Heat transfer by radiation MCQs With Answer

Heat transfer by radiation MCQs With Answer

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Heat transfer by radiation MCQs With Answer is a focused set of practice questions designed for B. Pharm students to master radiative heat transfer concepts relevant to pharmaceutical processes. This introduction covers fundamental ideas like blackbody radiation, emissivity, Stefan–Boltzmann law, spectral dependence, view factors, and radiative exchange in vacuum or participating media. Emphasis is placed on applications in drying, sterilization, lyophilization chambers, and thermal stability of drug formulations where radiation controls energy balances. Clear, exam-style MCQs with concise options help reinforce theory and calculation skills for practical pharmaceutical engineering problems. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the definition of a blackbody in heat transfer by radiation?

  • An object that reflects all incident radiation perfectly
  • An object that transmits all incident radiation without absorption
  • An idealized object that absorbs all incident radiation and emits maximum possible radiation at a given temperature
  • A real material with emissivity greater than 1

Correct Answer: An idealized object that absorbs all incident radiation and emits maximum possible radiation at a given temperature

Q2. Which law gives the total radiant energy emitted per unit area of a blackbody as a function of temperature?

  • Kirchhoff’s law
  • Planck’s law
  • Stefan–Boltzmann law
  • Lambert’s cosine law

Correct Answer: Stefan–Boltzmann law

Q3. The Stefan–Boltzmann law for a blackbody is E = σT^4. What does σ represent?

  • Stefan–Boltzmann constant
  • Emissivity of surface
  • Wien’s displacement constant
  • Planck’s constant

Correct Answer: Stefan–Boltzmann constant

Q4. Emissivity of a real surface is defined as:

  • The ratio of reflected to incident radiation
  • The ratio of emitted radiation of the surface to that of a blackbody at same temperature
  • The amount of transmitted radiation through a material
  • The absolute radiative flux from the surface

Correct Answer: The ratio of emitted radiation of the surface to that of a blackbody at same temperature

Q5. Which law describes the spectral distribution of blackbody radiation?

  • Lambert’s cosine law
  • Planck’s law
  • Kirchhoff’s law
  • Newton’s law of cooling

Correct Answer: Planck’s law

Q6. Wien’s displacement law relates the peak wavelength of emission to temperature. As temperature increases, the peak wavelength:

  • Shifts to longer wavelengths
  • Shifts to shorter wavelengths
  • Remains constant
  • Oscillates periodically

Correct Answer: Shifts to shorter wavelengths

Q7. Kirchhoff’s law of thermal radiation states that, for a body in thermal equilibrium, emissivity equals:

  • Reflectivity at all wavelengths
  • Absorptivity at the same wavelength and temperature
  • Transmissivity at the same wavelength
  • Emissive power divided by temperature

Correct Answer: Absorptivity at the same wavelength and temperature

Q8. For a gray body, emissivity is assumed to be:

  • Dependent on wavelength and direction
  • Constant and less than or equal to 1 across all wavelengths
  • Greater than 1 at high temperatures
  • Zero for all wavelengths

Correct Answer: Constant and less than or equal to 1 across all wavelengths

Q9. Which parameter quantifies the fraction of incident radiation absorbed by a surface?

  • Emissivity
  • Reflectivity
  • Absorptivity
  • Transmissivity

Correct Answer: Absorptivity

Q10. Lambert’s cosine law pertains to:

  • The directional distribution of emitted radiation from a diffuse surface
  • The spectral variation of emissivity
  • The relation between temperature and emissive power
  • The conservation of energy in radiative exchange

Correct Answer: The directional distribution of emitted radiation from a diffuse surface

Q11. Radiosity from a surface is defined as:

  • The net radiative heat received by the surface
  • The total radiation leaving the surface including emitted and reflected components
  • The fraction of radiation transmitted through a surface
  • The monochromatic emission at a single wavelength

Correct Answer: The total radiation leaving the surface including emitted and reflected components

Q12. View factor (shape factor) between two surfaces is:

  • The fraction of radiation leaving one surface that strikes the other
  • The emissivity product of two surfaces
  • The difference in radiator temperatures
  • The spectral absorptivity ratio

Correct Answer: The fraction of radiation leaving one surface that strikes the other

Q13. In vacuum or transparent medium with no absorption, radiative heat transfer between surfaces depends mainly on:

  • Conduction coefficients
  • View factors, surface emissivities, and temperature differences
  • Convective heat transfer coefficients
  • Mass transport of gases

Correct Answer: View factors, surface emissivities, and temperature differences

Q14. The net radiative heat exchange between two infinite parallel black plates at temperatures T1 and T2 is given by:

  • σ(T1 + T2)
  • σ(T1^4 – T2^4)
  • σ(T1^2 – T2^2)
  • σ(T1^3 – T2^3)

Correct Answer: σ(T1^4 – T2^4)

Q15. Which of the following best describes a gray surface?

