Models of Motion – 2014-15

Week 4: Diodes and power supplies

Reading:

–          Horowitz and Hill, Art of Electronics, pp. 43-53.

–          Horowitz and Hayes, Lab Manual, pp. 61-74

Exercises:

–          Due before class Tue: HH Ch 1: 1.26, and also end-of-chapter Additional Exercises (p. 58-9) #3, #7.

–          Due before class Thursday: Additional Exercises (p. 58-9) #2, #4, as well as 1.23, 1.26, 1.29. Also: Neil Problem #1:

o   You want to make an LED flashlight using a 9V battery. LEDs are diodes, but they have a “forward voltage drop” that is higher – usually about 2.5V. In other words, there is a 2.5V drop across the diode in the figure below:

A) To get light out of the LED, you run a current through it. For small LEDs, the maximum current they can take is about 20mA. What R value should you use to limit the current to no more than 20mA? (this is known as a “current-limiting resistor”.)

B) How much power is dissipated in the resistor? How much is dissipated in the LED? What fraction of the total power dissipated is used in the LED (that’s the fraction of the power that goes to generate light)? Based on that result, does this seem like a good circuit if you want your flashlight to last a long time?

Seth has regular tutoring hours on Wednesdays from 2 – 4 in Sem 2 D2105.

The 16th Annual Meeting of the Northwest Section of the American Physical Society is rapidly approaching! Held May 14 – 16 at Washington State University in Pullman, WA, you can find more information here. At that site, you’ll find a link for Registration. Early registration deadline is April 24, and will be $25 for undergraduate students; after that it will be $30 (addendum 4/17: see below).

As discussed in class, some of your classmates will investigate housing options in addition to staying in the Olympia Residence Hall (dorm-style accommodations). We’ll update once we have some info.

You’ll be responsible for the cost of registration, lodging, and meals. It’s your choice if you want to go to the Friday banquet (I tend not to go). We’ll arrange for vans (leaving late afternoon/early evening on Thu. May 14 arriving in Pullman very late that evening, returning late afternoon on Sat. May 16 arriving back in Olympia Saturday night).

Please ask if you have any questions. Physics! Conference!

addendum 4/17: One of your classmates provided this very helpful (thank you!) information “APS membership is required in order to receive the $25 rate for student registration. Otherwise it is $50 [$25 for membership + $25 registration]. Students can register for free by going to the APS site and selecting Free Trial Student Membership. Then select Online Free Trial Student Membership Application. When the application opens, select yes for this prompt: Are you joining to submit an abstract, register for a meeting at the member rate, or be eligible for fellowship?. This will make the membership start date April 1. After submitting the application, students can register for the conference (it is necessary to sign in when asked to receive the rate).”

For Tuesday April 14, Physics problem session will be a lecture, and will meet in CAL West from 10:15 – 11:15.

For those of you required to submit Academic Statements, your updated draft is due May 14. If not submitted on time, a hold will be placed on your account that will prevent you from registering for any future quarter (or for students hoping to graduate, prevent you from doing so). Please make sure you address this right away.

I think that all of you have a draft that is of sufficient quality to submit, and until you are ready to graduate, this is an internal document for planning and advising purposes only. I am happy to meet with you individually if you’d like feedback on your latest draft before you submit it, so please feel free to ask.

Solutions to the Week 22 Calculus and Physics problem sets are posted at the Week 22 Calendar page. No-one (!) voted for more detailed solutions to any of the Calculus problems, so I didn’t produce any. Please take advantage of this opportunity in the future.

A (very preliminary, so please check back) Week 23 Calendar page is available. (updated 4/12 with calculus and physics problem sets).

You can vote here for those odd numbered problems for which you would like detailed solutions produced. Voting open until noon Sat. Apr. 11.

The circuits assignment for next week is as follows:

Week 3: AC circuits

Note: The 3^rd edition of The Art of Electronics (ISBN 0521809266 ) has just been released, and so far looks to be as good or better than the last. The (amazing) Lab Manual (the real key to learning electronics – the book is best after one’s been through the manual, or in conjunction with it) has apparently now been expanded to a stand-alone book, “Learning the Art of electronics,” by Hayes & Horowitz… but unfortunately has not yet been released. My recommendation is to wait for that (apparently due out later this year) and get it first, then see if you want/need the main text.

Reading:

–          New posting of (complete) Ch 1 from the 2^nd ed. is here.

–          My suggestion: Skim 32-50 of the lab manual, then read carefully 20 – 42 of the Ch 1 from textbook, then go back and read carefully 32-50 from the lab manual. The lab manual has lots of good worked examples; it will help a lot.

–          Readings:

o   Horowitz and Hayes, Lab Manual, pp. 32 – 50.

o   Horowitz and Hill, Art of Electronics 2^nd ed, pp. 20-42.

• Optional / aside: Mazur, Ch. 32 has a lot of material on phasor diagrams and semiconductors. I recommend looking it over only after reading the Lab Manual pages; it would mainly help with 1.23, which will be assigned for Week 4.

Exercises:

–          Due before class Tue: HH Ch 1: 1.16, 1.17, 1.21

–          Due before class Thursday: HH Ch1, 1.13, 1.14 (read the section immediately before the question :), 1.15, 1.22, 1.24

o   Notes:

• 1.14: You need to make the assumption that the V_in = 0 until you connect it at t = 0, so at t = 0 V_out (V(t) in Fig 1.34) = 0 too. Then You connect V_in(t) , which may vary in time… what is V_out(t)? Hint: consider the Thevinin equivalent looking into the two resistors, then compare to the ‘Time Constant’ example on p. 34.
• 1.17: This one is now visible in the new chapter pdf.
• 1.21: They are looking for the magnitude of the response here, like they found for the high-pass filter on pp. 35-6. Show this by following that example, adapting it to the low-pass filter geometry (complex voltage divider in Fig 1.58), using the complex impedance for a capacitor.
• 1.22 (show by calculating the phase shift (in degrees – you’ll have to convert from radians) at 0.1 f_3DB (for a high-pass filter) and 10 f_3DB (for a low-pass filter

We will finish this chapter next week, and then prepare to move on to active circuits (like amplifiers)!

• This one is straightforward: Ch. 19, #95(b) should be 1.19 x 10³ J, not 1.19 x 10^-21 J.
• Ch. 19, #35(b) is a more substantial error. The text takes a short-cut about equilibrium that I disagree with (though the short-cut gets quite close, and for larger systems is an excellent short-cut to get the equilibrium configuration). However, the short-cut does not agree with the method we outlined in class for small systems. I can’t fix the problem in MP since several people have already attempted it. If you attempted it and lost points because you followed our in-class method, simply let me know and show me some correct work and I’ll credit you any lost points.
  • Here’s the short-cut, which leads to how you should re-interpret the question. The text assumes that the equilibrium configuration proportionality splits the energy based on the relative size of each system. So as an easy example, if there q = 10 energy units to be split among N_A = 3 and N_B = 7, the short-cut assumes that q_A = 3 and q_B = 7. However, if we build up the table as we did in class on Wednesday, the most probable macrostate is actually q_A = 2 and q_B = 8. As another example (more relevant to the types of numbers you will see), if there are q = 7 energy units to be split among N_A = 2 and N_B = 5, the short-cut assumes that q_A = 2 and q_B = 5. However, looking at the table, the most probable macrostate is actually q_A = 1 and q_B = 6.
  • So, in summary, for 19.35(b), use the short-cut described above to approximate the equilibrium configuration.
  • If you need further clarification or explanation, please ask in Thursday’s problem session.