Electrostatics :Electrostatics :Charges at restCharges at rest

Electric ChargeElectric Charge

A property of matter that creates a force A property of matter that creates a force between objects.between objects.

Can be positive or negativeCan be positive or negative Like charges REPELLike charges REPEL Opposite charges ATTRACTOpposite charges ATTRACT

What objects (particles) do we know about What objects (particles) do we know about that carry a charge? that carry a charge?

Electric ChargeElectric Charge

An object’s charge depends on imbalance of An object’s charge depends on imbalance of protons (+) and electrons (‐)protons (+) and electrons (‐)

More protons than electrons More protons than electrons positive positive More electrons than protons More electrons than protons negative negative

Units of charge: Coulombs (C)Units of charge: Coulombs (C) Protons and electrons have exactly the same Protons and electrons have exactly the same

amount of charge: 1.6 x 10amount of charge: 1.6 x 10‐19‐19 C C

Differ only in sign (+ or -)Differ only in sign (+ or -)

ElectrostaticsElectrostatics – Elementary– Elementary ChargeCharge Since protons and electrons are the Since protons and electrons are the

smallest whole particles, the charge on smallest whole particles, the charge on any object is a multiple of 1.6 x 10any object is a multiple of 1.6 x 10-19-19 C C

Elementary Charge (e) = 1.6 x 10Elementary Charge (e) = 1.6 x 10-19 -19 CC

1C of charge is made up of 6.25 x101C of charge is made up of 6.25 x101818 electrons.electrons.

Charging an ObjectCharging an Object

Objects become charged if they have an imbalance Objects become charged if they have an imbalance of protons and electrons.of protons and electrons.

Can an object gain or lose protons?Can an object gain or lose protons? (Think: Can protons MOVE?)(Think: Can protons MOVE?)

Can an object gain or lose electrons?Can an object gain or lose electrons? (Think: Can electrons MOVE?)(Think: Can electrons MOVE?)

The total sum of the charge a body has is the result of a loss or gain of electrons

ElectrostaticsElectrostatics

When there is an imbalance in protons When there is an imbalance in protons and electrons the atom will have a total and electrons the atom will have a total sum of sum of chargecharge Total positive sum of the charges – more Total positive sum of the charges – more

protons than electronsprotons than electrons

Total negative sum of the charges – more Total negative sum of the charges – more electrons than protonselectrons than protons +

+--

- -

+++- -

ElectrostaticsElectrostatics

For an object to gain a total sum of For an object to gain a total sum of charge, there must be an imbalance in the charge, there must be an imbalance in the number of protons and electrons.number of protons and electrons.

For the imbalance to occur, the atom can For the imbalance to occur, the atom can only gain or lose electrons.only gain or lose electrons.

If the atom lost a proton, it would change If the atom lost a proton, it would change what it is.what it is.

Charging an ObjectCharging an Object

Conductors: materials that transfer and redistribute Conductors: materials that transfer and redistribute charge easily.charge easily.

Excess charge can move through the material.Excess charge can move through the material. Examples: ???Examples: ???

Insulators: materials that do not transfer charge easily; Insulators: materials that do not transfer charge easily; retain charge within a localized region.retain charge within a localized region.

Excess charge stays on the surface of the material.Excess charge stays on the surface of the material. Examples: ???Examples: ???

·Semiconductors: materials that behave as either ·Semiconductors: materials that behave as either insulators or conductors, depending on temperatureinsulators or conductors, depending on temperature

Examples: ???Examples: ???

Electrostatics and ChargingElectrostatics and Charging Charge can move by many methodsCharge can move by many methods

Methods: Methods: • FrictionFriction• ConductionConduction• Induction Induction

For charge to move, electrons must move For charge to move, electrons must move from atoms. from atoms. Positive charge does not Positive charge does not move!move!

Charging by FrictionCharging by Friction

The electrons are literally rubbed off The electrons are literally rubbed off of one object and move to the other of one object and move to the other Charging a balloonCharging a balloon

• Hair Hair positive positive Balloon Balloon Negative Negative

Using the fur to charge the plastic rod.Using the fur to charge the plastic rod.• Fur Fur positive positive Rubber rod Rubber rod Negative Negative

balloon charging

Charging by ConductionCharging by Conduction Charge is transferred from one object to Charge is transferred from one object to

another object by direct contactanother object by direct contact

Walk across the carpet in socks and touch Walk across the carpet in socks and touch the doorknob… ZAP!the doorknob… ZAP! Charge is transferred and you experience a Charge is transferred and you experience a

shock.shock.