  • A surface with emissivity varying strongly with wavelength
  • A surface with reflectivity equal to emissivity
  • A surface with constant emissivity independent of wavelength over the considered range
  • A surface that transmits radiation perfectly

Correct Answer: A surface with constant emissivity independent of wavelength over the considered range

Q16. For non-black surfaces, effective emissive power is given by εσT^4 where ε is:

  • Emissivity of the surface
  • Absorptivity of the surface at all wavelengths
  • Reflectivity of the surface
  • Transmissivity of the surface

Correct Answer: Emissivity of the surface

Q17. In pharmaceutical dryers using radiant heaters, increasing emissivity of product surfaces will:

  • Decrease radiative heat absorption by products
  • Increase radiative heat absorption and speed drying
  • Have no effect on radiative heat transfer
  • Only affect convective drying, not radiative

Correct Answer: Increase radiative heat absorption and speed drying

Q18. Which statement is true about spectral emissivity?

  • It is independent of wavelength
  • It describes emissivity as a function of wavelength
  • It equals absorptivity only for transparent materials
  • It can exceed 1 at some wavelengths

Correct Answer: It describes emissivity as a function of wavelength

Q19. Planck’s law predicts that at a given temperature, the spectral radiance of a blackbody:

  • Is uniform over all wavelengths
  • Has a peak at a certain wavelength and decays on either side
  • Increases linearly with wavelength
  • Depends only on emissivity

Correct Answer: Has a peak at a certain wavelength and decays on either side

Q20. Which constant appears in Wien’s displacement law?

  • Stefan–Boltzmann constant
  • Wien’s displacement constant (b)
  • Planck’s constant divided by Boltzmann constant
  • Specific heat capacity

Correct Answer: Wien’s displacement constant (b)

Q21. Radiative heat transfer between surfaces with participating media (e.g., air with smoke) requires accounting for:

  • Only surface emissivities
  • Absorption, emission, and scattering within the medium
  • Only conduction across the medium
  • Only convective coefficients at surfaces

Correct Answer: Absorption, emission, and scattering within the medium

Q22. Kirchhoff’s law implies that a good absorber at a given wavelength is also a good:

  • Conductor at that wavelength
  • Reflector at that wavelength
  • Emitter at that wavelength
  • Transmitter at that wavelength

Correct Answer: Emitter at that wavelength

Q23. Which of these quantities is dimensionally same as radiative heat flux?

  • Temperature (K)
  • W/m^2
  • J/kg
  • m/s

Correct Answer: W/m^2

Q24. In radiation exchange between two concentric spheres, the view factor from inner sphere to outer sphere is:

  • Zero
  • One
  • Equal to the ratio of radii
  • Dependent on temperature only

Correct Answer: One

Q25. Which method is commonly used to account for surface properties and geometry in radiative heat exchange problems?

  • Finite element conduction only
  • Use of view factors and surface emissivities
  • Assuming convective coefficients are dominant
  • Mass transfer correlations

Correct Answer: Use of view factors and surface emissivities

Q26. A real surface at temperature T has emissive power E = εσT^4. If emissivity ε = 0.6 and T doubles, the emitted power changes by a factor of:

  • 2
  • 4
  • 8
  • 16

Correct Answer: 16

Q27. The net radiative heat transfer from a surface depends on which of the following?

  • Only its emissivity
  • Emissivity, surrounding surface temperatures, and view factors
  • Only its temperature
  • Only the medium’s conductivity

Correct Answer: Emissivity, surrounding surface temperatures, and view factors

Q28. Which principle explains why spectral absorptivity equals spectral emissivity at thermal equilibrium?

  • First law of thermodynamics
  • Kirchhoff’s radiation law
  • Lambert’s cosine law
  • Conservation of mass

Correct Answer: Kirchhoff’s radiation law

Q29. In pharmaceutical sterilization by infrared radiation, depth of penetration depends primarily on:

  • Wavelength and material optical properties
  • Ambient pressure only
  • Convection around the product
  • Mass of the product alone

Correct Answer: Wavelength and material optical properties

Q30. A surface with emissivity 0.9 is heated to 400 K. Which expression gives its emitted power per unit area?

  • 0.9 σ (400)^4
  • σ (400)^4
  • 0.1 σ (400)^4
  • 9 σ (400)^4

Correct Answer: 0.9 σ (400)^4

Q31. In enclosure radiation problems, energy balance for a surface commonly includes which terms?

  • Only conduction into the surface
  • Emission, reflection, and irradiation
  • Only convective heat transfer
  • Only stored thermal energy

Correct Answer: Emission, reflection, and irradiation

Q32. Which expression denotes monochromatic emissive power?

