ElectrostaticsElectrostatics

The study of electrical charges at The study of electrical charges at restrest

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

The Greeks observed that when the The Greeks observed that when the amber was polished with a piece of fur, it amber was polished with a piece of fur, it would attract small leaves and dust would attract small leaves and dust particlesparticles

We now know that this is caused by static We now know that this is caused by static electricityelectricity

Electrical ForcesElectrical Forces

Electrical forces arise from the interactions Electrical forces arise from the interactions between electrically charged particlesbetween electrically charged particles Postive charges – protons (pPostive charges – protons (p++)) Negative charges – electrons (eNegative charges – electrons (e--))

Opposite charges attractOpposite charges attract

Similar charges repelSimilar charges repel

AtomsAtoms

Atoms are made of a nucleus containing Atoms are made of a nucleus containing protons and neutrons (nprotons and neutrons (n00) surrounded by ) surrounded by electronselectronsThe atom as a whole usually contains the The atom as a whole usually contains the same number of protons, neutrons, and same number of protons, neutrons, and electrons.electrons.If there are the same amount of positives If there are the same amount of positives than negatives…the atom is electrically than negatives…the atom is electrically neutral in charge.neutral in charge.

Charging objectsCharging objects

If an electron is removed then it is no If an electron is removed then it is no longer neutrallonger neutral

The atom now has one more positive The atom now has one more positive charge than negative so it is positively charge than negative so it is positively charged.charged.

Positively and negatively charged atoms Positively and negatively charged atoms are called ions.are called ions.

Charging objectsCharging objects

When objects become charged, ONLY When objects become charged, ONLY electrons are either gained or lost.electrons are either gained or lost.

Why would electrons be the ONLY particle Why would electrons be the ONLY particle to be transferred?to be transferred?

Electrons are more easily stripped off of Electrons are more easily stripped off of atoms than protons because they are atoms than protons because they are found on the outside of the atomfound on the outside of the atomLaw of Conservation of ChargeLaw of Conservation of Charge No electrons are created or destroyed when No electrons are created or destroyed when

objects become charged…only transferredobjects become charged…only transferred The net amount of electric charged produced The net amount of electric charged produced

in any process is zeroin any process is zero

BalloonsBalloons

When objects become charged, they tend When objects become charged, they tend to lose their charge over time.to lose their charge over time.

Where does the charge go?Where does the charge go? The charge usually leaks off into water The charge usually leaks off into water

molecules in the airmolecules in the air Water molecules polar, meaning their charge Water molecules polar, meaning their charge

is not distributed evenly and have a positive is not distributed evenly and have a positive and negative side (similar to a bar magnet)and negative side (similar to a bar magnet)

Static ElectricityStatic Electricity

The word ‘static’ means not movingThe word ‘static’ means not moving

So static electricity is defined as charges So static electricity is defined as charges that are not moving that are not moving

It is easier to hold a static charge on a dry It is easier to hold a static charge on a dry day rather than a humid day since the day rather than a humid day since the polar water molecule will steal the stored polar water molecule will steal the stored charge when colliding with the charged charge when colliding with the charged objectobject

Insulators and ConductorsInsulators and Conductors

Insulators are materials that do not allow Insulators are materials that do not allow electric charge to move through it easilyelectric charge to move through it easily Examples?Examples?

Conductors are materials that allow Conductors are materials that allow electric charge to move through it easilyelectric charge to move through it easily Examples?Examples?

When a piece of metal is placed across When a piece of metal is placed across the two spheres, charge is allowed to the two spheres, charge is allowed to move easily into the other object.move easily into the other object.

The positive particles want to repel each The positive particles want to repel each other and get as far away from each other other and get as far away from each other as possible forcing their way throught the as possible forcing their way throught the conductor.conductor.

The charges can not move through the air The charges can not move through the air or through the wood to spread out, or through the wood to spread out, because they are not good conductorsbecause they are not good conductors

What makes some materials good What makes some materials good conductors is the way the electrons are conductors is the way the electrons are bound very loosely to the atoms and can bound very loosely to the atoms and can move freely within the materialmove freely within the material

In insulators the electrons are tightly In insulators the electrons are tightly bound to the atombound to the atom

Charging by ContactCharging by ContactWhen two substances are in contact, one When two substances are in contact, one of them has to give up some of its loose of them has to give up some of its loose electrons to the otherelectrons to the other

Usually charges are stripped away through Usually charges are stripped away through friction caused when two objects are friction caused when two objects are rubbed togetherrubbed together

Charging by conductionCharging by conductionAny excess charges that are gained on a good Any excess charges that are gained on a good conductor always resided on its outer surface conductor always resided on its outer surface regardless of shape.regardless of shape.

Charges simply push each other away to the Charges simply push each other away to the very extremities of the objectvery extremities of the object

John John TravoltageTravoltage

If the object is not a good conductor, the object If the object is not a good conductor, the object might have to be charged in various locations might have to be charged in various locations along the surface to spread the chargealong the surface to spread the charge

Charging by ConductionCharging by Conduction

Charging by InductionCharging by Induction

An object can become charged without An object can become charged without contact, a charge can be ‘induced’.contact, a charge can be ‘induced’.

