Unit 14: Electrostatics
Units of Chapter 16
• Static Electricity; Electric Charge and Its
Conservation
• Electric Charge in the Atom
• Insulators and Conductors
• Induced Charge; the Electroscope
• Coulomb’s Law
• Solving Problems Involving Coulomb’s Law and
Vectors
• The Electric Field
Units of Chapter 16
• Field Lines
• Electric Fields and Conductors
Do Now
1. How do protons and electrons differ in their
electrical charge?
2. Is an electron in a hydrogen atom the same as
an electron in a uranium atom?
3. Which has more mass – a proton or an
electron?
4. In a normal atom, how many electrons are
there compared to protons?
Do Now
1. How do protons and electrons differ in their
electrical charge?
Same magnitude, but opposite charge.
2. Is an electron in a hydrogen atom the same as
an electron in a uranium atom?
Yes.
3. Which has more mass – a proton or an
electron?
Proton
4. In a normal atom, how many electrons are
there compared to protons?
Same number, no net charge.
Atomic Structure
16.1 Static Electricity; Electric Charge
and Its Conservation
Objects can be charged by rubbing
Triboelectric Series
• Friction can cause electrons to transfer
from one material to another
• Different materials have a different degree
of attraction for electrons
• The triboelectric series determines which
materials have a greater attraction.
• When two materials are rubbed together,
the one with the higher attraction will end
up getting some of the electrons from the
other material
Triboelectric Series
If two materials are rubbed together, the one that is higher in
the series will give up electrons and become more positive.
MORE POSITIVE
Human Hands (if very dry)
Leather
Rabbit Fur
Glass
Human Hair
Nylon
Wool
Fur
Lead
Silk
Aluminum
Paper
Cotton
Steel (neutral)
Wood
Amber
Hard Rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Saran Wrap
Polyurethane
Polyethylene (scotch tape)
Polypropylene Vinyl (PVC)
Silicon
Teflon
MORE NEGATIVE
Question
• If fur is rubbed on glass, will the glass
become positively charged or negatively
charged?
• Glass – positive
• Fur -negative
16.1 Static Electricity;
Electric Charge and Its
Conservation
Charge comes in two
types, positive and
negative; like charges
repel and opposite
charges attract
Interaction Between Charged
and Neutral Objects
• Any charged object - whether positively
charged or negatively charged - will have
an attractive interaction with a neutral
object.
16.1 Static Electricity; Electric Charge
and Its Conservation
Electric charge is conserved – the
arithmetic sum of the total charge cannot
change in any interaction.
16.2 Electric Charge in the Atom
Atom:
Nucleus (small,
massive, positive
charge)
Electron cloud (large,
very low density,
negative charge)
16.2 Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons
from one to the other.
16.2 Electric Charge in the Atom
Polar molecule: neutral overall, but charge not
evenly distributed
16.3 Insulators and Conductors
Conductor:
Insulator:
Charge flows freely
Almost no charge flows
Metals
Most other materials
Some materials are semiconductors.
How Charge Is Transferred
• Objects can be charged by rubbing
• Metal objects can be charged by
conduction
• Metal objects can be charged by
• induction
16.4 Induced Charge; the Electroscope
Metal objects can be charged by conduction:
Charging by Induction
• When an object gets charged by induction, a
charge is created by the influence of a
charged object but not by contact with a
charged object. The word induction means to
influence without contact.
• If a rubber balloon is charged negatively
(perhaps by rubbing it with animal fur) and
brought near (without touching) the spheres,
electrons within the two-sphere system will be
induced to move away from the balloon.
Charging By Induction
With Negatively Charged Object
In step iii, why is the charge on the right sphere
almost uniformly distributed?
Charging By Induction
With Negatively Charged Object
What was the source of positive charge
that ended up on sphere B?
Source of charge in induction
• In induction, the source of charge that is
on the final object is not the result of
movement from the charged object to the
neutral object.
Ground
• An infinite source or sink for
charge
16.4 Induced Charge; the Electroscope
They can also be charged by induction:
16.4 Induced Charge; the Electroscope
The electroscope
can be used for
detecting charge:
16.4 Induced Charge; the Electroscope
The electroscope can be charged either by
conduction or by induction.
16.4 Induced Charge; the Electroscope
The charged electroscope can then be used to
determine the sign of an unknown charge.
16.4 Induced Charge; the Electroscope
Nonconductors won’t become charged by
conduction or induction, but will experience
charge rearrangement. The atoms or molecules
become polarized.
:
16.4 Induced Charge; the Electroscope
Nonconductors won’t become charged by
conduction or induction, but will experience
charge separation:
Do Now
True or False? Explain your reasoning.
1. An object that is positively charged contains
all protons and no electrons.
False
Positively charged objects have electrons; they
simply possess more protons than electrons.
2. An object that is electrically neutral contains
only neutrons.
False
Electrically neutral atoms simply possess the
same number of electrons as protons.
Coulomb’s Law
• The French physicist Charles Coulomb (1736 –
1806) investigated electric forces in the 1780s
using a torsion balance.
16.5 Coulomb’s Law
Experiment shows that the electric force
between two charges is proportional to the
product of the charges and inversely
proportional to the distance between them.
Coulomb’s Law
1. If two point charges Q1 and Q2 are a
distance r apart, the charges exert forces on
each object of magnitude:


