Sedimentation methods determine size from
Answer: C
Stokes's law relates settling velocity to an equivalent diameter, provided the flow stays laminar and the particles do not interfere with one another. Very fine particles settle so slowly that a centrifuge becomes necessary.
Please log in or register to report this question.
State whether the following is true or false.
Stokes's law applies to sedimentation analysis regardless of particle concentration.
Answer: D
Correct answer: False
Stokes’s law describes a single isolated sphere settling in an unbounded fluid. At any appreciable concentration the particles interfere with one another’s flow fields and hinder each other’s settling, so the measured velocity is lower and the sizes come out too small. Sedimentation analysis is therefore run at low concentrations, and the sample must be fully dispersed.
Please log in or register to report this question.
The Coulter or electrical sensing zone method sizes particles by measuring
Answer: A
Correct answer: the change in resistance as each passes through an orifice
Particles suspended in an electrolyte are drawn through a small orifice across which a current flows, and each one displaces electrolyte and produces a resistance pulse proportional to its volume. Because it counts and sizes particles one at a time it gives a true number distribution and a genuine count - which is why it remains the reference method in blood cell counting.
Please log in or register to report this question.
Microscopy as a sizing method has the particular advantage that it
Answer: B
Every other technique reports an equivalent diameter and says nothing about shape, whereas an image shows it directly. The corresponding weakness is that counting enough particles for a reliable distribution is laborious.
Please log in or register to report this question.
Assertion (A): Microscopy is often used alongside an automated sizing technique rather than instead of it.
Reason (R): It reveals particle shape and agglomeration that an equivalent diameter conceals.
Answer: C
Both are true and the reason gives the complementarity: the automated method supplies statistics on many particles and the microscope supplies the qualitative information needed to interpret them. Numbers without a picture can be seriously misleading.
Please log in or register to report this question.
State whether the following is true or false.
An automated particle sizer can distinguish an agglomerate from a single large particle.
Answer: D
It reports whatever passes through as one particle of that size, so a soft agglomerate is measured as though it were solid. Microscopy or a dispersion study is needed to tell the two apart. Agglomeration is invisible to an instrument that reports only size.
Please log in or register to report this question.
Broad particle size distributions are sometimes preferred in traditional ceramics because they
Answer: A
Fine particles occupy the voids between larger ones, so the green density is higher than a monosized powder achieves. Advanced ceramics often prefer narrow distributions instead, for uniformity of the final microstructure.
Please log in or register to report this question.
Assertion (A): Traditional and advanced ceramics often want opposite things from a particle size distribution.
Reason (R): One is chasing packing density while the other is chasing microstructural uniformity.
Answer: B
Both are true and the reason gives the divergence: a broad distribution packs better but sinters unevenly, and which of those matters more depends entirely on what the finished ceramic has to do. Neither choice is right in the abstract.
Please log in or register to report this question.
Spheres of a single size pack randomly to about 64 per cent of theoretical density. What porosity does that leave?
Answer: C
Subtracting 64 from 100 leaves 36 per cent porosity. Adding a finer fraction to fill those voids is how a bimodal or broad distribution reaches a higher green density. Less porosity to close means less shrinkage during firing.
Please log in or register to report this question.
State whether the following is true or false.
A bimodal powder can pack to a higher density than either of its components alone.
Answer: D
Correct answer: True
Fine particles fit into the spaces between coarse ones, so a well-chosen bimodal mixture packs denser than either fraction alone. The optimum is roughly a seven to one size ratio with about a quarter to a third fines by volume, and the principle extends to three or more sizes. It underlies the grading of refractory and concrete mixes and any body that must reach high green density.
Please log in or register to report this question.
