of heat transfer?
The space between the Sun and Earth is nearly a vacuum, so electromagnetic radiation is the primary way solar energy travels between them.
Q9. Why do polished metallic surfaces reduce radiative heat transfer?
Polished metals reflect much of the incident radiation and usually have low emissivity, reducing both their emission and absorption of thermal radiation.
Q10. A body emits radiation at a rate of 500 W while its surroundings emit 300 W toward it. What is the net radiative energy loss from the body?
The net radiative loss is the body's emission minus the absorbed incoming radiation, so ##500 - 300 = 200## W.
Q11. Which quantity describes the fraction of incident radiation absorbed by a surface?
Absorptivity is the fraction of incident radiant energy absorbed by a surface. Reflectivity and transmissivity describe reflected and transmitted fractions.
Q12. For an opaque surface, if its absorptivity is 0.30, its reflectivity is:
For an opaque surface, transmissivity is zero, so absorptivity plus reflectivity equals one. Therefore reflectivity is ##1 - 0.30 = 0.70##.
Q13. A blackbody is best described as an object that:
An ideal blackbody absorbs all incident radiation and, at a given temperature, emits the maximum possible thermal radiation across all wavelengths.
Q14. Two objects have the same surface area and temperature, but one has emissivity 0.4 and the other has emissivity 0.8. Assuming identical surroundings, the object with emissivity 0.8 will generally:
For the same area and temperature, emitted power is proportional to emissivity. Thus, 0.8 is twice 0.4, so the emitted power is approximately doubled.
Q15. Why can a thermal camera detect a warm object in darkness?
Warm objects emit infrared radiation, and a thermal camera detects differences in this radiation without requiring visible light.
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