Charging by Conduction: DemoCharging by Conduction: Demo

Demonstration: Touch the negatively Demonstration: Touch the negatively charged magic wand to the mylar butterfly.charged magic wand to the mylar butterfly. Some electrons move from wand to butterfly.Some electrons move from wand to butterfly. Butterfly repels wand (both slightly negative).Butterfly repels wand (both slightly negative).

Static Discharge: movement of charge Static Discharge: movement of charge from one object to another by conduction from one object to another by conduction using the air. Charge can JUMP! using the air. Charge can JUMP! (Lightning)(Lightning)

Charging by InductionCharging by Induction

A temporary charge can be induced in a A temporary charge can be induced in a neutral object by bringing a charged object neutral object by bringing a charged object close to it.close to it. The charges in the neutral object move in The charges in the neutral object move in

response to the external charge. Result: response to the external charge. Result: induced charge (POLARIZATION!!!)induced charge (POLARIZATION!!!)

If a path is provided, this moved charge If a path is provided, this moved charge will escape. Result: body has a total sum will escape. Result: body has a total sum of chargeof charge

Charging by Induction: DemoCharging by Induction: Demo Demonstration: Bring negatively charged magic Demonstration: Bring negatively charged magic

wand near soda can.wand near soda can. Electrons in soda can repel.Electrons in soda can repel. Protons in soda can are attracted and cause the can Protons in soda can are attracted and cause the can

to move forwardto move forward

Demonstration: Bring negatively charged balloon Demonstration: Bring negatively charged balloon near wall.near wall. Electrons in wall repel.Electrons in wall repel. Protons in wall attract to balloon, and it sticks to wallProtons in wall attract to balloon, and it sticks to wall

Charging by InductionCharging by Induction

If a charged object is If a charged object is brought near the brought near the spheres, the charges spheres, the charges will POLARIZE.will POLARIZE.

The negative charges The negative charges move towards the move towards the positive charges and positive charges and away from other away from other negatives.negatives.

Measuring ChargeMeasuring Charge

ElectroscopeElectroscope Movement of leaves is a “rough estimate” of Movement of leaves is a “rough estimate” of

amount of chargeamount of charge Other electroscopes areOther electroscopes are

more precise.more precise.• When a charge is present, When a charge is present,

the straw rotates.the straw rotates. More rotation = more chargeMore rotation = more charge Can detect + and – charge, Can detect + and – charge,

cannot differentiatecannot differentiate

http://www.youtube.com/watch?v=vzFnUtP_wEg

Electric ForceElectric Force

Electric Force: force of attraction or repulsion Electric Force: force of attraction or repulsion between objects due to chargebetween objects due to charge Depends on CHARGE and DISTANCEDepends on CHARGE and DISTANCE

• Increase charge Increase charge force increases force increases• Increase distance Increase distance force decreases force decreases

Forces can be exerted by one charge on Forces can be exerted by one charge on another from a distance through a FIELDanother from a distance through a FIELD Electric Field: region around a charged object in Electric Field: region around a charged object in

which other charged objects experience an which other charged objects experience an electric forceelectric force

Coulomb’s LawCoulomb’s Law

F = electric force in NewtonsF = electric force in Newtons k = constant (just a #) = 9.0x109 Nm2/C2k = constant (just a #) = 9.0x109 Nm2/C2 q1 = charge of object #1 in Coulombs (C)q1 = charge of object #1 in Coulombs (C) q2 = charge of object #2 in Coulombs (C)q2 = charge of object #2 in Coulombs (C) r = radius between two charges in metersr = radius between two charges in meters

F = k q1q2

r2

Coulomb’s Law calculates a Coulomb’s Law calculates a forceforce

If If the calculated force is:the calculated force is:

• NegativeNegative The force is The force is attractiveattractive between between particlesparticles

• PositivePositive The force is The force is repulsiverepulsive between between particlesparticles

Effect of Coulomb’s LawEffect of Coulomb’s Law

If the charge of one object doubles…If the charge of one object doubles…

Force doubles (x2)Force doubles (x2) If the charges of both objects double…If the charges of both objects double…

Force quadruples (x4)Force quadruples (x4) If the distance between the charges If the distance between the charges

doubles…doubles…

Force is quartered (divided by 4)Force is quartered (divided by 4)

EXAMPLEEXAMPLE Find the force exerted by one electron on Find the force exerted by one electron on

another separated by a distance of 2.0 m.another separated by a distance of 2.0 m. Draw a pictureDraw a picture

Table of KnownsTable of Knowns

(What is the charge of each electron?)(What is the charge of each electron?)

