Falling sphere viscometer MCQs With Answer

Falling sphere viscometer MCQs With Answer

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Introduction: The falling sphere viscometer MCQs with answer collection helps B. Pharm students master viscosity measurement using the falling (or falling ball) viscometer, a key tool in pharmaceutical formulation and quality control. This concise, SEO-friendly introduction covers principles like Stokes’ law, terminal velocity, Reynolds number, wall-correction factors, and practical considerations for suspensions, syrups, and non-Newtonian fluids. Ideal for exam prep, lab work, and concept reinforcement, these MCQs emphasize calculations, instrument setup, error sources, and interpretation of results. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What physical principle primarily governs the falling sphere viscometer for Newtonian fluids?

  • Bernoulli’s principle
  • Stokes’ law relating drag to viscosity
  • Hooke’s law for elastic deformation
  • Fick’s law of diffusion

Correct Answer: Stokes’ law relating drag to viscosity

Q2. In the ideal Stokes regime, terminal velocity of a sphere is achieved when which two forces balance?

  • Buoyancy and surface tension
  • Gravitational and viscous drag
  • Inertial and magnetic forces
  • Centrifugal and Coriolis forces

Correct Answer: Gravitational and viscous drag

Q3. Which formula gives the dynamic viscosity η for a sphere falling at terminal velocity in the Stokes regime?

  • η = (2/9) * r^2 * (ρ_s – ρ_f) * g / v_t
  • η = 6πr v_t / (ρ_s – ρ_f)
  • η = v_t / (r^2 (ρ_s – ρ_f) g)
  • η = (ρ_f – ρ_s) g / (6πr v_t)

Correct Answer: η = (2/9) * r^2 * (ρ_s – ρ_f) * g / v_t

Q4. In the Stokes formula η = (2/9) r^2 (ρ_s − ρ_f) g / v_t, what does ρ_s represent?

  • Density of the falling sphere
  • Density of the fluid
  • Viscosity of the fluid
  • Radius of the sphere

Correct Answer: Density of the falling sphere

Q5. What is the typical Reynolds number range for valid Stokes flow (laminar creeping flow) in falling sphere viscometry?

  • Re < 1
  • Re ≈ 10–100
  • Re > 1000
  • Re between 100 and 500

Correct Answer: Re < 1

Q6. Which factor causes the need for wall-correction factors in falling sphere viscometry?

  • Thermal convection in the fluid
  • Finite container diameter affecting flow around the sphere
  • Electrical charges on the sphere surface
  • Evaporation of the test fluid

Correct Answer: Finite container diameter affecting flow around the sphere

Q7. Which correction is commonly applied to account for wall effects in a vertical cylinder viscometer?

  • Faxén or Ladenburg wall-correction factor
  • Arrhenius temperature correction
  • Henry’s law correction
  • Van’t Hoff factor

Correct Answer: Faxén or Ladenburg wall-correction factor

Q8. How does increasing temperature generally affect the viscosity measured by a falling sphere viscometer for liquids?

  • Viscosity increases with temperature
  • Viscosity decreases with temperature
  • Viscosity remains unchanged
  • Viscosity oscillates unpredictably

Correct Answer: Viscosity decreases with temperature

Q9. Which property of the sphere must be known precisely to calculate viscosity using the falling sphere method?

  • Electrical conductivity of the sphere
  • Density and radius of the sphere
  • Magnetic susceptibility of the sphere
  • Optical reflectivity of the sphere

Correct Answer: Density and radius of the sphere

Q10. If the fluid density ρ_f approaches the sphere density ρ_s, what happens to the terminal velocity and viscosity calculation?

  • Terminal velocity increases and viscosity calculation is easier
  • Terminal velocity decreases and viscosity calculation becomes unreliable due to small driving force
  • Terminal velocity is unaffected
  • Viscosity becomes independent of densities

Correct Answer: Terminal velocity decreases and viscosity calculation becomes unreliable due to small driving force

Q11. Which unit is commonly used for dynamic viscosity in pharmaceutical contexts?

  • Pascal-second (Pa·s) or milliPascal-second (mPa·s)
  • Newton (N)
  • Kilogram per mole (kg/mol)
  • Joule per Kelvin (J/K)

Correct Answer: Pascal-second (Pa·s) or milliPascal-second (mPa·s)

Q12. Why is it important to ensure the sphere reaches terminal velocity before measuring speed?

