2/3/15 do now – on a new sheet

• What are the particles and their charges in an atom?

• Homework – castle learning

• Midterm Exam – part I – tomorrow – in class

Static Electricity - Chapter Outline

Lesson 1: Basic Terminology and Concepts

Lesson 2: Methods of Charging

Lesson 3: Electric Force

Lesson 4: Electric Fields

Lesson 5: Electric Potential

Lesson 1: Basic Terminology and Concepts

1. The Structure of Matter 2. Neutral vs. Charged Objects 3. Charge as a Quantity4. Charge Interactions 5. Conductors and Insulators 6. Polarization and electroscope

The Structure of Matter• ATOMS- All material objects are composed of

atoms.

• Atoms contain a dense center called the nucleus and a larger surrounding of mostly empty space that contains the electrons.

Summary of Subatomic Particles

Nucleus Electron

Proton Neutron Outside nucleusWeakly BoundNeg. Charge

Not very massive

(9.11X10-31kg)Can be added or removed

easily

Tightly Bound + Charge

Massive( 1.67 X10-27 kg)

Can not be added or removed

easily

Tightly Bound No Charge

Massive(1.67X10-27kg)

Can not be added or removed

easily

Neutral vs. Charged Objects

• Note: protons and neutrons can not be removed, only electrons can be removed or Added!

Positively-Charged

Negatively-Charged

Uncharged

Possesses more protons than

electrons

Possesses more electrons than

protons

Equal numbers of protons and

electrons

Charged objects contain unequal numbers of protons and electrons

example

• Which part of an atom is most likely to be transferred as a body acquires a static electric charge?

1. proton

2. neutron

3. electron

4. positron

example• A glass rod is given a positive charge by

rubbing it with silk. The rod has become positive by

1. gaining electrons

2. gaining protons

3. losing electrons

4. losing protons

Charge as a Quantity

• There are two ways to express the quantity of charge on an object.

• ELEMENTARY CHARGE (e). This is the charge of an electron or a proton. For example, if an object have +5.6 x 107 e, this means that the object has 5.6 x 107 more protons than electrons.

• COULOMB (C). There are 6.25 x 1018 elementary charge in 1 C of charge. For example, if an object’s charge is - 0.2 C, this means that the object has )0.2 x 6.25 x 1018( excess electrons.

The relationship between Coulomb and elementary charge

• 1C = 6.25 x 1018 e• 1e = 1.6 x 10-19 C• The charge of 1 electron is -1e, or -1.60 x 10-19 C• The charge of 1 proton is +1e, or +1.60 x 10-19

• One Coulomb of charge is an abnormally large quantity of charge. Smaller values such as µC and nC are often used.

• Since objects are charged through electron transfers, the charged objects must have either excess of absence whole number of electrons. It can not have a fraction of an electron. For Example, an object may have an excess of 501 electrons, but not 501.4 electrons. Or an object may have an absence of 423 electrons, but not 423.2 electrons.

• An charged object can only have a charge that is multiple of the elementary charge – multiple of 1.6 x 10-19 C.

• The smallest charge any object can have is 1.6 x 10-19 C

Possible charges on any object

• An object has three excess electrons.– What is its “elementary charge”?

– What is its charge in coulombs?

-3e

q = -3e x (1.6 x 10-19 C)/e = -4.8 x 10-19 C

Example

• An object has 75 protons and 65 electrons– What is its “elementary charge”?

– What is its charge in coulombs?

+10 e

q = +10e x (1.6 x 10-19 C)/e = +16 x 10-19 C

Example

An object can not have a charge of

1. 3.2 × 10-19 C

2. 4.5 × 10-19 C

3. 8.0 × 10-19 C

4. 9.6 × 10-19 C

Example

Charge Interactions• The electric force is a non-contact force. Any charged

object can exert this force upon other objects - both charged and uncharged objects.

• Two simple and fundamental statements can be made about the nature of the electric force.

• Opposites attract. likes repel.

