Exam #1 Information.

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rjagodowski
Posts: 2371
Joined: Fri Sep 04, 2015 6:59 pm

Exam #1 Information.

Post by rjagodowski »

Exam #1 will be distributed in class on 3/7/2016. It is a take home exam and is due on or before 4:00 p.m., Thursday, March 31, 2016. This exam will cover content from Chapters 1 through 5 in the text with some additional related content.
rjagodowski
Posts: 2371
Joined: Fri Sep 04, 2015 6:59 pm

Exam #1 Results & Common Mistakes

Post by rjagodowski »

Class Average: 77.4%, Standard Deviation: 9.5 There was a 4 pt. correction added to everyone's exam to compensate for various mis-interpretations of a couple of the questions.

Common mistakes made on Exam #1:

Both 555 circuits with the diode: The diode by-passes R2 during the charge cycle, so the capacitor charges ONLY through R1 and discharges ONLY through R2. That changes the equations to eliminate R2 from the charging time (output high time). This link explains it: 555 Astable Circuits, scroll down a bit. They show a circuit with an extra diode, but that second diode doesn't change things much.

And here's a link with the 555 Calculator...forgive the "horror" graphics, it's a good site otherwise.


The "best" Ideal Gas Law constant to use was the constant which was already in the units given in the original problem. They were all valid constants, but the CORRECT answer was the one with the units which didn't require ANY additional unit conversions, other than the temperature from Celsius to Kelvin. In other words, the correct answer had the units of liters, moles, atmospheres & Kelvin.


One of the 555 questions asked for the output pulse-width answer in milliseconds. Most people gave the answer in seconds, which is what the formula gives. You had to multiply your answer by 1,000 to give the answer in milliseconds.

There is a similar question which asked for the Duty Cycle as a percentage, and many gave an answer as a ratio. If you had the ratio, you had to multiply it by 100 to convert it to a percentage.


The U-shaped manometer: the Process Pressure has to be LESS THAN the Reference Atmospheric Pressure for the liquid to be higher in the tube on the Process side. Therefore, a gauge pressure reading (where at atmospheric pressure = 0 psig) of the Process Pressure would have to be NEGATIVE if it was less than ZERO.

The question: "There can be ____ resistive pressure elements in the bridge circuit of a semiconductor strain gauge." The correct answer is "All of the above", because there can be 1, 2 or 4. MOST will have 4 as that gives the greatest sensitivity by having four sensing elements. However, the bridge circuit CAN BE designed to use 2 or even 1. Though less sensitive, it is also much lower in cost to implement.



If you have questions about any of the other answers, please feel free to post the question here and I'll respond ASAP, or see me in my office.
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