Week 24

Physics

  • Reading: Knight, Ch 32. We’re starting magnetism!
  • Problems: Ch 32, problems 1, 2, 10, 14, 24, 29, 33, 35, 61, 62.
    • Optional challenge problem: 65.
    • Notes:
      • #10 is directly related to the NMR magnet in the big silver dewar in the room near the stairs on the second floor of Lab I; it has a superconducting niobium electromagnet (coil of wire) inside.
      • #33 has additional support available. The most precise balances (scales) in the world – the ones used to compare the international standard kilograms, which are accurate to a bit better than one part in 10^12, which is to say 1 ng for a 1kg mass – use magnetic force balancing a current (as in this problem) to measure the mass difference of the two sample kilograms.
Circuits
  • Reading:
  • Problems:
    • Due 5PM Wed, box outside my office: HH Ch 1: 1.26, and also end-of-chapter Additional Exercises (at end of chapter, 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: Word Problem #1:
  • Word Problem:
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:
 
week 4
 
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?
 

Calculus

  • Reading: 14.4 – 14.6
  • Problems:
    • Monday Spring Drills 4, due 10 am Mon. April 25 via WileyPlus:
      • 14.4: 15, 18, 21, 23, 26, 29, 31, 34, 37
      • 14.5: 24, 31, 33, 37, 38
      • 14.6: 1, 2, 4, 7, 13, 18
      • Solutions
    • Wednesday Spring Problem Set #4, due noon Wed. April 27 outside Lab 2 3255:
      • 14.4: 51, 54, 74, 76
      • 14.5: 54, 62, 65, 66
      • 14.6: 20, 43
      • Solutions