The Electric Force and Newton's Third Law

This electric force exerted between two charged objects is a force in the same sense that friction, tension, gravity and air resistance are forces. And being a force, the same laws and principles that describe any force describe the electrical force. One of those laws was Newton's law of action-reaction. )balloons(

Force of B upon A is the same in magnitude as Force of A upon B. they are action and reaction forces.

Force of D upon C is the same in magnitude as Force of C upon D. they are action and reaction forces.

Interaction Between Charged and Neutral Objects

• Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object. – Positively charged objects and neutral objects

attract each other; – Negatively charged objects and neutral

objects attract each other.• In accordance with Newton's law of action-

reaction, the neutral object attracts the charged object.

Charge detection

• If two objects repel each other, one can conclude that both objects are charged and charged with the same type of charge. One could not conclude if they are both positively charged or both negatively charged.

• If two objects attract each other, one can conclude that at least one of the objects is charged. The other object is either neutral or charged with the opposite type of charge. You cannot draw a conclusion about which one of the objects is charged or what type of charge )positive or negative( the charged object possesses.

example• A lightweight sphere hangs by an insulating

thread. A student wishes to determine if the sphere is neutral or electro statically charged. She has a negatively charged hard rubber rod and a positively charged glass rod. She does not touch the sphere with the rods, but runs tests by bringing them near the sphere one at a time. The student notes that the sphere is attracted to both rods. This test result shows that the charge on the sphere is

1. positive 2. negative 3. neutral

example• A negatively charged plastic comb is

brought close to, but does not touch, a small piece of paper. If the comb and the paper are attracted to each other, the charge on the paper

1. may be negative or neutral

2. may be positive or neutral

3. must be negative

4. must be positive

Midterm Exam Part I

• After you finish the midterm, you should work on packets:

• Static electricity pp. #1-4

• Worksheet 4.1.1 #1-8

Please Recycle – the box is in front of the classroom

2/5

• Static electricity pp. #1-4

• Worksheet 4.1.1 #1-8

Homework - Castle learning

2/6 do now

1. An object possessing an excess of 6.0 × 106 electrons. What is net charge in coulombs?

2. An object has 6.08 μC of negative charge. How many excess electrons does the object have?

Home work – castle learningTotal castle learning assignments for this unit 2

If there is a snow day on Monday – Worksheets )packet( 3.2.1 – 3.2.5 are due Tuesday

Conductors and Insulators• The behavior of a charged object depends on whether the

object is made of a conductive or a nonconductive material.

• Conductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule.

• In contrast to conductors, insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule.

conductorinsulator

Charge on an insulator will remain at the initial location of charging.

Charge on a conductor is quickly distributed across the entire surface of the object. This electron migration happens across the entire surface of the object, until the overall sum of repulsive affects between electrons across the whole surface of the object are minimized.

insulators vs. conductors

Examples of conductors and insulators• Examples of conductors include

– metals, – aqueous solutions of salts – graphite, – water– human body.

• Examples of insulators – plastics, – Styrofoam, – paper, – rubber, – glass – dry air.

The division of materials into the categories of conductors and insulators is a somewhat artificial division. It is more appropriate to think of materials as being placed somewhere along a continuum.

Check your understanding• Suppose that a conducting sphere is charged

positively by some method. The charge is initially deposited on the left side of the sphere. Yet because the object is conductive, the charge spreads uniformly throughout the surface of the sphere. The uniform distribution of charge is explained by the fact that ____.

a. the charged atoms at the location of charge move throughout the surface of the sphere

b. the excess protons move from the location of charge to the rest of the sphere

c. excess electrons from the rest of the sphere are attracted towards the excess protons

• A conductor differs from an insulator in that a conductor ________.

a. has an excess of protons

b. has an excess of electrons

c. can become charged and an insulator cannot

d. has faster moving molecules

e. does not have any neutrons to get in the way of electron flow

f. none of these

Polarization and Electroscope

• The interaction between a neutral object and any charged object can be explained using our usual rules of opposites attract and likes repel.

