Physics 201- Introduction to Electricity & Magnetism
Homework Assignments & Solutions

 

 

 
             Here are the assigned problems. The problem solving handout gives specific instructions on how to solve these problems (as well as others you may encounter in other coruses or later in life).  The exercises will be handed out in class and PDF copies will be posted to the web.  They will be in the class notes of the day the exercises were handed out.
 

Charges Unit
Due September 6
        You and a friend are doing the laundry when you unload the dryer and the discussion comes around to static electricity. Your friend wants to get some idea of the amount of charge that causes static cling. You immediately take two empty soda cans, which each have a mass of 120 grams, from the recycling bin. You tie the cans to the two ends of a string (one to each end) and hang the center of the string over a nail sticking out of the wall. Each can now hangs straight down 30 cm from the nail. You take your flannel shirt from the dryer and touch it to the cans, which are touching each other. The cans move apart until they hang stationary at an angle of 10° from the vertical. You now calculate the amount of charge transferred from your shirt. Also, what does this tell you about the amount of charge that was on your shirt when your first took it out of the dryer?

Due Septermber 11
         For this next summer you were able to land an internship with LA Department of Water and Power.  They have asked you to help design the air cleaners that will be used on a new coal burning power plant.  This is a significant project as nearly half of all of the electricity they generate comes from burning coal.  If the pollutants are not removed from the exhaust before being released into the air, the city would suffer from even worse pollution.  Burning coal produces carbon dioxide (CO2), sulfur dioxide (SO2) and nitrogen oxides (NOx).  These gases are vented from the boiler.  Bottom ash, which is made of coarse fragments that fall to the bottom of the boiler, is removed.  Fly ash, which is very light (typically between 1 x 10-3 g and 1 x 10-6 g) and small in diameter (typically between 1 mm and 0.1mm), exits the boiler along with the hot gases.  It is this fly ash with which you are concerned.  Current plants are using electrostatic precipitators to remove the fly ash.  They wish to continue using this technology as it removes over 99% of these particles, but you must redesign the system to match the specifications of the new plant. 
         An ash precipitator removes solid emissions from smokestacks by electrostatic means.  As the gas, laden with fly ash, leaves the boiler it is sent through a system that first gives the particles a net negative charge.  The average net charge is 10-10 C/ particle.  The gas then proceeds to a vertical stack.  There the small particles move upward with the gases at 2 -5 feet/ second.  In the stack, two charged parallel, vertical plates are placed so that the smoke passes through them.  (The plates have opposite charges.)  The horizontal electric field between the plates deflects the ash and traps it on one of the plates.  For your system you know that the space between the vertical plates will be 2 feet.  The height of the stack (and therefore the region with an electric field) is 50 feet.  The air that leaves the plates is now free from harmful particles.  You want to know how powerful the electric field needs to be such that most of the particles deflect into the plates and never make it out of the stack.
 

Electricity Unit
Due September 25
         You have a great summer job in a research laboratory with a group investigating the possibility of producing power from fusion.  The device being designed confines a hot gas of positively charged ions, called plasma, in a very long cylinder with a radius of 2.0 cm.  The charge density of the plasma in the cylinder is 6.0 x 10-5 C/m3.  Positively charged tritium ions are to be injected into the plasma perpendicular to the axis of the cylinder in a direction toward the center of the cylinder.  Your job is to determine the speed that a tritium ion should have when it enters the plasma cylinder so that its velocity is zero when it reaches the axis of the cylinder.  Tritium is an isotope of hydrogen with one proton and two neutrons.  (solution)

Due October 2
      These exercises will comprise our weekly "problem".  The solutions should include all parts of the problem solving algorithm- write out your assumptions, explain the physics concepts, etc.

24-18P, 24-22P, 24-26P, 24-34P (solution)
 

Circuits Unit
Due October 23
        You and a friend are studying for a midterm and the session goes until the early morning.  At about 4 am you decide to cook some breakfast.  Despite being sleepy, things are going well-- the waffles are cooking and the coffee is perking.  Should you make some toast now?  The 1000watt waffle iron and the 600watt coffee maker are plugged into kitchen wall electrical outlets.  You will also use a kitchen wall outlet for the toaster.  The kitchen wall outlets are all part of the same 110V circuit which has a 20A circuit breaker (with negligible resistance) to protect the wire carrying the largest current from getting too hot.   You know that if you plug in too many appliances you will overload the circuit breaker.  The toaster label says that its power output is 700 watts.  (solution)
 

Magnetism Unit
Due November 8
        You have landed a great summer job at UC-Irvine's medical school assisting in a research group investigating short lived radioactive isotopes which might be useful in fighting cancer.  Your group is working on a way of transporting alpha particles (helium nuclei) from where they are made to another room where they will collide with other material to form the isotopes.  Since the radioactive isotopes are not expected to live very long, it is important to know precisely how much time it will take to transport the alpha particles.  Your job is to design that part of the transport system which will deflect the beam of alpha particles (m = 6.64 x 10-27 kg, q = 3.2 x 10-19 C) through an angle of 90o by using a magnetic field.  The beam will be traveling horizontally in an evacuated tube.  At the place the tube is to make a 90o turn you decide to put a dipole magnet which provides a uniform vertical magnetic field of 0.030 T.  Your design has a tube of the appropriate shape between the poles of the magnet.  Before you submit your design for consideration, you must determine how long the alpha particles will spend in the uniform magnetic field in order to make the 90o-turn.  (solution)

Due November 18
What is Science? essay

Due November 25
Read the article on power lines and cancer that was handed out in class.  Then write a two page reaction paper with your thoughts.  The paper should be typed, double spaced and free of spelling and grammatical errors.


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Jeff Phillips
phillips@lmu.edu
Loyola Marymount University
Fall 2002