  • Eλ: emissive power per unit wavelength
  • E: total emissive power over all wavelengths
  • q: convective heat flux
  • k: thermal conductivity

Correct Answer: Eλ: emissive power per unit wavelength

Q33. For two infinite parallel plates with emissivities ε1 and ε2, the net exchange factor includes term:

  • 1/(1/ε1 + 1/ε2 – 1)
  • ε1 + ε2
  • (ε1 – ε2)
  • 1/ε1 + 1/ε2

Correct Answer: 1/(1/ε1 + 1/ε2 – 1)

Q34. Radiative heat transfer is most significant compared to conduction and convection at:

  • Low temperatures and small surfaces
  • Very high temperatures and large temperature differences
  • When fluids are highly conductive
  • Only in liquids

Correct Answer: Very high temperatures and large temperature differences

Q35. The radiosity-irradiation relationship for a diffuse gray surface is given by:

  • J = εEb + (1 − ε)G, where J is radiosity and G is irradiation
  • J = G − εEb
  • J = εG + (1 − ε)Eb
  • J = Eb/G

Correct Answer: J = εEb + (1 − ε)G, where J is radiosity and G is irradiation

Q36. Which is true for a perfectly diffuse emitter?

  • Emission intensity is independent of direction and follows Lambert’s law
  • Emission is stronger in specular directions only
  • It transmits all incident radiation
  • Emissivity varies strongly with angle

Correct Answer: Emission intensity is independent of direction and follows Lambert’s law

Q37. The unit of emissive power is:

  • W
  • W/m
  • W/m^2
  • W/m^3

Correct Answer: W/m^2

Q38. Which phenomenon reduces net radiative heat transfer between two surfaces?

  • Increasing surface emissivities
  • Adding a low-emissivity radiation shield between them
  • Increasing view factor
  • Raising temperature of the hotter surface

Correct Answer: Adding a low-emissivity radiation shield between them

Q39. In the presence of a semitransparent medium, radiative transport equation must consider:

  • Only boundary emission
  • Absorption coefficient, emission term, and scattering term
  • Only Planck’s function at surfaces
  • Only geometric view factors

Correct Answer: Absorption coefficient, emission term, and scattering term

Q40. The inverse-square law for radiation intensity applies to:

  • Large extended surfaces in close proximity
  • Point sources where intensity decreases with square of distance
  • Blackbody emission regardless of geometry
  • Enclosure radiative exchange with view factors

Correct Answer: Point sources where intensity decreases with square of distance

Q41. Which of these is NOT a typical application of radiative heat transfer in pharmaceutical engineering?

  • Infrared drying of granules
  • Sterilization using radiant heaters
  • Convection-only mixing in fermenters
  • Heat shielding in lyophilization chambers

Correct Answer: Convection-only mixing in fermenters

Q42. Spectral transmissivity of a material equals:

  • 1 − spectral absorptivity − spectral reflectivity
  • Spectral absorptivity times spectral emissivity
  • Spectral emissivity alone
  • Always zero for opaque materials

Correct Answer: 1 − spectral absorptivity − spectral reflectivity

Q43. Which of the following increases radiative heat transfer between a hot surface and the surroundings?

  • Decreasing surface temperature
  • Using a polished low-emissivity surface
  • Increasing surface emissivity and temperature
  • Placing a reflective shield nearby

Correct Answer: Increasing surface emissivity and temperature

Q44. For a diffuse-gray enclosure, which numerical method is often used to solve complex radiative exchange?

  • Monte Carlo ray tracing
  • Simple Fourier transform
  • Laplace transforms
  • Reynolds averaging

Correct Answer: Monte Carlo ray tracing

Q45. When designing an infrared sterilizer, which property of the target material is most important to know?

  • Electrical conductivity
  • Spectral absorptivity at sterilizer wavelengths
  • Bulk modulus
  • Magnetic permeability

Correct Answer: Spectral absorptivity at sterilizer wavelengths

Q46. Which of the following statements about radiative heat transfer in vacuum is correct?

  • Radiation cannot occur in vacuum
  • Only conduction is possible in vacuum
  • Radiation is the dominant mode of heat transfer in vacuum between surfaces
  • Convection dominates in vacuum

Correct Answer: Radiation is the dominant mode of heat transfer in vacuum between surfaces

Q47. The emissive power of a blackbody at temperature T doubles when:

  • Temperature is multiplied by sqrt(2)
  • Temperature is doubled
  • Temperature is multiplied by fourth root of 2 (2^(1/4))
  • Temperature is halved

Correct Answer: Temperature is multiplied by fourth root of 2 (2^(1/4))

Q48. Which parameter would you change to reduce radiation heat loss from a heated vessel?

  • Increase vessel surface emissivity
  • Decrease vessel surface temperature
  • Remove insulation
  • Increase view factor to cold surroundings

Correct Answer: Decrease vessel surface temperature

Q49. In evaluating radiation heat flux from a surface, which environmental factor matters most?

  • Surrounding surface temperatures and emissivities
  • Ambient sound level
  • Air humidity only (if transparent and non-participating)
  • Gravitational acceleration

Correct Answer: Surrounding surface temperatures and emissivities

Q50. When two surfaces are small and far apart compared to their sizes, the view factor between them approximates:

  • Zero
  • One
  • Infinite
  • Equal to emissivity product

Correct Answer: Zero

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