Exam #1:  Friday, February 9, 2001, 8:30 am in class

Covers all of chapters 18-20, but see details below


This handout is an attempt to give you a sense of what’s important to study for and what’s less important.  However, ultimately you are responsible for the reading material (even if not discussed in class) and the material covered in lecture (even if not in the reading).  If you missed any lectures, try to get the notes from someone who was there.  You are welcome to ask me questions about anything you didn’t understand.


Homework #3:  Material on HW3 will be covered on the test, although not emphasized as much.  I will try to have the solutions on the web page by 5 pm Thursday, so please try to look at them that evening.  More importantly, if you are finding that homework confusing please be sure to get help from me during the week.  Everything you need to know to do HW3 should be covered by the end of the lecture on Monday, Feb 2, so that gives you several days to figure out what’s confusing or what you still don’t understand, so make sure you are comfortable with that material by Friday.


Calculators:  You may not use a programmable calculator on the exam; you will not need a calculator with trigonometric functions, although you may use one if you want.


Stuff not on exam:

-      electronic ink (p524), personal electronic assistants (p599)

-        Names of physicists (Charles-Augustin Coulomb, etc)

-        Prefixes like “micro” or “nano” – you should know these, but if I use them on the exam I will remind you what they mean.

-        Parts of the textbook not assigned (page 545, sections 18.10 & 20.5, etc)

-        Resistivity, temperature dependence of resistors

-        The details of how voltmeters & ammeters work (section 20.11)

-        Sections 20.13 & 20.14 will not be on this exam, but may show up in the future


Stuff that will be on the exam:  I’m not going to make a specific list, as I hope it’s fairly obvious – for example, anything covered in homework.  Additionally, all of the “suggested problems to look at” are worth looking at.  Hypothetically, if you work all those problems you’ll do great on the exam, and perhaps more realistically you should at least feel like you know how to set up all of those problems.  Note that many of them are odd-numbered (and have the answers in the back of the book) and some of them are also problems discussed in the Student Solution Manual.


Stuff you shouldn’t overlook – topics that may not have been emphasized but which you should know something about:

-        properties of conductors (section 18.8)

-        dielectric breakdown and basic concepts of sparks & lightning

-        Section 19.6: you should know the body is a reasonably good conductor, but it’s not at equipotential.  It has small voltage differences (microVolts to milliVolts) which are physiologically useful.  The details beyond that, you don’t need to know for this exam.

Office hours:  I will have the usual help sessions Tuesday and Wednesday evenings at 7 pm.  I may have additional office hours which will be discussed in class and probably posted on the web.


Electric flux:  You’ve seen the formula      Flux = EA cos q

But, all you really need to know are two cases:

1.   If the electric field is perpendicular to a surface, Flux = EA

2.     If the electric field is tangent (parallel) to a surface, Flux = 0


Sign errors:  I am less likely to ask questions such as “What is the difference in potential between point A and point B” and more likely to ask you “What is VA – VB” or “How much kinetic energy does a negative charge gain moving from point A to point B.”  That is, you should know the formulas, and know information like what directions + and – charges move in E and V fields.  I will be more likely to ask about directions of motion than “just give me a number and make sure the sign is right.”  The additional concept questions below present good examples of the type of question I’m likely to ask.


Kirchhoff’s Rules:  Know what these rules are; know how to use them to set up equations that describe a circuit.  At most on the exam you might be asked to set up the equations, you would not be asked to solve them.


Format of exam:  I haven’t made the exam, but it will be similar to the homework.  At least 1/3 of the test will be concept questions and perhaps as much as ½.  For the quantitative problems, as with the homework, my tendency is to not take off too much if you just make math errors.   On the test, if you are pressed for time, at least write down the correct equations, draw a free body diagram if appropriate, and indicate how to get the solution – this will get you substantial partial credit if you are writing down correct things.


Some additional concept questions to help you review:


-        I have three metal balls, A, B, and C.  One or more of these objects may be charged.  I observe that A and B attract each other, and B and C attract each other.  Can objects A and C both be neutral?  Can they both be positive?  Can they be oppositely charged?  Explain your answers to these three questions.


-        In class, I demonstrated a device called the Windhurst generator (although I didn’t mention its name that day).  It was the demonstration that I turned a crank by hand which then generated a spark between two metal balls.  The spark had a certain amount of charge transferred, a certain voltage between the two metal balls, and a certain electric field between the metal balls.  How do these three quantities (Q, V, E) compare with that of a lightning bolt – that is, obviously the lightning bolt is bigger in some respects, but which respects?  Is the lightning bolt the roughly same in any of these quantities?


-        An electron starts at rest in an electric field, and then moves from x=0 m to x=1 m (to the right).  What is the direction of the electric field?  Where is the electric potential higher, at x=0 m or x=1 m?


-        A small coin with a charge of Q = -2 mC starts with velocity v=1 cm/s at point A, where the potential is 7 V.  It moves for a while and then we observe it’s at point B, where the potential is 5V.  Has the coin sped up or slowed down by the time it’s at point B?  What if it’s original velocity is not in the same direction as the electric field, does that change your answer?


-        Suppose we want to place four charges onto the corners of a long, skinny rectangle.  Two of the four charges are +1 mC, the other two are –1 mC.  Where should the four charges be placed to maximize the stored electrical potential energy, using the usual convention that two charges infinitely far apart have zero EPE.


-        I have a cubical box, 1x1x1 meter.  I place this box in a uniform electrical field E=45 N/C pointing to the left.  What is the net electrical flux coming through this box, and is it into the box or out of the box?


-        I have a cubical box, 1x1x1 meter.  Inside this box I place a +1 C charge.  Is the net electrical flux through the box zero or not?  If not, is it going into or out of the box?  Does the answer change if it’s a –1 C charge?


-        What’s the difference between an insulator and a conductor?  If we had an object, what sort of tests could we do to determine which one it is?


-        I have three objects that are all charged, and each of them repels the other two.  Is this possible?  If so, what are the signs of the charges on the objects?


-        I have three objects that are all charged, and each of them attracts the other two.  Is this possible?  If so, what are the signs of the charges on the objects?