(What is a constant? k= 9x10(What is a constant? k= 9x1099 N*m N*m22 /C/C22))

Is the force repulsive or attractive? Is the force repulsive or attractive?

Q1 Q2 D (m)Force (N) Attractive or Repulsive?

+2C +2C 0.5

-3C -3C 0.1

-1C +2 2

1 x 10-3 C 2 x 10-3 C 0.5

-4 x 10-3 C -3 x 10-3 C 0.75

6 x 10-6 C -4 x 10-3 C 0.002

3 x 105 C 7 x 105 C 2.5

450C -25 C 5.2

67 C -67 C 6.8

-4 C 2 C 0.7

Electric ForceElectric Force Electric Force – the force of attraction or Electric Force – the force of attraction or

repulsion between objects due to repulsion between objects due to charge.charge. Like charges repel.Like charges repel. Unlike charges attract.Unlike charges attract.

Depends on size of charge and distanceDepends on size of charge and distance Act over a distance through an electric fieldAct over a distance through an electric field

Coulomb’s LawCoulomb’s Law Electric force is given by Coulomb’s LawElectric force is given by Coulomb’s Law

Where :Where : qq11 and q and q22 are the charges are the charges

r is the radius between the chargesr is the radius between the charges

Force decreases as r gets bigger but never Force decreases as r gets bigger but never will be zero – Like gravitywill be zero – Like gravity

221

2

29 )*

109(d

qq

C

mNxF

Electric FieldElectric Field

Electric Field – An electric field is the Electric Field – An electric field is the region around a charge in which the region around a charge in which the electrostatic force is felt by other electrostatic force is felt by other charges.charges.

Electric fields are drawn and 'mapped' Electric fields are drawn and 'mapped' in diagrams using “field lines”..in diagrams using “field lines”..

Electric Field LinesElectric Field Lines

Always extend from a positively charged Always extend from a positively charged object to a negatively charged objectobject to a negatively charged object

From a positively charged object to infinityFrom a positively charged object to infinity

From infinity to a negatively charged From infinity to a negatively charged object.object.

Electric Field LinesElectric Field Lines

Electric field lines never cross each other.Electric field lines never cross each other.

At locations where electric field lines meet At locations where electric field lines meet the surface of an object, the lines are the surface of an object, the lines are perpendicular to the surface.perpendicular to the surface.

Electric Field LinesElectric Field Lines

The number of field lines that meet a charged The number of field lines that meet a charged particle represents the size of the charge, particle represents the size of the charge, higher charge amounts have more field lines.higher charge amounts have more field lines.

Electric Field LinesElectric Field Lines

The closer together field lines are to each The closer together field lines are to each other, the greater the strength of the field other, the greater the strength of the field at that point.at that point.

Electric Field Lines Electric Field Lines

Between ParticlesBetween Particles

Electric ForceElectric Force

Electric field is ‘seen’ by the force on a Electric field is ‘seen’ by the force on a charged body, test charge.charged body, test charge.

Electric field is depicted through Electric Electric field is depicted through Electric Field lines.Field lines. Field lines point in direction of force on Field lines point in direction of force on

positive chargepositive charge Field lines never crossField lines never cross Field lines travel away from positive chargeField lines travel away from positive charge Field lines travel toward negative chargeField lines travel toward negative charge

Electric FieldElectric Field

Electric Field LinesElectric Field Lines

When two opposite charges are When two opposite charges are brought close together, brought close together,

The field lines travel from positive to The field lines travel from positive to negative.negative. The lines connect the charges showing The lines connect the charges showing

attractionattraction

Slide 36

Fig 15.29a, p.551

Electric Field LinesElectric Field Lines

When like charges are brought near When like charges are brought near each other,each other,

The fields lines repel each other.The fields lines repel each other. Reason for the repelling force between Reason for the repelling force between

like charges.like charges.

httphttp://www.falstad.com/vector3de/index.html

Electric Field EquationsElectric Field Equations E = F/qE = F/qtest test

E = kqE = kqsourcesource/r/r22

The electric field only The electric field only depends on the source depends on the source chargecharge

It does NOT depend on It does NOT depend on the test charge – see how the test charge – see how the field is only created by the field is only created by the extra protons and the extra protons and electrons, NOT the puckelectrons, NOT the puck