★Specialized Examination for Chemical Engineering ★ — One question per day (2015-11-19) — Points are awarded for participation

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This post was last edited by zshiwei2008 on 2015-11-23 at 13:51. Points system for the daily question: Participation – 1~3 points; Correct answer – 5 points; In-depth analysis – 5~10 points. The answer is provided the next day. Daily Question (2015-11-18): 1. Which of the following statements is incorrect? A As a fundamental dimension, it should have the characteristic of being independent of one another and not derivable from each other. B There are three fundamental dimensions in mechanics: mass, length, and time. C There are four fundamental dimensions in thermodynamics: mass, length, time, and temperature. D Dimensionlessization involves using physical quantities that must be fundamental dimensions as initial variables, so that the dimensions of these initial variables remain independent of each other and no dimensionless group can be formed. 2. Some people want to make the pipe walls smoother, so they coat the inner surface of the pipes with paraffin. If the conveying task remains unchanged and the fluid flows in a stagnant manner, how will the flow resistance change? A Unchanged B Increases C Decreases D The change in resistance depends on the wettability between paraffin and the fluid. Answer: For the first question, the answer is D. Dimensionless quantities used as initial variables do not necessarily have to be fundamental dimensionless physical quantities. This question seems quite unfamiliar to me, and I don’t really have any concept of dimensions either. Here is an explanation taken from the Internet: All physical quantities are composed of two factors – their physical properties (or categories) and the measurement standards (or units) defined to quantify those physical properties. For example, length is a physical property that constitutes a linear geometric quantity, and its units of measurement include various units such as meters, centimeters, feet, and light-years. The general components of a physical quantity are its properties and measurement units. We call the attributes (categories) of physical quantities dimensions, also known as units. Usually, L represents the length dimension, M represents the mass dimension, and T represents the time dimension. Physical quantities that are dimensionless are called adimensional quantities; they are pure numbers, such as pi π and the Reynolds number Re. In any physical equation that correctly reflects objective laws, the dimensions of its terms must be consistent. Dimensional analysis was developed based on this principle; it is one of the important discoveries in mechanics at the beginning of the 20th century. Answer to question 2: A, unchanged. The format of this question is quite innovative and closely related to practical applications, but it tests basic concepts; it’s easy to get the answer right once distracting factors are excluded. The factors that affect fluid resistance are mainly Re and the friction coefficient, with the specific analysis varying depending on the flow state of the fluid. In a stagnant flow state, the fluid friction coefficient depends only on the Reynolds number, not on relative roughness. Therefore, changing the smoothness of the tube wall has no effect on fluid resistance.