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Assignment 5
Assignment is due at 11:00pm on Sunday, February 25, 2007
Credit for problems submitted late will decrease to 0% over the course of 10 hour(s) after the
deadline has passed.
The wrong answer penalty is 1% per part. Multiple choice questions are penalized as described in
the online help.
The unopened hint bonus is 1% per part.
You are allowed 10 attempts per answer.
Relationship between Electric Force and Electric Potential Conceptual Question
Three points (A, B, and C) are located on equipotential lines as shown.
Part A

A proton is released from Point A. Indicate the direction of the electric force vector acting on the proton.
Hint A.1
Determining the direction of the electric force
Hint not displayed
ANSWER:
The electric force vector at Point
A
Part B
An electron is released from Point B. Indicate the direction of the electric force vector acting on the
electron.
Hint B.1
Determining the direction of the electric force
Hint not displayed
ANSWER:
The electric force vector at Point B
Part C
An electron is released from Point B and a second electron is released from Point C. What can you say
about the electric forces experienced by these electrons the instant they are released?
Hint C.1
Relating electric field strength to change in electric potential
Hint not displayed
ANSWER:
The electron released at Point B experiences a greater force.
The electron released
at Point C experiences a greater force.
Electrons released from Points B and C would
experience equal forces.
The relationship bewteen the two forces cannot be
determined.
PSS 29.2: The Hot Rod

Learning Goal:
To practice Problem-Solving Strategy 29.2 for problems involving the electric potential
due to continuous charge distributions.
A straight rod of length
has a positive charge
distributed along its length. Find the electric potential due
to the rod at a point located a distance
from one end of the rod along the line extending from the rod.
MODEL:
Model the charges as a simple shape, such as a line or a disk. Assume the charge is uniformly
distributed.
VISUALIZE:
For the pictorial representation:
1.
Draw a picture and establish a coordinate system.
2.
Identify the point P at which you want to calculate the electric potential.
3.
Divide the total charge
into small pieces of charge
using shapes for which you already
know how to determine
. This division is often, but not always, into point charges.
4.
Identify distances that need to be calculated.
SOLVE:
The mathematical representation is
.
■
Use superposition to form an algebraic expression for the potential at P.

■
Let the
coordinates remain as variables.
■
Replace the small charge
with an equivalent expression involving a charge density and a
coordinate, such as
, that describes the shape of charge
.
This is the critical step in making
the transition from a sum to an integral
because you need a coordinate to serve as an integration
variable.
■

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