PHYS 221 General Physics II
Coulomb’s Law
Spring 2015
Assigned Reading: 17.1-17.2
Lecture 1
Electricity, Light and Modern Physics
Prof. Leonid Rokhinson
… is a condensed matter physicist
single electron counting
magnetic materials and devices
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superconductivity
Quantum computing
Want to learn more? Visit https://www.physics.purdue.edu/leogroup/
PHYS 221 General Physics: Electricity,
Light and Modern Physics
• Physics Department Home page: http://www.physics.purdue.edu/
• Course Home page: http://www.physics.purdue.edu/phys221
• CHIP Home page: http://chip.physics.purdue.edu/public/221/spring2015/
• Room 144- Physics Undergraduate Office
• Room 11- Physics Help Center
will be posted on the course home page
• Room 290- Physics Library
Register iClickers by Wednesday on CHIP!
Syllabus
You may find syllabus (as well as all course-related
notes, calendar, lecture notes, link to CHIP (HW), etc)
on PHYS 221 home page.
http://www.physics.purdue.edu/phys221
READ IT, PLEASE
Unsolicited Tips
• Pre-lecture reading
– Come to the lectures prepared!
• Note taking
– Lecture notes can be downloaded from the Phys221 Home page.
– But they are sketchy … you need to fill in details in class.
• Post-lecture reading
– Go over ppt notes and your own notes.
– Use ppt notes as a checklist to see if things are clear.
– Read the relevant sections of the textbook on things that are not.
• Homework
– Do homework only after post-lecture reading.
– Go back to the notes when you get stuck on a problem.
– DO IT YOURSELF
• Seeking help
– Set up an appointment to see me
– Physics Help Center (the schedule will be posted)
Lecture 1 Electric Charges & Coulomb’s Law
http://www.diyhappy.com/wpcontent/images/Lightning.bmp
www.ehow.com/how_2180464_ reducestatic-cling.html
Electric Charge
Electric charge is an intrinsic property of the fundamental
particles that make up objects.
Positive Charge
Negative Charge
+
 Electrically neutral: object contains equal amounts
of positive and negative charges
 Net charge: imbalance in charge
Electric Charge
Net charge of a system:
algebraic sum of all the charges
Law: Conservation of charge
The net charge of a closed system never changes
Charge Carriers
neutral atom
e
–
e–
+Ze
e–
Particles
Symbol
Charge
Electron
e or e-
e-
Proton
p
e+
Neutron
n
0
nucleus
electron
orbits
An atom consists of the same number of electrons and
proton and is neutral, so a proton must carry the same
amount of charge as an electron.
Electric Charge
Electric charge is quantized
q = ne,
n = 0,±1,±2,±3,.....
Elementary charge:
e = 1.60602176462(63) x 10–19 C
Coulomb (C): one coulomb is the amount of charge that is
transferred through the cross section of a wire in 1 second
when there is a current of 1 ampere in the wire.
Robert Millikan, (Chicago University)
Oil drop experiments (1913), Nobel Prize 1923
A. Free fall (terminal velocity)
𝐹𝑑𝑟𝑎𝑔 = 6𝜋𝒓𝜂𝑣1
4𝜋 3
𝐹𝑏𝑜𝑦𝑎𝑛𝑡 =
𝑟 𝜌𝑎𝑖𝑟 𝑔
3
𝐹𝑔𝑟𝑎𝑣
4𝜋 3
=
𝑟 𝜌𝑜𝑖𝑙 𝑔
3
B. Up-floating in electric field
𝑉
𝐹𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = 𝑞𝐸 = 𝒒
4𝜋 3 𝑑
𝐹𝑏𝑜𝑦𝑎𝑛𝑡 =
𝑟 𝜌𝑎𝑖𝑟 𝑔
3
4𝜋 3
𝐹𝑔𝑟𝑎𝑣 =
𝑟 𝜌𝑜𝑖𝑙 𝑔
3
𝐹𝑑𝑟𝑎𝑔 = 6𝜋𝑟𝜂𝑣2
Coulomb’s Law of Electro-static Force
r
q2
q1
The electro-static force of attraction/repulsion has a
magnitude:
Coulomb’s Law
where:
k=
1
4pe o
= 8.99x10 9 Nm 2 / C 2
and the permittivity constant is
e o = 8.85x10 C / Nm
-12
2
2
Coulomb’s Law of Electro-static Force
q1 q2
F =k
2
r
*Each particle exerts a
force of this magnitude on
the other particle.
*The two forces form an
action-reaction pair.
Coulomb’s Law of Electrostatic Force
Force exerted by q1 on q2 at a distance r12
kq1q2
F12 = 2 r̂1,2
r1,2
q1, q2 in Coulombs (C)
r12 in meters (m)
F12 in Newtons (N)
F12
Same sign charges: F12 is in the direction of r1,2 .
Opposite sign charges: F12 is in the direction opposite to r1,2 .
Coulomb’s Law Analogous to Newton’s
Equation of Gravitation
m1m2
F =G 2
r
q1 q2
F =k
2
r
Analogous
* k electro-static constant
* Inverse Square Law
* Charge
* G gravitational constant
* Inverse Square Law
* Mass
DIFFERS
*Attractive/repulsive
depending on sign of
charges
*Two kinds of charges
*Dominates on small scale
*Always attractive
*One kind of mass
*Dominates on large scales
Three charges on a line
Fnet
R
net electric force on q1
Fnet = F21 + F31
kq2 q1
F21 = 2 r̂1,2
r1,2
kq3q1
F31 = 2 r̂1,3
r1,3
Problem Solving Strategies:
• Draw a free-body diagram
• Use consistent units (SI: meter, Coulomb,
Newton)
• Remember that the force is a vector
i>Clicker question
i>Clicker question
Conductors vs Insulators
• Conductors: material in which electric charges can
move around “freely.
(metals, tap water, body)
• Insulators: material in which electric charges are
“frozen” in place.
(air, glass, plastic)
• Semi-conductor: material in which electric charges
can move around but not as freely as in conductors.
(silicon, germanium)
• Super-conductor: no resistance to the movement of
charge.
Mobility of Charge
Conductors: material in which electric charges can move
around “freely”.
• Negatively charged plastic rod
will attract either end of the
electrically isolated copper rod
• Reason: charges in copper
rod can redistribute
themselves.
Charging by Induction
1. Bring a charged rod
close to conductor.
3. Break connection to
ground, keeping the
charged rod in place
2. Ground the conductor.
4. Remove
the rod. The
sphere is
charged.
Interaction of Charges: Insulators
Insulators: material in which electric charges are
“frozen” in place.
Conductors vs Insulators
• Demo 5A-04: Charges are more readily
transferred by conductors