Exam #2:  Friday, March 2, 2001, 8:30 am in class

Covers all assigned sections of chapters 21-23, 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.


Notecard:  As for the first exam, you are allowed one 4”x6” card with any notes you care to write on it, both sides.


Homework #6:  Material on HW6 will be covered on the test, although not emphasized as much.  Note that after the Friday Feb 23 lecture, we will have covered most of the material necessary for that homework, and the problems in Chapter 23 are easier and will be covered by the Monday Feb 26 lecture.  So, you should have plenty of time to figure out what is confusing, and learn it before the exam.  I will try to have the solutions on the web page by 5 pm Thursday, so please try to look at them that evening.


Stuff not on exam:

-      Ampere’s Law

-        Section 23.3:  you don’t need to know anything here except Concept Example 4

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

-        Details of Earth’s magnetic field (page 630) not important


Stuff that will be on the exam:  Similar to exam #1, previous homework, suggested problems, and problems/concepts discussed in lecture are the highest priority.  Try not to merely memorize solutions, but to make sure you understand how to set up problems.  Be sure you know how to use both right hand rules.  RHR#1 deals with forces due to magnetic fields and RHR#2 deals with magnetic fields caused by wires with current.


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

-        Why iron filings are attracted to magnets (February 16 lecture, handout)

-        What the magnetic field looks like near and inside of circular loops, solenoids (Figs 21.30, 21.34)

-        Particles moving in circular trajectories in magnetic fields – especially make sure you know when they do move in circular trajectories, and when they don’t.

-        In section 22.2 there is a discussion of the relationship between motional EMF, work, and electrical energy.  This has been mentioned in class but not discussed heavily; be sure you understand this stuff.

-        Section 20.13 (RC circuits) may be on this exam.


Earlier Material:  Clearly some of the material from chapters 18-20 is relevant – you should definitely know the formula F = q E for example, and Ohm’s Law reappears in chapter 23 which is included in this exam.  While there’s a lot of the earlier material you don’t need to know for this exam – for example, Coulomb’s Law, EPE, electric potential – be sure you are comfortable with the homework problems that have involved the previous material.  An exception to this is problem 21.64 which involves EPE, that type aspect of that problem will not be on this exam.  Mainly, I want to draw the distinction between what you should not worry about – don’t write down all the formulas you used the first exam on your notecard, for example! – and what you should know, which includes topics like EMF, and the velocity selector discussed in section 21.3.


Tricky questions which may be on the exam

-        Be sure if the problem involves an electron, you remember that the direction of the force is reversed.

-        If the velocity of a particle is parallel or anti-parallel to the magnetic field, then sin theta = 0 and there is no magnetic force.  Likewise for wires carrying current.

-        Faraday’s Law involves magnetic flux.  Don’t forget the flux is zero if the magnetic field is tangential to the loop.

-        In class we have gone over many cases of loops of wire that feel a net torque, or a net force, or neither a torque nor a net force.  Be sure these cases are clear – and be aware especially of the cases where there is neither a net torque nor a net force.

-        If a coil of wire has N loops, and you’re finding the induced EMF by Faraday’s Law, be sure to multiply by N for your final answer


Office hours:  As usual, plus additional office hours 1 – 6 pm on Thursday (with a short break for me from 2:50 to 3:20).


Format and grading of exam:  It will be similar to Exam #1.  On the test, if you are pressed for time, at least write down the correct equations, draw a diagram if appropriate, and indicate how to get the solution – this will get you substantial partial credit if you are writing down correct things.  Writing down incorrect or random formulas will not help you, but otherwise it’s easy to get partial credit.


Some additional concept questions to help you review:


-        A proton is sitting motionless in a uniform magnetic field.  I turn an electric field on for 10 seconds, that points in the same direction as the magnetic field.  After the electric field is turned off, what does the motion of the proton look like?


-        I repeat the above experiment, but this time the electric field points in a direction perpendicular to the magnetic field.  After I turn the electric field off, what does the motion of the proton look like?


-        Infamous Fig 21.28a shows two wires with currents in opposite directions.  They each feel a repulsive force.  Let’s assume the currents are equal.  Is it possible to put a third wire in between them, with a current through the third wire, so as to cancel out the repulsive forces each of them feel?


-        What about in Fig. 21.28b, with the currents in the same direction?  Now the original two wires are feeling an attractive force; can we put a third wire between them to cancel out this attraction?


-        If you have an isolated loop of wire with current through it, and there is no external magnetic field, the loop of wire feels no net force and no net torque.  Yet, the loop of wire itself is generating a magnetic field, as discussed in section 21.7.  Explain why the magnetic field the loop of wire generates does not create a net force or a net torque on the loop.  Probably the simplest way to explain it is to use a rectangular loop.