Lesson 16 Electrostatics

LESSON 16

ELECTROSTATICS

Eleanor Roosevelt High SchoolChin-Sung Lin

Electrostatics

Electrostatics is the electricity at rest It involves electric charges, the force

between them, and their behavior in the materials

History

The word electricity comes from the Greek “elektron” which means “amber”

The “amber effect” is what we call static electricity

History

Based on the experiment situations, there must be two types of charges. Benjamin Franklin arbitrarily call one positive and another one negative

He also argued that when a certain amount of charge is produced on one body, an equal amount of the opposite charge is produced on the other body…

ELECTRIC CHARGE

Electric Charges

The fundamental rule of all electrical phenomena is

Like charges repelopposite charges

attract

Thomson’s Cathode Ray Experiment

Discover electrons in a gas discharge cathode-ray tube, and prove that the charge of electrons are negative

Millikan’s Oil Drop Experiment

The charge of oil drops are always the multiples of -1.60 x 10-19 C

Charge histogram in units of e of the 42.5 million drops measured during the 36 month experimental run

Millikan’s Oil Drop Experiment Summary as of January 2007

Total mass throughput for all experiments- 351.4 milligrams of fluid

Total drops measured all experiments - 105.6 million

Millikan’s Oil Drop Experiment What conclusion can you draw from

these data?

Electric charges are quantized (discrete)

Charges are always multiple of fixed charges

No fractional charge particles was found

Prove the existence of unit charge

Electric Charges

The SI unit of charge is the coulomb (C) The charge of an electron is -1.60 x 10-19

C 1 C is the charge of 6.24 x 1018 electrons Symbol: q, Q

Electric Charges

By convention, electrons are negatively charged protons are positively chargedneutrons have no charge

All electrons are identical All protons are identical All neutrons are identical A proton’s charge is equal in magnitude

to the negative charge of the electron

Electric Charges

Particle Charge (C) Mass (kg)

electron -1.6x10-19 9.109x10-31

proton +1.6x10-19 1.673x10-27

neutron 0 1.675x10-27

CHARGE BY FRICTION

Charge by Friction

Why is the rod got charged simplily by rubbing it with fur?

Charge by Friction

What decides which will gain or loss electrons during friction?

Charge by Friction

What happens when electrons are excited to such an extent that they leave the atoms?

Charge by Friction

Why won’t protons move?

Charge by Friction

Electrons are being transferred by friction when one material rubs against another

The one that gains electrons becomes negative. The one that lose electrons becomes positive

Different materials have different tendency to gain or lose electrons

Triboelectric Series

The triboelectric series is a list that ranks various materials according to their tendency to gain or lose electrons

The tendency of a material to become positive or negative after triboelectric charging has nothing to do with the level of conductivity of the material

The greater the relative position, the larger the expected electric charge

Triboelectric Series

Charging by Friction

CHARGE BY CONTACT

Charge by Contact

Electrons can be transferred from one material to another by simply touching

Charge by Contact

A positively charged metal ball with charge Q contacts with an identical electrically neutral metal ball

The charges equally redistribute to both balls

Separate these two balls and each one has charge Q/2

Electric Charges Distribution If the object is good conductor, the

charges will spread to all part of its surface because the like charges repel each other. Net charge inside is zero

Electric Charges Distribution If we dump a

bunch of electrons to the statue of liberty, and the statue is insulated from the ground, where will those electrons go?

Electric Charges Distribution On symmetric objects, charge

distributes uniformly, on non-symmetric objects, charges stay at sharp corners

Electric Charges Distribution

Why do electrons stay at sharp corners?

Electric Charges Distribution If the object is a poor conductor,

charge cannot move, so it remains localized around the contact region

LAW OF CONSERVATION OF CHARGE

Law of Conservation of Charge The net amount of electric charge

produced in any process is zero In an isolated system the total

charge is conserved Electrons are neither created nor

destroyed but are simply transferred from one material to another

Law of Conservation of Charge An object that has unequal

numbers of electrons and protons is electrically charged

In a neutral atom, there are as many electrons as protons, so there is no charge

A charged atom is called an ion. An imbalance comes about by adding or removing electrons

CHARGE BY INDUCTION

Charge by Induction

Why does the balloon stick to the wall?

Charge by Induction

If we bring a charged object near a conducting surface, even without physical contact, electrons will move in the conducting surface

Charge by Induction

The introduction of a charge into another body without contact being made

Temporary charging of this type produces Temporary Polarity

Charge by Induction

Two neutral metal balls A & B are placed next to each other and contact each other

Approach a negatively charged rod to ball A and different charges will be induced on each ball

Charge by Induction

Separate the two metal balls while the rod is still there

Remove the rod and the charges will be redistributed evenly on both balls

AIM: Charge by InductionDoNow: Started with a neutral ball with metal

paint and a negatively charged rod, how can you charge the ball with positive charges by induction?

