What Gives an Electric Charge?What Gives an Electric Charge? An imbalance of protons and electrons.An imbalance of protons and electrons.

Neutral objects have equal numbers of electrons and Neutral objects have equal numbers of electrons and protons.protons.

Positively charged objects have more protons than Positively charged objects have more protons than electrons.electrons.

Negatively charged objects have more electrons than Negatively charged objects have more electrons than protons.protons.

When you rub a rubber rod with rabbit fur electrons are When you rub a rubber rod with rabbit fur electrons are transferred to the rubber rod and it becomes negatively transferred to the rubber rod and it becomes negatively charged.charged.

Conservations of chargeConservations of charge When electrons are transferred from one substance to When electrons are transferred from one substance to

another the overall charge is conserved.another the overall charge is conserved.

The SI Unit for Charge is the The SI Unit for Charge is the CoulombCoulomb

The charge of a proton is +1.6 x 10The charge of a proton is +1.6 x 10-19-19 Coulombs.Coulombs.

The charge of a electron is -1.6 x 10The charge of a electron is -1.6 x 10-19-19 Coulombs.Coulombs.

This is why an atom is neutral when there This is why an atom is neutral when there are equal numbers of protons and are equal numbers of protons and neutrons.neutrons.

ConductorsConductors

Conductors have electrons that are more Conductors have electrons that are more loosely held and therefore allow charges loosely held and therefore allow charges to flow more easily.to flow more easily.

Good conductors are:Good conductors are:MetalsMetalsWater and solutions with ions present.Water and solutions with ions present.

Wires are usually composed of copper wire.Wires are usually composed of copper wire.

InsulatorsInsulators

Insulators have more tightly held electrons and Insulators have more tightly held electrons and are not good conductors of electricity.are not good conductors of electricity. Good Insulators are:Good Insulators are:

RubberRubber PlasticPlastic AirAir SilkSilk

Good electrical insulators are usually also good Good electrical insulators are usually also good thermal insulatorsthermal insulators..

Copper wires are usually encased in rubber to Copper wires are usually encased in rubber to prevent shock.prevent shock.

MillikanMillikan In 1909 Robert Millikan a physicist at the In 1909 Robert Millikan a physicist at the

University of Chicago ran an experiment that University of Chicago ran an experiment that revealed that charges occur in discrete revealed that charges occur in discrete quantities.quantities. These quantities always occur in whole number These quantities always occur in whole number

intervals and are said to be “quantized”.intervals and are said to be “quantized”. These quantities are known as electrons which have a These quantities are known as electrons which have a

charge of charge of --1.6 x 101.6 x 10-19-19 Coulombs. Coulombs. Electrons cannot be divided into fractions. Any object that is Electrons cannot be divided into fractions. Any object that is

charges has a surplus or deficit of some whole number of charges has a surplus or deficit of some whole number of electrons.electrons.

# of electrons = total charge/charge of an electron.# of electrons = total charge/charge of an electron. If something has a If something has a --1C of charge it contains 6.2 x 101C of charge it contains 6.2 x 101818 electrons. electrons.

Millikan’s Oil Drop ExperiemntMillikan’s Oil Drop Experiemnt

Charging ObjectsCharging Objects

Charging by FrictionCharging by FrictionElectrical charges can be transferred from Electrical charges can be transferred from

one object to another.one object to another.Two substances can be rubbed together.Two substances can be rubbed together.

Rub a balloon with wool.Rub a balloon with wool. The wool will transfer electrons to the balloon and the The wool will transfer electrons to the balloon and the

balloon will become negatively charged.balloon will become negatively charged.

Static ClingStatic Cling Clothing in the dryer rub electrons off each other and Clothing in the dryer rub electrons off each other and

they become oppositely charged and stick together.they become oppositely charged and stick together.

ConductionConduction

Objects can be charged by contact.Objects can be charged by contact.Since electrons can move from one place to Since electrons can move from one place to

another they will move into another object to another they will move into another object to even out the overall charge of the two objects.even out the overall charge of the two objects.

Conservation of charge.Conservation of charge.

- --

- - - - - - -- -

InductionInduction

Charges within a neutral conductor can move.Charges within a neutral conductor can move. Objects can gain an Objects can gain an induced chargedinduced charged when a when a

charges object is brought near a neutral object.charges object is brought near a neutral object. Charges become polarized.Charges become polarized. This induced charge is temporary if the objects never This induced charge is temporary if the objects never

touch.touch. Once the charged object is removed the charges Once the charged object is removed the charges

evenly spread out again.evenly spread out again.