  • To avoid measuring buoyancy instead of drag
  • To ensure steady-state balance between forces so Stokes formula applies
  • To allow sphere to heat the fluid uniformly
  • To calibrate the thermometer

Correct Answer: To ensure steady-state balance between forces so Stokes formula applies

Q13. Which experimental factor can introduce systematic error if the sphere is not perfectly smooth?

  • Increased contact angle
  • Surface roughness increases drag leading to overestimation of viscosity
  • Change in sphere density
  • Reduction in gravitational acceleration

Correct Answer: Surface roughness increases drag leading to overestimation of viscosity

Q14. For non-Newtonian fluids, what limitation applies to the falling sphere viscometer?

  • It directly measures shear-thinning index accurately
  • It provides a single apparent viscosity at the shear conditions around the sphere, not a full flow curve
  • It cannot be used at all for non-Newtonian fluids
  • It measures only yield stress, not viscosity

Correct Answer: It provides a single apparent viscosity at the shear conditions around the sphere, not a full flow curve

Q15. How is the Reynolds number defined for a falling sphere experiment?

  • Re = ρ_f v_t r / η
  • Re = η / (ρ_f v_t r)
  • Re = v_t / (ρ_f r η)
  • Re = 6πr v_t / η

Correct Answer: Re = ρ_f v_t r / η

Q16. Which of the following is an advantage of falling sphere viscometry in pharmaceutical labs?

  • Requires large sample volumes only
  • Simple principle and visually verifiable terminal velocity measurement
  • Directly measures molecular weight
  • Insensitive to temperature changes

Correct Answer: Simple principle and visually verifiable terminal velocity measurement

Q17. What is the effect of wall proximity when the ratio of tube diameter D to sphere diameter d is small?

  • No effect; flow is unchanged
  • Wall increases drag on the sphere and slows terminal velocity
  • Wall decreases drag and speeds up terminal velocity
  • Wall changes sphere density

Correct Answer: Wall increases drag on the sphere and slows terminal velocity

Q18. What experimental measurement is directly observed to compute viscosity in a falling sphere viscometer?

  • Sphere rotation rate
  • Terminal velocity or descent time between marked points
  • Electrical resistance of the fluid
  • Optical density of the fluid

Correct Answer: Terminal velocity or descent time between marked points

Q19. If a 1 mm radius steel ball (ρ_s = 7800 kg/m3) falls in water (ρ_f = 1000 kg/m3) with measured v_t, which parameter must be corrected for temperature?

  • Sphere radius only
  • Fluid density and viscosity
  • Gravitational acceleration
  • Sphere density only

Correct Answer: Fluid density and viscosity

Q20. Which measurement technique can reduce random timing errors when determining terminal velocity?

  • Use manual stopwatch only
  • Use high-speed camera or electronic sensors to track descent
  • Estimate visually without timing
  • Use a heavier sphere always

Correct Answer: Use high-speed camera or electronic sensors to track descent

Q21. For accurate viscosity calculation, why must the sphere density be measured or known accurately?

  • Because sphere material determines surface tension
  • Because the buoyant force depends on density difference (ρ_s − ρ_f)
  • Because density controls electric charge
  • Because density influences optical properties only

Correct Answer: Because the buoyant force depends on density difference (ρ_s − ρ_f)

Q22. What is the primary reason falling sphere viscometers are not ideal for highly viscous pastes?

  • Pastes are Newtonian so the method is redundant
  • Spheres may not reach terminal velocity within reasonable time due to very high viscosity
  • Pastes dissolve the sphere
  • Pastes increase gravitational acceleration

Correct Answer: Spheres may not reach terminal velocity within reasonable time due to very high viscosity

Q23. When using the falling sphere method for a pharmaceutical suspension, which sample preparation step is critical?

  • Ensuring suspension has large air bubbles
  • Degassing and homogenizing to remove bubbles and settle lumps
  • Heating to boiling to sterilize
  • Adding surfactants to change sphere density

Correct Answer: Degassing and homogenizing to remove bubbles and settle lumps

Q24. If doubling the sphere radius r in the Stokes formula, how does the terminal velocity scale (assuming same viscosity and densities)?

  • v_t doubles
  • v_t increases fourfold (proportional to r^2)
  • v_t halves
  • v_t is unchanged

Correct Answer: v_t increases fourfold (proportional to r^2)

Q25. Which of the following is true about kinematic viscosity ν?