Charge by Induction

Approach a negatively charged rod to a neutral ball with metal paint and different charges will be induced on each side of the ball

Charge by Induction

While the rod is still there, ground the opposite side of the ball and electrons will flow to the ground

Charge by Induction

A charged object is needed to charge an object by induction (without any contact)

The object being charged ends up with a charge which is the opposite of the object being used to charge it

A ground must be used to charge on the object. The ground allows for electron movement into or out of the object being charged

Charge by Induction Touch the negatively charged rod with

the positive charged ball, then the excess electrons of the rod will flow to the ball

Now like charges repel

Charge by Induction Charging by induction is not limited to

conductors When a charged rod is brought near an

insulator, 0n side of the atom or molecular is more positive (or negative) than the other side

Charge by Induction This explains why neutral bits of paper

are attracted to a charged object or the negatively charged balloon attached to the neutral wooden wall

Charge by Induction Many molecules (H2O for example) are

electrically polarized in their normal states. The distribution of electric charge is not perfectly even. Such molecules are said to be electric dipoles

Charge by Induction Microwave oven uses oscillating electric

field to make the water molecules oscillating. The flip-flopping H2O molecules impart thermal motion to surrounding food molecules

Lightning

The negatively charged bottoms of clouds induced a positive charge on the surface of Earth below

Becomes very “negative”

Becomes very “positive”

Lightning Rod

A metal rod mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning

TYPES OF MATERIALS

Types of materials

Conductor: a material that transfers charge easily (ex. Metals)

Types of materials

Insulator: a material that does not transfer charge easily (ex. glass, water, air, ceramic, nonmetals)

Types of materials

Semiconductors: somewhere between 1 & 2 (ex. Silicon, carbon, germanium)

Types of materials

Superconductors: some metals become perfect conductors below certain temperatures

ELECTROSTATICS INSTRUMENTS

Electrophorus

An electrophorus is used to produce electrostatic charge via the process of electrostatic induction

It consists of a plastic plate and a metal disc with an insulating handle

Electrophorus

Charge by friction, induction, and then conduction

Electroscope

Electroscope used to detect the

presence and magnitude of electric charge on a body

It consists of a vertical metal rod, from the end of which hang two strips of thin gold leaf

A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied

Electroscope

ELECTRIC FORCE

Electric Force— The Coulomb’s LawCoulomb’s Law

Describing the electrostatic force between electrically charged particles

Published in 1785 by French physicist Charles Augustin de Coulomb

Torsion balance experiment

Electric Force— The Coulomb’s Law

Torsion Balance

Cylindrical Glass case & tube

Torsion head & fiber

Needle & spheres

Removable spheres

Torsion Balance Experiment(Video 4 minutes)

Write down:

What’s the purpose?

Three major steps

What’s the conclusion?

Fe ~ q1 q2

Coulomb’s Law

The electric force between two charges is proportional to the product of the two charges and

Fe ~ 1/r2

Coulomb’s Law The electric force between two charges is

inversely proportional to the square of the distance between the charges

Coulomb’s Law

Fe ~ q1 q2

Fe ~ 1/r2

Coulomb’s Law

Fe ~ q1 q2

Fe ~ 1/r2

Coulomb’s Law

Fe ~ q1 q2

Fe ~ 1/r2

Coulomb’s Law

Fe ~ q1 q2

Fe = k q1 q2

Fe: electric force (N)

k: electrostatic constant (N m2/C2) q1, q2: charge (C)

r: distance between charges (m)

Fe = k q1 q2

Coulomb’s Law

k — A very large constant 8.99x109

What’s the unit of k? (1 minute)

Fe = k q1 q2

Electrostatic constant k

k =Fe r2

k — A very large constant 8.99x109 N m2/C2

What’s the unit of k?

Fe = k q1 q2

Electrostatic constant k

k =Fe r2

[ k ] = [ ]N m2

Charge 1 Charge 2 Distance Electric Force

q1 q2 d Fe

2q1 q2 d

3q1 q2 d

q1 4q2 d

q1 5q2 d

2q1 3q2 d

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d

q1 4q2 d

q1 5q2 d

2q1 3q2 d

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d 3Fe

q1 4q2 d

q1 5q2 d

2q1 3q2 d

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d 3Fe

q1 4q2 d 4Fe

q1 5q2 d

2q1 3q2 d

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d 3Fe

q1 4q2 d 4Fe

q1 5q2 d 5Fe

2q1 3q2 d

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d 3Fe

q1 4q2 d 4Fe

q1 5q2 d 5Fe

2q1 3q2 d 6Fe

4q1 ½ q2 d

q1 q2 d Fe

2q1 q2 d 2Fe

3q1 q2 d 3Fe

q1 4q2 d 4Fe

q1 5q2 d 5Fe

2q1 3q2 d 6Fe

4q1 ½ q2 d 2Fe

q1 q2 d Fe

q1 q2 2d

q1 q2 3d

q1 q2 4d

q1 q2 ½ d

q1 q21/3 d

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d

q1 q2 4d

q1 q2 ½ d

q1 q21/3 d

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d Fe /9

q1 q2 4d

q1 q2 ½ d

q1 q21/3 d

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d Fe /9

q1 q2 4d Fe /16

q1 q2 ½ d

q1 q21/3 d

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d Fe /9

q1 q2 4d Fe /16

q1 q2 ½ d 4Fe

q1 q21/3 d

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d Fe /9

q1 q2 4d Fe /16

q1 q2 ½ d 4Fe

q1 q21/3 d 9Fe

q1 q21/4 d

q1 q2 d Fe

q1 q2 2d Fe /4

q1 q2 3d Fe /9

q1 q2 4d Fe /16

q1 q2 ½ d 4Fe

q1 q21/3 d 9Fe

q1 q21/4 d 16Fe

AIM: Electric Force— The Coulomb’s Law

DoNow: (3 minutes)