Charging through induction occurs when Charging through induction occurs when another charged object is brought next to a another charged object is brought next to a neutral object and the charges separateneutral object and the charges separate

If the object is moved away the charges return If the object is moved away the charges return to normalto normal

If the object was grounded, the electrons would If the object was grounded, the electrons would flow out of the object to leave a net positive flow out of the object to leave a net positive charge on the objectcharge on the object

Charge PolarizationCharge Polarization

When a charged object is brought near an When a charged object is brought near an insulator, there is no movement of free electronsinsulator, there is no movement of free electrons

Instead, there is a rearrangement of charges Instead, there is a rearrangement of charges within the individual atoms so that the atoms are within the individual atoms so that the atoms are electrically polarized (like the poles on a magnet)electrically polarized (like the poles on a magnet)

Check your UnderstandingCheck your Understandingfrom The Physics Classroom Tutorial from The Physics Classroom Tutorial

A negatively charged balloon is brought near a A negatively charged balloon is brought near a neutral conducting sphere as shown below. As it neutral conducting sphere as shown below. As it approaches, charge within the sphere will approaches, charge within the sphere will distribute itself in a very specific manner. Which distribute itself in a very specific manner. Which one of the diagrams below properly depicts the one of the diagrams below properly depicts the distribution of charge in the sphere? distribution of charge in the sphere?

Check your UnderstandingCheck your Understanding

Two neutral conducting pop cans are touching Two neutral conducting pop cans are touching each other. A positively charged balloon is each other. A positively charged balloon is brought near one of the cans as shown below. brought near one of the cans as shown below. The cans are separated while the balloon is The cans are separated while the balloon is nearby, as shown. After the balloon is nearby, as shown. After the balloon is removed the cans are brought back together. removed the cans are brought back together. When touching again, can X is ____.When touching again, can X is ____.

Check your UnderstandingCheck your Understanding

A positively charged balloon is brought near a A positively charged balloon is brought near a neutral conducting sphere as shown below. neutral conducting sphere as shown below. While the balloon is near, the sphere is touched While the balloon is near, the sphere is touched (grounded). (grounded).

At this point, there is a movement of electrons. At this point, there is a movement of electrons. Electrons move ____ . Electrons move ____ .

ElectroscopeElectroscope

Electric FieldElectric Field

The space around every electrically The space around every electrically charged body creates an electric field.charged body creates an electric field. Just as every object that has mass has a Just as every object that has mass has a

gravitational field around itgravitational field around it

Electric field is a vector quantityElectric field is a vector quantity

If a body with charge If a body with charge qq experiences a experiences a force force FF at some point in space the Electric at some point in space the Electric field field EE is is

q

FE

If a positive test charge was placed near If a positive test charge was placed near the charges below, and vectors are the charges below, and vectors are pointing in the direction of the electric pointing in the direction of the electric force on the test charge, what would be force on the test charge, what would be the charge of each?the charge of each?

+ -

Electric FieldElectric Field

Electric ForceElectric Force

The electric field acts similar to a field of gravity, The electric field acts similar to a field of gravity, so there will be a force acting on the chargeso there will be a force acting on the charge

Coulomb’s lawCoulomb’s law

q q represents the charge of the particle measure represents the charge of the particle measure in coulombs [C]in coulombs [C]

k k is Boltzman’s constant equal to 9x10is Boltzman’s constant equal to 9x1099 N N·m·m22/C/C22

221

d

qqkF

Quantized chargeQuantized charge

Charge in quantized so charge only exists Charge in quantized so charge only exists in discrete amounts (1in discrete amounts (1qq, 2, 2qq, 3, 3qq…n…nqq))

You can never lose a fraction of a chargeYou can never lose a fraction of a charge

The charge of an electron isThe charge of an electron is e = -e = -1.602x101.602x10-19-19 C C

The charge on a proton is the same The charge on a proton is the same amount but (+)amount but (+)

Electric PotentialElectric Potential

Just as mass has potential energy in a Just as mass has potential energy in a gravitational field, charge has potential gravitational field, charge has potential energy in the presence of an electric fieldenergy in the presence of an electric field

A positive charge A positive charge placed in this electric placed in this electric field will be pushed field will be pushed away buy the positive away buy the positive side and pulled side and pulled towards it by the towards it by the negative. negative.

The potential will The potential will change to kinetic as it change to kinetic as it moves. moves.

The opposite occurs if The opposite occurs if the charge is changed the charge is changed to a negative.to a negative.

The negative charge The negative charge will move in the will move in the opposite direction of opposite direction of the electric fieldthe electric field

What is going to happen?What is going to happen?

+ + + + + + + + + + + + +

- - - - - - - - - - - - - - -

- v

Acts like a projectile on Earth

What is going to happen?What is going to happen?

+ + + + + + + + + + + + +

- - - - - - - - - - - - - - -

+ v

Electric Potential EnergyElectric Potential Energy

The change in PE is equal to the work The change in PE is equal to the work done to move a chargedone to move a charge

W=FdW=Fd

And force is equal to Eq from And force is equal to Eq from

So we get the equationSo we get the equation

W=qEd W=qEd which is electric which is electric

potential energypotential energy

q

FE

A difference in electric potential energy is A difference in electric potential energy is called the called the potential differencepotential difference

Potential difference is measured in voltsPotential difference is measured in volts

It measures the difference in PE between It measures the difference in PE between two points per unit of chargetwo points per unit of charge

Equipotential LinesEquipotential Lines

All points between charged objects that All points between charged objects that have the same potentialhave the same potential

Hence Equal PotentialHence Equal Potential

An equipotential line must beAn equipotential line must be perpendicular to the electric fieldperpendicular to the electric field

Charge has same potential energy along this lineMoving it requires no Work, the voltage doesn’t change.

16V 10V14V 12V 8V

If the point charge moves between the charges then work will be done and the potential energy will change