  • ν = η / ρ_f, where η is dynamic viscosity and ρ_f fluid density
  • ν = η * ρ_f
  • ν is measured in Pascals
  • ν has units of kg/m3

Correct Answer: ν = η / ρ_f, where η is dynamic viscosity and ρ_f fluid density

Q26. In the context of falling sphere viscometry, what does terminal velocity v_t refer to?

  • The initial acceleration of the sphere
  • The constant velocity attained when net force is zero
  • The maximum possible speed in vacuum
  • The speed at which the sphere dissolves

Correct Answer: The constant velocity attained when net force is zero

Q27. Which phenomenon can cause deviation from Stokes’ law at higher Reynolds numbers?

  • Electrostatic attraction
  • Onset of inertial effects and flow separation increasing drag
  • Decrease in gravitational acceleration
  • Improved laminarity

Correct Answer: Onset of inertial effects and flow separation increasing drag

Q28. For a given sphere and fluid, if measured terminal velocity is too high due to measurement error, how does this affect computed viscosity?

  • Computed viscosity will be overestimated
  • Computed viscosity will be underestimated
  • Viscosity remains correct
  • Viscosity becomes negative

Correct Answer: Computed viscosity will be underestimated

Q29. Which material property of the test liquid is needed to compute buoyancy in the falling sphere equation?

  • Viscosity only
  • Density of the liquid
  • Color of the liquid
  • pH of the liquid

Correct Answer: Density of the liquid

Q30. What is a common laboratory method to measure sphere radius accurately?

  • Weighing the sphere only
  • Optical microscopy or calibrated micrometer measurement
  • Estimating by eye
  • Measuring terminal velocity first

Correct Answer: Optical microscopy or calibrated micrometer measurement

Q31. Which of the following is a key limitation when using metallic spheres for reactive pharmaceutical liquids?

  • Metallic spheres are too light
  • Possible chemical interaction or corrosion with the liquid
  • Metallic spheres reduce fluid viscosity permanently
  • Metallic spheres eliminate wall effects

Correct Answer: Possible chemical interaction or corrosion with the liquid

Q32. In practical falling sphere setups, why are multiple trials at different sphere sizes used?

  • To vary temperature responses
  • To check consistency and apply wall-correction factors across sizes for accuracy
  • To change fluid density
  • To avoid measuring terminal velocity

Correct Answer: To check consistency and apply wall-correction factors across sizes for accuracy

Q33. Which instrument component helps maintain isothermal conditions during a falling sphere viscometry experiment?

  • Rotating motor
  • Thermostatted jacket or water bath surrounding the cylinder
  • Optical sensor only
  • Electrostatic shield

Correct Answer: Thermostatted jacket or water bath surrounding the cylinder

Q34. In pharmaceutical formulations, why is viscosity measurement important for suspensions and syrups?

  • Viscosity influences taste only
  • Viscosity affects stability, sedimentation rate, dosing accuracy, and mouthfeel
  • Viscosity determines chemical potency
  • Viscosity controls microbial growth exclusively

Correct Answer: Viscosity affects stability, sedimentation rate, dosing accuracy, and mouthfeel

Q35. Which of the following best describes terminal settling in a viscometer when sphere density is much larger than fluid density?

  • Terminal velocity is proportional to r^3
  • Terminal velocity is larger due to larger net gravitational force per volume
  • Terminal velocity becomes zero
  • Terminal velocity is independent of sphere density

Correct Answer: Terminal velocity is larger due to larger net gravitational force per volume

Q36. Which correction is applied when the sphere does not fall along the central axis of the tube?

  • Temperature correction
  • Off-axis correction or ensuring centerline release to avoid biased drag
  • pH correction
  • Density correction for air

Correct Answer: Off-axis correction or ensuring centerline release to avoid biased drag

Q37. Which experimental practice minimizes start-up acceleration errors in falling sphere tests?

  • Dropping the sphere from the very top so it accelerates throughout
  • Releasing the sphere gently at or near the measurement region after allowing initial acceleration to decay
  • Heating the sphere before release
  • Using irregularly shaped particles

Correct Answer: Releasing the sphere gently at or near the measurement region after allowing initial acceleration to decay

Q38. When measuring high-viscosity liquids, what modification can improve measurement feasibility?

