Exam grades

Your first exam is graded, as of right this minute. The average was 83.6 with a standard deviation of 9.5, which implies you were more clever than expected (or I was more generous in partial credit, but let's be optimists here). In any case: well done, most of you will be happy.

Do show up tomorrow to get your graded exams, you'll also get back Quiz3 and Lab2, and possibly Quiz4 and Lab3.

See you in a few hours ... we'll go over the exam and then begin with induction and so forth. 

Tomorrow's lab

will be on simple transistor circuits. You will understand transistors, but be initially baffled by the protoboard of all things. It is just the way of the world, apparently, so don't feel bad about it.

Random exam information

Earlier a student asked by email what they should do to study, etc. Here's my response with the original questions paraphrased [in brackets].

1) [What to study/read] I think you'd probably be just as well off reading the notes - start with the chapter on electric forces and fields, and continue through dc circuits. Most of my problems will be close to the treatment in the notes. I think the notes are worth a quick read to make sure you get the ideas down, but the best thing is probably to work problems. Look at the old HW and exam problems and try to work through some of those, that will be closest to the real thing.


2) [What to focus on?] Some things to focus on in particular are:

• what is the E field from an arrangement of point charges, like a square?
• what is the potential energy of a system of charges, like a crystal (like the HW problem)?
• the equivalent capacitance of an arrangement of capacitors and the charge stored
• the equivalent resistance and the current through each resistor
• what is the power in resistors and how current relates to drift velocity and stuff (chapter on current)

3) [What to put on the formula sheet?] you'll have all the formulas you need on the test, so the best thing for the formula sheet is probably to have a few example problems and notes to yourself on when to use different formulas. A good example might be to have one of the crystal problems or equivalent resistor problems worked out so you'll know the steps if one of those shows up on the exam.


4) [When are you around?] I should be on campus by 10:15 and around my office, or at least I'll try to be. I will also try to leave a little time after class and end early so you can ask questions - I have a meeting at 1:15, but I can try to leave at least 12:30 or 12:45-1:15 free.

HW3 solutions

are out for your studying pleasure.

UPDATE: I corrected a dumb mistake in 3a - I forgot to multiply by 4 when I collected all of the q5 terms, so the answer should be 4-15*sqrt(2) times the energy of a single pair, or about -17.21 as pointed out in the comments. The link above has the corrected version.

UPDATE: Small mistake in 1b as well - the initial formula for acceleration is OK, but the final expression where I've plugged in the result for d was incorrect, as was the numerical answer. Fixed now. 

Quiz 4

and its solution are now posted.

Quiz 2 and 3 solutions

are here.

HW3 hints

1. First, remember that e=1.6x10^-19 when you plug in numbers for the charges. The alpha particle has a certain amount of kinetic energy, and the closest it can get to the gold nucleus is when it has spent all of its kinetic energy at the expense of electrical energy required. At closest approach, the electrical energy between the alpha particle and gold nucleus will be equal to the initial kinetic energy of the alpha particle. At that distance, acceleration is force per unit mass, and there is only one force present ...

2. Use Gauss' law - the potential due to a spherical grain at a distance r, its radius, would be the same as that of a point charge of the same magnitude. That gets the total charge, and you know the charge of one electron.

3. Sum up the energy of all unique pairs. See this example from a previous homework set to get started.

4. If S₁ is open, the battery is not connected to a closed circuit ... If S₁ is closed, and S₂ is open, C₃ does nothing, since it is connected on only one end. This leaves you with two capacitors in series ... find their equivalent, and then find the total charge. What do you know about the charge on capacitors in series?

5. Remember that series resistors split the total supply voltage while parallel resistors have the same voltage. Remember that power is current times voltage, and power determines brightness. Given the same battery voltage in each case and the same resistance R for each bulb, find the voltage and current for each bulb to get the power and then rank them.

Monday's lab

will be all about resistors and RC circuits. It will use the lab boxes like last time, hopefully things will be a bit easier the second time around.