• In an atom, the protons are tightly bound in a nucleus and incapable of movement. In conducting objects, electrons are so loosely bound that they may be induced into moving from one portion of the object to another portion of the object. By placing a charged object near a neutral conducting object you can create electron movement.

• Polarization is inducing movement of electrons within an object by a charged body.

Polarization

• No electrons have been added to or subtracted during polarization. The charges are simply separated to the opposite ends of the object. The overall charge of the object is electrically neutral.

Polarization is not charging

Neutral

The Electroscope • An electroscope is a device which is capable of detecting

the presence of a charges in an object through polarization.

An Insulator can be Polarized• In an insulator, electrons merely redistribute

themselves within the atom or molecules nearest the outer surface of the object.

Polarization of water molecules

example• An inflated balloon which has been rubbed against a

person's hair is touched to a neutral wall and remains attracted to it.  Which diagram best represents the charge distribution on the balloon and the wall?

a b c d

example• The diagram below shows three neutral

metal spheres, x, y, and z, in contact and on insulating stands. Which diagram best represents the charge distribution on the spheres when a positively charged rod is brought near sphere x, but does not touch it?

A B

C

D

Lesson 2: Methods of Charging

1. Charging by Friction

2. The Law of Conservation of Charge

3. Charging by Conduction

4. Charging by Induction

5. Grounding - the Removal of a Charge

Charging by Friction • When two objects are rubbed together electrons may be

transferred from one object to another. One object gains electrons and the other object loses electrons, so both objects have a charge.

• The property of electron affinity refers to the relative amount of love that a material has for electrons. High affinity means the material has more pull to electrons.

• The more love of electrons a material has the more likely it is to steal electrons from the other object during charging by friction.

electron affinity

Triboelectric series

• A triboelectric series is an ordering of substances with high affinities on top.

• When any two materials in the table are rubbed together, the one which is higher can be expected to pull electrons from the material which is lower.

Law of Conservation of Charge

• The total amount of charge in a closed system remains constant – charge is not created or destroyed, it only moves from one object to another

• Charge “moves” as a result of ELECTRON movement ONLY!!!

example

• The diagram shows four charged metal spheres suspended by strings.  The charge of each sphere is indicated. If spheres A, B, C, and D simultaneously come into contact, the net charge on the four spheres will be

1. +1 C

2. +2 C

3. +3 C

4. +4 C

• During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct explanation for why the plastic strip becomes positively charged is that ...

a. the plastic strip acquired extra protons from the cotton.

b. the plastic strip acquired extra protons during the charging process.

c. protons were created as the result of the charging process.

d. the plastic strip lost electrons to the cotton during the charging process.

example

Charging by Conduction • Charging by conduction involves the contact of a

charged object to a neutral object.

• When charging by conduction both object have the same type of charge when separated.

• To charge by conduction successfully, your charged and neutral object must be conductors!

Law of Conservation of Charge

• In each of the methods of charging: - charging by friction and charging by induction – and charging by conduction, the law of conservation of charge is observed. The law of conservation of charge states that charge is always conserved. The total amount of charge among the objects is the same before the process starts as it is after the process ends.

example• Two metal spheres having charges of

+4.0 × 10-6 coulomb and +2.0 × 10-5 coulomb, respectively, are brought into contact and then separated.  After separation, the charge on each sphere is

1. 8.0 × 10-11 C

2. 8.0 × 10-6 C

3. 2.1 × 10-6 C

4. 1.2 × 10-5 C

• A physics student, standing on the ground, touches an uncharged plastic baseball bat to a negatively charged electroscope. This will cause ___.

a. the electroscope to be grounded as electrons flow out of the electroscope.

b. the electroscope to be grounded as electrons flow into the electroscope.

c. the electroscope to be grounded as protons flow out of the electroscope.

d. the electroscope to be grounded as protons flow into the electroscope.

e. the baseball bat to acquire an excess of protons.f. absolutely nothing )or very little( to happen since the

plastic bat does not conduct.

example

2/9 do now

1. An object possessing an excess of 3.0 × 103 electrons. What is net charge in coulombs?

2. An object has 5.08 nC of negative charge. How many excess electrons does the object have?

Home work – castle learning

Charging by Induction• charging by induction method is to charge an object

without actually touching the object to any other charged object.