1. Write down the “Coulomb’s Law”.

2. The particle A carries 1.2 x 10-4 C, the particle B carries 2.4 x 10-4 C, and the particle C carries 9.7 x 10-8 C. The three particles form a equilateral triangle with the measure of the side 0.45 m. What’s the ratio of the repelling forces between A-C and B-C?

q1 q2 d Fe

2q1 q2 2d

q1 3q2 2d

2q1 2q2 2d

3q1 2q2 2d

3q1 ½ q2 ½ d

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d

2q1 2q2 2d

3q1 2q2 2d

3q1 ½ q2 ½ d

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d 3Fe /4

2q1 2q2 2d

3q1 2q2 2d

3q1 ½ q2 ½ d

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d 3Fe /4

2q1 2q2 2d Fe

3q1 2q2 2d

3q1 ½ q2 ½ d

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d 3Fe /4

2q1 2q2 2d Fe

3q1 2q2 2d 3Fe /2

3q1 ½ q2 ½ d

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d 3Fe /4

2q1 2q2 2d Fe

3q1 2q2 2d 3Fe /2

3q1 ½ q2 ½ d 6Fe

½ q1 ½ q2 ½ d

q1 q2 d Fe

2q1 q2 2d Fe /2

q1 3q2 2d 3Fe /4

2q1 2q2 2d Fe

3q1 2q2 2d 3Fe /2

3q1 ½ q2 ½ d 6Fe

½ q1 ½ q2 ½ d Fe

Determine the electrical force of attraction between two balloons with separate charges of 3.5 x 10-8 C and -2.9 x 10-8C when separated a distance of 0.65 m.

Fe = k q1 x q2/r2

Fe = 8.99 x 109 Nm2/C2 x (3.5 x 10-8 C)(-2.9 x 10-8 C) / (0.65 m)2

= -2.16 x 10-5 N

Determine the electrical force of attraction between two balloons with separate charges of 3.5 x 10-8 C and -2.9 x 10-8C when separated a distance of 0.65 m.

Electric Force Exercise

Each of the two identical hot-air balloons acquires a charge of 3.2 x 10-6 C on its surface as it travels through the air. How far apart are the balloons if the electrostatic force between them is 4.5 x 10-2 N?

Electric Force Exercise

Fe = k q1 x q2/r2

4.5 x 10-2 N = 8.99 x 109 Nm2/C2 x (3.2 x 10-6 C)(3.2 x 10-6 C) / r2

r = 1.43 m

Each of the two identical hot-air balloons acquires a charge of 3.2 x 10-6 C on its surface as it travels through the air. How far apart are the balloons if the electrostatic force between them is 4.5 x 10-2 N?

Electrical & Gravitational Forces

Fg: gravitational force (N)

G: Universal gravitational constant, 6.77x10-11 N m2/kg2

m1, m2: mass (kg)

r: distance between masses (m)

Fg = G m1 m2

Law of Universal Gravitation

Electric Force vs. Gravitational Forces

Force Electric Force

Gravitational Force

Fg = G m1 m2

Law of Universal Gravitation Coulomb’s Law

Fe = k q1 q2

Electric Force

Electrostatic constant (k) = 8.99 x 109 Nm2/C2

k is a very, very large number!

Compare to G = 6.67 x 10 -11 N m2/kg2

Electrical force is much stronger than the gravity

The reason why we don’t feel it most of the time is because that the attracting force and repelling force cancel each other

Electric Force

Comparison of four basic forces

Force Strong

Electromagnetic Weak Gravit

yStrengt

h 1 1/137 10-6 6x10-39

Range 10-15 m

∞10-18 m

Electric Force

Model of a helium atom: Based on the atomic model proposed by Rutherford and Bohr, a positively charged nucleus is surrounded by electrons

Electrons are attracted to protons in the nucleus, but electrons repel other electrons

+Model of a Helium atom

Electric Force

Why don’t protons pull the oppositely charged electrons into the nucleus?

Model of a Helium atom

Electric Force

The reason why don’t protons pull the oppositely charged electrons into the nucleus is in the domain of quantum physics. An electron behaves like a wave and has to occupy a certain amount of space related to its wavelength

Electric Force

Why the protons in the nucleus do not mutually repel and fly apart?

Electric Force

The reason why the protons in the nucleus do not mutually repel and fly apart is that in addition to electrical forces in the nucleus, there are even stronger forces (strong forces) that are non-electrical in nature

The End

Lesson 16 Electrostatics

charge of electrons

negative charge

identicala protons charge

coulomb cthe charge

charges repelopposite

chargeall electrons

loss electrons

types of charges

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