Static Electricity

Physics

http://www.uwec.edu/jolhm/EH3/Group2/Pictures/lightning.jpg

History• Electron means “amber” in Greek

• Thales of Miletos 600 BC discovered properties by Greek.

• He rubbed amber (mineral) with cat fur and attracted feathers.

Ben Franklin’s Kite Experiment (1740’s)

Leyden Jar

Benjamin Franklin

• 1740’s lightning experiment with kite, key and Leyden jar (stores static electricity).

• Franklin developed the lightning rod.

• Proposed conservation of charge.

• Saw electricity as a flowing fluid and called the flow direction positive.

Law of Conservation of Electrical Charge

• The net charge of an isolated system remains constant.

Example:

• An object of +10C touched an identical object that was neutral. What is the charge of each object?

Law of Charges

• Like charges repel

• Opposite charges attract

J J Thomson (1897, England)

• He discovered the electron.

• He found that the mass of the electron is about 1/1800 of the mass of a hydrogen atom.

• He won the Nobel Prize (1906) for his discovery of the electron.

JJ Thomson with the CRT

Cathode Ray Tube

Cathode Ray

Electrical Charge

• Symbol: q

• Unit : Coulomb, C

Charge and Mass of the Electron

• Charge of Electron:

1.6 x 10-19 C (Coulombs)

• Mass of Electron:

9.11 x 10-31 kg.

First We Have to Remember Matter

How It Relates to Charge

Charged versus Uncharged Particles

Positively Charged Negatively Charged Uncharged

Possesses more protons than electrons

Possesses more electrons than protons

Equal numbers of protons and electrons

It’s All About Electrons

• Electrons within atoms can be persuaded to leave their own electron shells and become members of the electrons shells of other atoms of different materials.

• In short, electrons are migrants - constantly on the move and always ready to try out a new atomic environment.

It’s All About Electrons

• For electrons to make a move from the atoms of one material to the atoms of another material, there must be:– an energy source– a motive– and a low-resistance pathway.

Positively charged

objects lost electrons.

Example: Rubbing a glass rod with silk. • Rod becomes + (loses electrons)• Silk becomes - (gains electrons). 

Negatively charged objects have gained electrons.

Example: rubbing a rubber rod with fur. • Rubber Rod: - charged• Fur: + charged

Note

• Negatively charged objects have more mass than an identical neutral object, since each extra electron has a mass of 9.11 x 10-31 kg.

Types of Materials in terms of Electrical Conductivity

• Conductors (metals)

• Semiconductors (germanium, silicon)

• Insulators (wood, glass, rubber)

Conductors

• Materials that permit electrons to flow freely from atom to atom and molecule to molecule.

• Due to the metal’s “sea of electrons”

Conductors

• Outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but are free to roam in the material.

• Materials through which electric charge can flow are called conductors.

• Metals are good conductors for the motion of electric charges because their electrons are “loose.”

Insulators

• Electrons in other materials—rubber and glass, for example—are tightly bound and remain with particular atoms.

• They are not free to wander about to other atoms in the material.

• These materials, known as insulators, are poor conductors of electricity.

Electrostatic Charging Methods

• Friction

• Conduction

• Induction

Charging by Friction

• The two objects wind up with opposite charges.

We can stroke a cat’s fur and hear the crackle of sparks that are produced.We can comb our hair in front of a mirror in a dark room and see as well as hear the sparks of electricity. We can scuff our shoes across a rug and feel the tingle as we reach for the doorknob.Electrons are being transferred by friction when one material rubs against another.

Charging by Friction and Contact

If you slide across a seat in an automobile, you are in danger of being charged by friction.

Charging by Friction and Contact

Triboelectric Series+ Fur (rabbit)

GlassWoolFur (cat)LeadSilkHuman skin, AluminumCottonWoodAmberNickel, Copper, Brass, GoldRubberSulfur

- Celluloid

Charging by Conduction (direct contact)

• The objects end up with the same type charge.• If the charges are equal in size, they share the charge

equally.

http://www.ap.smu.ca/demonstrations/index.php?option=com_content&view=article&id=106&Itemid=85

Charging by Induction

• Objects ends up with opposite charge.• Involves grounding.

http://www.physics.sjsu.edu/becker/physics51/elec_charge.htm

When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground. When we allow charges to move off (or onto) a conductor by touching it, we are grounding it.

