Electrostatics

Electrostatics Electrostatics is

electricity at rest It involves electric

charges, the forces between them, and their behavior in material

An understanding of electricity takes a step-by-step approach

Electrostatics is the first step in understanding electricity as a whole

Electrical Forces and Charges You are familiar with the force of gravity Now imagine a force that acts on you that is

billions upon billions of times stronger Electrical attracting force

This force is so strong it could compress you to the thickness of a sheet of paper

Luckily, there is another force acting on you, equally as strong Electrical repelling force

These two forces balance so you do not notice their effects at all

These forces are electrical forces

Electrical Forces and Charges Electrical forces arise from particles in atoms In atoms, the nucleus is made up partially of

protons, which are positively charged Around the nucleus is a cloud of electrons, which

are negatively charged The positively charged protons are attracted to

the negatively charged electrons This attracting and repelling behavior is

attributed to a property called charge Opposite charges attract; Like charges repel

The number of protons and the number of neutrons in each atom are equal, so there is a balance of charge

Conservation of Charge Matter is made of atoms, and atoms are made

form protons and electrons (neutrons as well) An object that has an equal number of protons and

electrons has no net electric charge If there is an imbalance in the numbers, however, an

object is then electrically charged An imbalance comes about by adding or removing

electrons The outermost electrons in atoms can easily be

removed because they are farther from the positively charged nucleus

How easily electrons are removed varies with different substances

Conservation of Charge For example, electrons are held more firmly in

rubber than in hair If a rubber balloon is rubbed on human hair, electrons

will be physically transferred from the hair to the balloon The balloon then has an abundance of electrons and is

negatively charged The hair then has a deficiency of electrons and is

positively charged The negatively charged balloon will stick to a neutrally

charged wall because of its difference in charge Electrons are not created or destroyed, only

transferred There is always a conservation of charge

Questions Beneath the complexities of electrical

phenomena, there lies a fundamental rule from which nearly all other effects stem. What is this fundamental rule?

How does a charge of an electron differ from the charge of a proton (magnitude and sign)?

If you scuff electrons onto your feet while walking across a rug, are you negatively or positively charged?

Coulomb’s Law The relationship between the electrical forces of two

objects was discovered by the French physicist Charles Coulomb in the 18th century

Coulomb’s Law states that for charged particles, the force between the charges is directly proportional to the product of the charges, and inversely proportional to the square of the distance between them

Fe = k [(q1q2)/d2] d is the distance between the charged particles q1 is the quantity of charge of one particle q2 is the quantity of charge of the second particle k is the proportionality constant, 9.0 x 109

The SI unit of charge is the Coulomb (C)

Example What is the electrical force between the proton

and the electron of a hydrogen atom? The distance between the proton and the electron is 5.3

x 10-11 m The charge of a proton is 1.6 x 10-19 C The charge of an electron is -1.6 x 10-19 C

Fe = k [(q1q2)/d2]

Fe = -8.2 x 10-8

It is NOT necessary to carry the negative sign throughout the equation

Just remember likes repel, opposites attract

Conductors and Insulators Electrons are more easily moved in some materials than in

others Materials with outer electrons that are loosely bound to the

nucleus are good conductors of electricity The electrons are free to move within the material Metals

Materials with outer electrons that are closely bound to the nucleus are good insulators Insulators do not conduct electricity well Rubber, Glass

Semiconductors can be made to behave sometimes as conductors and sometimes as insulators

I.E. computer chips that are silicon based Superconductors are materials that have indefinite

conductivity once electric current is established by having very low resistance

I.E. Power transmission lines, MRI machines, and Maglev Trains

Charging by Friction and by Contact Charging by friction occurs

when electrons are transferred when one material rubs against another Scuff socks along a rug in the

dark and see and hear sparks

Charging by contact occurs when electrons are transferred from one material to another by simply touching A charged metal rod comes in

contact with a neutral object; some of the charge transfers

Charging by Induction If a charged object is brought near a

conducting surface, even without physical contact, electrons will move in the conducting surface

If a negatively charged metal rod is brought near a can of pop, electrons in the can near the rod are repelled and a separation of charge is induced in the can

The can will roll toward the charged rod!

Why? As long as the rod doesn’t touch the

can, the rod retains its original charge

When we touch the conducting surface with a finger, the charges that repel each other have a conducting path to the ground

This is called grounding the conductor

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

The kind we are most familiar with is the electrical discharge between the clouds and the oppositely charged ground

Most lightning is an electrical discharge between oppositely charged parts of clouds

Lightning

As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface. This leaves the ground as positive.

A streamer of negative charges is repelled by the bottom of the cloud and is attracted by the ground.

As this streamer approaches the ground, a streamer of positive charges is repelled by the ground and attracted to the negative streamer.

When the two streamers connect, they have created a fairly conductive path which allows a sudden down surge of electrons to jump to the ground. This is the lightning. The rapidly moving electrons excite the air along the path so much that it emits light and causes the air to expand so rapidly it causes

thunder.

Charged Polarization Charging by induction is

not limited to conductors 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

The atom or molecule is then said to be electrically polarized