Physics Electrostatics Symbolic Expression Word Problems

Name: Lab Day/Time:
Homework 1 Electrostatics
Homework is due at the beginning of the Wednesday lecture. It must be handwritten, not typeset. The multiplechoice answers must be circled. In the space after the problem, a short justification of each multiple-choice the
answer must be included. The open-response answers must be worked out clearly using good physics presentation
and will be graded on correctness and how carefully the work is explained. The problems should be worked in
the space after the problem on the assignment printout; additional paper may be used if needed. No credit will
be given for answers without appropriate supporting work. Minimum good presentation requires the following:
(1) Symbolic expression for any formula, (2) Manipulation of symbolic expressions, not numeric expressions, (3)
Substitution of numbers with units, (4) Reporting final answers with correct units and vector expressions, (5)
Enough English description to allow the reader to have some idea what you are doing without looking at the
math.
Multiple Choice Problems
The questions in this section are to be answered by circling the correct multiple-choice answer AND
providing a short justification of your answer in the space after the problem..
Homework Problem 1.1 A patch of positive charge is placed on a conducting sphere. Where will the positive
charge be at a later time? Assume no charge is lost to the environment.
Select One of the Following:
(a) The charge will remain in the same place.
(b) The charge will stay bunched together but will move around the surface of the conductor.
(c) The charge will separate as much as possible spreading over the surface of the conductor.
(d) The charge will transform into neutral charge and disappear.
(e) The charge will spread apart, but will eventually come back together.
Homework Problem 1.2 A cylinder of radius 1.5mm and height 13cm has a volume charge density of about
0.40 × 10−6C/m3
. What is the total charge of the cylinder?
Select One of the Following:
(a) Q = 3.3 × 10−5C
(b) Q = 3.7 × 10−13C
(c) Q = 6.5 × 10−5C
(d) Q = 1.2 × 10−13C
(e) Q = 5.7 × 10−12C
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Homework Problem 1.3 A conducting can is brought near a negatively charged insulated rod. The can is then
grounded. What is the direction of the force on the can after the ground is removed while the rod is still near?
Select One of the Following:
(a) The force on the can is directed towards the rod.
(b) The force on the can is directed away from the rod.
(c) The rod exerts zero net force on the can.
Homework Problem 1.4 Which of the following describes an experiment that demonstrates that there are at
least two different types of electric charge?
Select One of the Following:
(a) Rub one pair of rods made of the same material – for example, glass – with felt. Observe that the glass rods
repel one another.
(b) Charge a pair each of glass and rubber rods by rubbing them with felt. Observe that that (1) the glass rods
repel each other, (2) the glass rods attract the rubber rods, and (3) the rubber rods repel each other.
(c) Charge a pair each of glass and quartz rods by rubbing them with felt. Observe that that (1) the glass rods
repel each other, (2) the quartz rods repel the glass rods, and (3) the quartz rods repel each other.
Homework Problem 1.5 A positively charged rod is used to charge a metal sphere by induction. What is the
sign of the charge transferred to ground in the process?
Select One of the Following:
(a) positive
(b) negative
(c) No charge is transferred to ground while charging the sphere by induction.
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Homework Problem 1.6 ZZ Top’s Billy Gibbons owns a unique piece of art, a spherical VW Microbus ball.
Treating this piece as a spherical shell of charge with surface charge density σ = 55µC/m2
compute the total charge.
Select One of the Following:
(a) 7 × 10−4C
(b) 7 × 10−6C
(c) 7 × 10−7C
(d) 7 × 10−10C
(e) −7 × 10−3C
Homework Problem 1.7 The dielectric plate of an electrophorus is given a negative charge. The conducting
plate of the electrophorus is initially neutral. The plate is held by its insulating handle and placed on the negatively
charged dielectric. The handle is then released and no other interaction with the electrophorus occurs. The charge
of the electrophorus after it is placed on the dielectric is
Select One of the Following:
(a) positive (b) negative (c) approximately zero
Homework Problem 1.8 A charged insulating rod is brought near a light, neutral styrofoam ball which is
suspended by a string. As the rod nears the ball, which of the following could be observed. Assume no charge is
lost to the enviroment over the course of the experiment.
Select One of the Following:
(a) The ball will not move; there is no interaction between the rod and the ball.
(b) The ball will swing away from the rod.
(c) The ball will swing away from the rod, and then return to its original position.
(d) The ball will swing toward the rod initially, but will swing away after it makes contact with the rod.
Homework Problem 1.9 Copper has an atomic mass of 63.546 g
mol . Each copper atom has twenty-nine protons
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and twenty-nine electrons. How many coulombs, Q, of positive charge are there in the protons of a copper test
Select One of the Following:
(a) 2.5 × 103C
(b) 2.5 × 106C
(c) 3.2 × 106C
(d) 9.9 × 106C
(e) 9.9 × 109C
Homework Problem 1.10 How does the mass of the electron compare with the mass of the proton?
Select One of the Following:
(a) The two masses are equal.
(b) The electron is slightly more massive than the proton.
(c) The electron is slightly less massive than the proton.
(d) The electron is greatly more massive than the proton.
(e) The electron is greatly less massive than the proton.
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Open Response Questions
Homework Problem 1.11 A solid, insulating sphere has a radius of 15cm.
(a)If it carries a uniform surface charge density of σ = −0.5µC/m2
, calculate the total charge of the
sphere.
(b)If it carries a uniform volume charge density of ρ = 250nC/m3
, calculate the total charge of the
sphere.
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Homework Problem 1.12 Explain how you would build and charge an electrophorus. Be careful to draw
each stage of the charging process including the location of charge on each object.
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