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Unformatted text preview: ent in w hich to learn. 8 LAB 1: ELECTRIC FIELD S AN D FORCES
The m ost fund am ental forces are characterized as “action -at-a-d istance”. This
m eans that an object can exert a force on another object that is not in contact w ith it.
You have alread y learned about the gravitational force, w hich is of this type. You
are now learning the electric force, w hich is another one. Action -at-a-d istance forces
have tw o features that require som e getting used to. First, it is hard to visualize
objects interacting w hen they are not in contact. Second , if objects that interact by
these action-at-a-d istance forces are grouped into system s, the system s have
potential energy. But w here d oes the potential energy resid e?
Inventing the concept of a field solves the conceptual d ifficu lties of both the force
and the potential energy for action-at-a-d istance interactions. With a field theory, an
object affects the space around it, creating a field . Another object entering this space
is affected by that field and experiences a force. In this picture the tw o objects d o
not d irectly interact w ith each other: one object causes a field and the other object
interacts d irectly w ith that field . The m agnitud e of the force on a p articular object is
the m agnitud e of the field (caused by all the other objects) at the p articular object’s
position, m ultiplied by the property of that object that causes it to interact w ith that
field . In the case of the gravitational force, that property is the m ass of the object.
(The m agnitud e of the gravitational field near the earth’s surface is g = 9.8 m / s 2.) In
the case of the electrical force, that property is the electric charge. The d irection of
the force on an object is d eterm ined by the d irection of the field at the space the
object occupies. When a system of tw o, or m any, objects interact w ith each other
through a field , the potential energy resid es in the field .
Thinking of interactions in term s of field s is a very abstract w ay of thinking about
the w orld . We accept the burd en of this ad d itional abstraction because it lead s u s to
a d eeper und erstand ing of natural phenom ena and insp ires the invention of new
applications. The problem s in this laboratory are prim arily d esigned to give you
practice visualizing field s and u sing the field concept in solving problem s.
In this laboratory, you w ill first explore electric field s by build ing d ifferent
configurations of charged objects and m app ing their electric field s. In the last tw o
problem s of, you w ill m easure the behavior of electrons as they m ove through an
electric field and com pare this behavior to your calcula tions and your experience
w ith gravitational field s. O BJECTIVES
A fter successfully com pleting this laboratory, you should be able to:
• Qualitatively construct the electric field caused by charged objects based on the
geom etry of those objects. 9 LAB 1: ELECTRIC FIELD S AN D FORCES
• D eterm ine the m agnitud e and d irection of the force on a charged particle in an
electric field . PREPARATION
Read Tipler & Mosca: Chapter 3, sections 3-1 & 3-2; Chapter 21.
Before com ing to lab you should be able to:
• Apply the concepts of force and energy to solve problem s. • C alculate the m otion of a particle w ith a constant acceleration. • Write d ow n Coulom b's law and und erstand the m eaning of all quantities
involved . 10 PROBLEM #1: ELECTRIC FIELD VECTORS
You have been assigned to a team d eveloping a new ink -jet printer. Your team is
investigating the use of electric charge configurations to m anipulate the ink particles.
To begin d esign w ork, the com pany w ants to use a com puter program to sim ulate the
electric field for arbitrary charge configurations. Your task is to evaluate such a
program . To test the program , you use it to qualitatively pred ict the electric field of
three d ifferent sim ple charge configurations (single positive charge, single negative
charge, and d ipole) to see if the sim ulations correspond to your expectations. Initially,
you sketch the electric field vector for each of the three cases.
Instructions: Before lab, read the required reading from the textbook and the laboratory in its
entirety. In your lab notebook, respond to the warm up questions and derive a specific prediction
for the outcome of the lab. During lab, compare your warm up responses and prediction in your
group. Then, work through the exploration, measurement, analysis, and conclusion sections in
sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to
perform data analysis, rather than doing it by hand. A t the end of lab, disseminate any
electronic copies of your results to each member of your group.
Read : Tipler & Mosca Chapter 21 sections 21-1 – 21-5. It also m ight be a good id ea to
review Chapter 1 Section 1-6 & 1-7. EQUIPMEN T
You w ill use the com puter application EM Field . This program w ill d raw the electric
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
- Spring '14