  • Use smaller spheres only
  • Use larger spheres or higher density spheres to achieve measurable terminal velocity
  • Increase ambient pressure drastically
  • Use lower temperature without calibration

Correct Answer: Use larger spheres or higher density spheres to achieve measurable terminal velocity

Q39. Which assumption about the fluid is required for direct use of Stokes’ law?

  • Fluid must be inviscid
  • Fluid must be Newtonian and flow must be laminar (creeping flow)
  • Fluid must be compressible gas
  • Fluid must be a non-Newtonian yield-pseudoplastic solid

Correct Answer: Fluid must be Newtonian and flow must be laminar (creeping flow)

Q40. How does particle aggregation in suspensions affect falling sphere viscometry readings?

  • Aggregation has no effect
  • Aggregation alters apparent viscosity and can produce inconsistent descent behavior
  • Aggregation reduces fluid density only
  • Aggregation speeds up terminal velocity always

Correct Answer: Aggregation alters apparent viscosity and can produce inconsistent descent behavior

Q41. Which measurement is most useful to check for laminar flow in a falling sphere test?

  • Reynolds number calculation using measured v_t
  • Colorimetric pH test
  • Optical transmittance alone
  • Sphere hardness test

Correct Answer: Reynolds number calculation using measured v_t

Q42. Which of the following is an appropriate calibration check for a falling sphere viscometer?

  • Run the instrument with air only
  • Measure viscosity of a standard reference liquid (e.g., glycerol-water mixture) and compare with known value
  • Use an arbitrary unknown fluid
  • Skip calibration and assume accuracy

Correct Answer: Measure viscosity of a standard reference liquid (e.g., glycerol-water mixture) and compare with known value

Q43. In pharmaceutical labs, which safety concern is relevant when using falling sphere viscometers with volatile solvents?

  • Magnetic interference
  • Flammability and solvent vapor exposure requiring fume hood and grounding
  • Excessive sound production
  • Electrical shock from spheres

Correct Answer: Flammability and solvent vapor exposure requiring fume hood and grounding

Q44. What is the typical effect of adding a polymer thickener to a pharmaceutical syrup on falling sphere results?

  • Apparent viscosity decreases
  • Apparent viscosity increases and terminal velocity decreases
  • Sphere density changes to match fluid
  • No change in terminal velocity

Correct Answer: Apparent viscosity increases and terminal velocity decreases

Q45. How can one verify experimentally that a sphere has reached terminal velocity in a transparent viscometer?

  • Observe constant slope of distance vs time plot over the measurement region
  • Check sphere color change
  • Measure pH during descent
  • Time only the first millimeter of travel

Correct Answer: Observe constant slope of distance vs time plot over the measurement region

Q46. For a falling sphere test in a cylindrical tube, which geometric ratio is important to report?

  • The ratio of sphere density to fluid density
  • The ratio of tube diameter to sphere diameter (D/d)
  • The ratio of fluid pH to temperature
  • The ratio of sphere mass to gravitational constant

Correct Answer: The ratio of tube diameter to sphere diameter (D/d)

Q47. What practical step reduces evaporative concentration changes during a long falling sphere experiment?

  • Leave the tube open to air
  • Cover the cylinder to minimize evaporation and maintain concentration
  • Add more solvent while measuring
  • Shake the tube during measurement

Correct Answer: Cover the cylinder to minimize evaporation and maintain concentration

Q48. Which parameter is directly proportional to the drag force on a sphere in creeping flow?

  • 6π η r v
  • ρ_f g r^3
  • v^2 only
  • Surface tension times area

Correct Answer: 6π η r v

Q49. In reporting viscosity results for regulatory submissions, what additional information is important besides the viscosity value?

  • Temperature, sphere material/size, tube dimensions, method details and uncertainty
  • Only the date of the test
  • Color of the sample only
  • Name of the operator only

Correct Answer: Temperature, sphere material/size, tube dimensions, method details and uncertainty

Q50. Which alternative viscometer is often used when falling sphere limitations make it unsuitable for certain pharmaceutical fluids?

  • Osmometer
  • Rotational (cone-and-plate or rotational cup-and-bob) viscometer for wide shear range
  • Gas chromatograph
  • pH meter

Correct Answer: Rotational (cone-and-plate or rotational cup-and-bob) viscometer for wide shear range

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