GroundingGrounding

Grounding an object provides a conduit in which Grounding an object provides a conduit in which the excess electrons can leave the object.the excess electrons can leave the object.

Objects that are charged by induction can gain Objects that are charged by induction can gain a permanent charge by grounding.a permanent charge by grounding.

Objects that are charged by induction and then Objects that are charged by induction and then grounded end up with an opposite charge than grounded end up with an opposite charge than the conductor.the conductor.

Objects charged by conduction end up with the Objects charged by conduction end up with the same charge as the original conductor.same charge as the original conductor.

GroundingGrounding

GroundingGrounding

Charging by InductionCharging by Induction

Electric ForcesElectric Forces

Electric Force Electric Force Force between chargesForce between charges

Repulsion between like charges and attraction Repulsion between like charges and attraction between opposite charges.between opposite charges.

Coulomb’s LawCoulomb’s LawFFee = k = k qq11qq22

dd22

The electric force decreases with the square of the The electric force decreases with the square of the distance between the chargesdistance between the charges

Look Familiar?Look Familiar? Remember FRemember Fgg = G = G mm11mm22

dd22

Quantity of charge is like the mass of an objectQuantity of charge is like the mass of an objectLet’s compare G and kLet’s compare G and k

G = 6.67 x 10G = 6.67 x 10-11-11 N m N m 22/kg/kg

k = 8.99 x 10k = 8.99 x 1099 N m N m 22/C/C22

Which force is stronger?Which force is stronger?A pair of positively charged particles of 1 C each 1A pair of positively charged particles of 1 C each 1meter apart would experience a repulsive force of meter apart would experience a repulsive force of 9.0 x 109.0 x 1099NN

The weight of a battleship!The weight of a battleship!

Law of SuperpositionLaw of Superposition

The resultant force on any given charge is The resultant force on any given charge is the vector sum of all the forces acting on the vector sum of all the forces acting on that charge.that charge.

Applied to three or more charges with a Applied to three or more charges with a certain distance to each other.certain distance to each other.Must consider whether or not the force is Must consider whether or not the force is

attractive or repulsive.attractive or repulsive.

Gravitational and Electric FieldsGravitational and Electric Fields

Review of gravitationReview of gravitation Gravitational field strength “g” gives the ratio of force Gravitational field strength “g” gives the ratio of force

to mass.to mass. FFgg = = mmbodybodymmearthearth Fg is the force of gravity (weight)Fg is the force of gravity (weight)

dd2 2 d is the distance from the center of the earthd is the distance from the center of the earth

We can think of mWe can think of mearthearth as creating the field and m as creating the field and mbodybody “experiencing the” force in response to the field.“experiencing the” force in response to the field.

““g” is the g” is the field strengthfield strength of the earth’s gravitational field of the earth’s gravitational field (9.8N/kg at the surface).(9.8N/kg at the surface).

We can calculate “g” by using the equation:We can calculate “g” by using the equation: g = G g = G mmearthearth

dd22

Electric Field StrengthElectric Field Strength

Electric field strength Electric field strength EE gives the ratio of force to charge. gives the ratio of force to charge.

FFee = q = q 22E E like Flike Fgg = mg = mg

FFee = electrostatic force = electrostatic force

qq22= charge experiencing the field= charge experiencing the field

E = field strengthE = field strength E = kE = kqq1 1 gives the electric field strength at any given gives the electric field strength at any given

dd22 distance from the charge creating the field distance from the charge creating the field ..

qq11 is the charge creating the field is the charge creating the field K is 8.99 x 10K is 8.99 x 109 9 N m/CN m/C22

d is the distance from the center of the charge to any given point in the field.d is the distance from the center of the charge to any given point in the field. Si unit for field strength is N/CSi unit for field strength is N/C

Field Lines Associated with Electric Field Lines Associated with Electric FieldsFields

Stronger charges have more concentrated field Stronger charges have more concentrated field lines.lines.

A positive charge creates a field that emanates A positive charge creates a field that emanates outward from the charge.outward from the charge.

A negative charge creates a field that is directed A negative charge creates a field that is directed towards the charge from the charge.towards the charge from the charge.

Field lines are always perpendicular to the Field lines are always perpendicular to the charge or charged object.charge or charged object.

Field lines never intersect. (this would indicate Field lines never intersect. (this would indicate two different values of force at the same point)two different values of force at the same point)