Charging by induction using negatively charged object

The law of conservation of charge is easily observed in the induction charging process. The overall charge on the system of two objects is the same after the charging process as it was before the charging process

Charging by induction using positively charged object

Charging a single sphere by induction

The Importance of a Ground in Induction Charging

• In the charging by induction cases, charge is never transferred from the charged object to the neutral object… They do not touch!

• The charged object causes the neutral object to become polarized.

• The object that touches the now polarized object serves as a supplier or receiver of electrons. This electron source and receiver is known as a ground.

Grounding - the Removal of a Charge

• Grounding is a way of uncharging an object. It is the process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size. When a charged object is grounded, the excess charge is balanced by the transfer of electrons between the charged object and a ground.

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

• Any object can be grounded provided that the charged atoms of that object have a conducting pathway between the atoms and the ground.

A ground is simply a large object that serves as an almost infinite source or sink of electrons.

Check Your Understanding• A positively charged pop can is touched by a

person standing on the ground. The pop can subsequently becomes neutral. The pop can becomes neutral during this process because ______.

a. electrons pass from the pop can to the person )ground(

b. electrons pass from the person )ground( to the pop can

c. protons pass from the pop can to the person )ground(

d. protons pass from the person )ground( to the pop can

If grounding removes charge how does it aid in charging an object during induction?

•When the object is polarized, one side has a positive charge and the other has a negative charge.

•The ground is removing the excess electrons on the sphere on one side but it can’t remove the positive charges because these charges are being balanced by the negative charged balloon that is near the sphere.

Charging an electroscope by induction

1. Bring a charged object near the electroscope

2. The electroscope is being polarized.

3. Touch the part of the electroscope that is away from the charged object.

4. Remove your hand.

5. Remove the charged object.

fundamental principles regarding induction charging

1. The charged object never touches to the object being charged by induction.

2. The charged object does not transfer electrons to or receive electrons from the object being charged.

3. The charged object serves to polarize the object being charged.

4. The object being charged is touched by a ground; electrons are transferred between the ground and the object being charged )either into the object or out of it(.

5. The object being charged ultimately receives a charge that is opposite that of the charged object which is used to polarize it.

example

• A charged body may cause the temporary redistribution of charge on another body without coming in contact with it. This process is called

1. conduction

2. potential

3. Charging by friction

4. induction

Class work

• Electricity packet – pp. 5-12

• Worksheet 4.1.2 Transfer of Charges

Lesson 3: Electric Force

1. Charge Interactions Revisited

2. Coulomb's Law

3. Inverse Square Law

4. Electrical Force and Newton's Laws

• The two fundamental charge interactions are: – oppositely charged objects attract – like charged objects repel.

• These mutual interactions resulted in an electrical force between the two charged objects.

Charge Interactions Revisited

The electrical forces, like all forces, are vector quantity.

The electrical force is a non-contact force - it exists despite the fact that the interacting objects are not in physical contact with each other.

example• An electron is located 1.0 meter from a +2.0-coulomb charge, as shown in the diagram. The electrostatic force acting on the electron is directed toward point1.  A 2.  B 3.  C 4.  D

A

B

C

D

example• Two plastic rods, A and

B, each possess a net negative charge of 1.0 × 10-3 coulomb.  The rods and a positively charged sphere are positioned as shown in the diagram.  Which vector below best represents the resultant electrostatic force on the sphere?

a b c d

Coulomb's Law

charge

distance

charge

•k is a proportionality constant known as the Coulomb's law constant. k = 8.99 x 109 N • m2 / C2. •Fe: force between two charges, (in Newtons)

rq2q1

• The interaction between charged objects is a non-contact force that acts over some distance of separation. The force between two charged objects depends on three variables:

– The ______________on object 1, )q1(

– The ______________ on object 2, )q2(

– The _________________ between them. )r(

221

r

qkqFe

• Coulomb's law states that the electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects.