Q1 Q2
F1on 2  F2 on1  k
2
r
These forces are an action/reaction pair,
equal in magnitude but opposite in direction.
16.5 Coulomb’s Law
2. The forces are along the line connecting the
charges, and is attractive if the charges are
opposite, and repulsive if they are the same.
16.5 Coulomb’s Law
Unit of charge: coulomb, C
The proportionality constant in Coulomb’s
law is then:
When we only need two significant figures:
k  9.0 10 N  m / C
9
2
2
Charges produced by rubbing are
typically around a microcoulomb:
16.5 Coulomb’s Law
Charge on the electron:
Electric charge is quantized in units
of the electron charge.
This is the smallest charge in nature –
fundamental or elementary charge.
The net charge of any object must be a multiple
of that charge.
16.5 Coulomb’s Law
The proportionality constant k can also be
written in terms of
, the permittivity of free
space:
(16-2)
16.5 Coulomb’s Law
Coulomb’s law strictly applies only to point charges.
Superposition: for multiple point charges, the forces
on each charge from every other charge can be
calculated and then added as vectors.
16.6 Solving Problems Involving
Coulomb’s Law and Vectors
The net force on a charge is the vector
sum of all the forces acting on it.
16.6 Solving Problems Involving
Coulomb’s Law and Vectors
Vector addition review:
Example 1
Suppose that two point charges, each with a
charge of +1.00 Coulomb are separated by
a distance of 1.00 meter. Determine the
magnitude of the electrical force of repulsion
between them.
Given:
Find: F - ?
IQI I= 1.00 C
Q2 I= 1.00 C
r = 1.00 m
Solve:
F k
Q1 Q2
r
2
(9.0 10 )(1)(1)
9
F
2
1
 9.0 10 N
9
The force of repulsion of two +1.00
Coulomb charges held 1.00 meter apart is
9 billion Newton. This is an incredibly large
force that compares in magnitude to the
weight of more than 2000 jetliners.
Example 2
Determine the magnitude and direction of
the electric force on the electron of a
hydrogen atom exerted by the single proton
that is the atom’s nucleus. Assume the
average distance between the revolving
10
electron an the proton r  0.53 10 m
Given:
Find: F-?
Q1  Q2  1.6 10 19 C
r  0.531010 m
Solution:
F k
Q1 Q2
r2
(9.0 109 N  m 2 / C 2 )(1.6 10 19 C )(1.6 10 19 C )
8


8
.
2

10
N
10
2
(0.53 10 m)
Problem 1
A balloon with a charge of 4.0 10 5 C is held a
distance of 0.10m from a second balloon
having the same charge. Calculate the
magnitude of the electrical force between
the charges. Draw a diagram.
F k
Q1 Q2
r2
(9.0 109 N  m 2 / C 2 )(4.0 10 5 C )(4.0 10 5 C )
1


14400

10
 1440 N
2
(0.10m)
Problem 2
• Calculate the electrical force exerted
between a 22-gram balloon with a charge
of -2.6μC and a wool sweater with a
charge of +3.8μC; the separation
distance is 75cm. (Note: 1C  1106 C)
F k
Q1 Q2
r2
(9.0 109 N  m 2 / C 2 )(2.6 10 6 C )(2.6 10 6 C )

 158.08 10 3 N  0.158 N
2
(0.75m)