Charging by Induction

Charging by induction occurs during thunderstorms. The negatively charged bottoms of clouds induce a positive charge on the surface of Earth below. Most lightning is an electrical discharge between oppositely charged parts of clouds. The kind of lightning we are most familiar with is the electrical discharge between clouds and oppositely charged ground below.

Charging by Induction

If a rod is placed above a building and connected to the ground, the point of the rod collects electrons from the air.This prevents a buildup of positive charge by induction. The primary purpose of the lightning rod is to prevent a lightning discharge from occurring. If lightning does strike, it may be attracted to the rod and short-circuited to the ground, sparing the building.

Charging by Induction

Electroscopes are used to test the charge of an object.

When a charged object is brought near the electroscope,

its leaves spread apart.

http://upload.wikimedia.org/wikipedia/commons/e/ec/Electroscope_showing_induction.png

When a charged object touches an electroscope, the

electroscope is now charged.

What was the charge of the object that touched this

electroscope?

Charging by induction is not restricted to conductors. Charge polarization can occur in insulators that are near a charged object.When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material. Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves.

Charge Polarization

One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side.The atom or molecule is said to be electrically polarized.

Charge Polarization

a. When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange.

b. All the atoms or molecules near the surface of the insulator become electrically polarized.

Charge Polarization

Examples of Charge Polarization

Polarization explains why electrically neutral bits of paper are attracted to a charged object, such as a charged comb.Molecules are polarized in the paper, with the oppositely charged sides of molecules closest to the charged object.

Charge Polarization

The bits of paper experience a net attraction. Sometimes they will cling to the charged object and suddenly fly off. Charging by contact has occurred; the paper bits have acquired the same sign of charge as the charged object and are then repelled.

Charge Polarization

A charged comb attracts an uncharged piece of paper because the force of attraction for the closer charge is greater than the force of repulsion for the farther charge.

Charge Polarization

Rub an inflated balloon on your hair and it becomes charged. Place the balloon against the wall and it sticks. The charge on the balloon induces an opposite surface charge on the wall. The charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign.

Charge Polarization

Electric Dipoles

Many molecules—H2O, for

example—are electrically polarized in their normal states. The distribution of electric charge is not perfectly even. There is a little more negative charge on one side of the molecule than on the other. Such molecules are said to be electric dipoles.

Charge Polarization

Polarization

http://www.csiro.au/helix/sciencemail/activities/WaterBend.html

In summary, objects are electrically charged in three ways.

• By friction, when electrons are transferred by friction from one object to another.

• By contact, when electrons are transferred from one object to another by direct contact without rubbing.

• By induction, when electrons are caused to gather or disperse by the presence of nearby charge without physical contact.

Charge Polarization

If the object is an insulator, on the other hand, then a realignment of charge rather than a migration of charge occurs. This is charge polarization, in which the surface near the charged object becomes oppositely charged.

Charge Polarization

A ground is simply an object that serves as a seemingly infinite reservoir of electrons; the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object.

Grounding

Part II

• Coulomb’s Law

Coulomb’s Law

• 1785,Charles Augustin Coulomb (French scientist)

F = k q1q2

______

d2

• k = 9 x 109 Nm2/C2

• d (or r): distance between the charges.• q : charge of each object.

The Direction of the Electrical Force

Coulomb’s Law is an Inverse Square Law

http://web.ncf.ca/ch865/graphics/Coulomb.jpeg

The electrical force is one of the four fundamental forces.

Comparison with Gravitational Force

• What are 3 differences between the electrical force and the gravitational force?

Comparison with Gravitational Force

• What are 3 differences between the electrical force and the gravitational force?– 1) Electrical forces are greater than

gravitational forces.– 2) Electrical forces can repel or attract.

Gravitational forces are only attractive.– 3) Gravitational forces apply to masses;

electrical forces apply to charges.

Example 1

• A charge of 2C is 0.5 m from a charge of 3C. Find the electric force.

Example 2

• Three charges are positioned as shown. Find the force acting on the 2 C charge.

• +1 C

• 0.5 m

• +2 C

• 0.5 m -3 C

Example 3 Two equal charges are located 1m from

each other. The force acting between them is 2N.

How many Coulombs is each charge?

Answer: 15μC