• The force value is positive )repulsive( when q1 and q2 are of

like charge - either both "+" or both "-".

• The force value is negative )attractive( when q1 and q2 are of

opposite charge - one is "+" and the other is "-".

221

r

qkqFe

• Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 Newtons. Determine the separation distance between the two balloons.

Given: Find: d = ?q1 = +3.37 µC = +3.37 x 10-6 C q2 = -8.21 µC = -8.21 x 10-6 CFe = -0.0626 N )negative sign indicate attractive force(

r2 • Fe = k • q1 • q2

r2 = k • q1 • q2 / Fe

r = √)k • q1 • q2 / Fe

r = +1.99 m

Example

221

r

qqkFe

• Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them.

Given:q1 = 1.00 C q2 = 1.00 Cr = 1.00 mFind: Fe =?

Fe = k • q1 • q2 / d2 Fe = )8.99 x 109 N•m2/C2( • )1.00 C( • )1.00 C( / )1.00 m(2

Fe = 9.0 x 109 NThis is an incredibly large force which compares in magnitude to the weight of more than 2000 jetliners. Objects simply do not acquire charges on the order of 1.00 Coulomb. In fact, Charge is often expressed in units of microCoulomb )µC( and nanoCoulomb )nC(.

1 C = 106 μC  1 C = 109 nC

Example

Electrical vs. Gravitational Forces

The similarities:

Both equations have same form.

Both equations show an inverse square relationship between force and separation distance.

both equations show that the force is proportional to the product of the quantity that causes the force.

Both electrical force and gravitational force are non-contact forces.

221

r

mmGFe

2

21

r

qqkFe

k = 8.99 x 109 N·m2/C2 G = 6.67 x 10-11 N·m2/kg2

The difference:Coulomb's law constant )k( is significantly greater than Newton's universal gravitation constant )G(. Subsequently the force between charges – electric force - are significantly stronger than the force between masses – gravitational force. Gravitational forces are only attractive; electrical forces can be either attractive or repulsive.

example

• The diagram below shows two identical metal spheres, A and B, separated by distance d. Each sphere has mass m and possesses charge q.

•                                                   

• Which diagram best represents the electrostatic force Fe and the gravitational force Fg acting on sphere B due to sphere A?

A B C D

example

• Two protons are located one meter apart. Compared to the gravitational force of attraction between the two protons, the electrostatic force between the protons is

1.stronger and repulsive

2.weaker and repulsive

3.stronger and attractive

4.weaker and attractive

• That is, the factor by which the electrostatic force is changed is the inverse of the square of the factor by which the separation distance is changed.

• If the separation distance is doubled )increased by a factor of 2(, then the electrostatic force is decreased by a factor of four (22)

• If the separation distance is tripled )increased by a factor of 3(, then the electrostatic force is decreased by a factor of nine (32).

F

d

Coulomb’s Law – force and distance have inverse squared relationship

221

r

qqkFe

example• Two charges that are 2 meters apart

repel each other with a force of 2x10-5 newton. If the distance between the charges is decreased to 1 meter, the force of repulsion will be

1. 1 x 10-5 N 2. 5 x 10-6 N 3. 8 x10-5 N 4. 4 x 10-5 N

example

• If the charge on each of two small spheres a fixed distance apart is doubled, the force of attraction between the spheres will be

1. quartered

2. doubled

3. halved

4. quadrupled

example• Which graph best represents the electrostatic force

between an alpha particle with a charge of +2 elementary charges and a positively charged nucleus as a function of their distance of separation?

A B C D

• Electrostatic force is directly proportional to the charge of each object. So if the charge of one object is doubled, then the force will become two times greater. If the charge of each of the object is doubled, then the force will become four times greater.

Coulomb’s law – force and charge have direct relationship

221

r

qqkFe

example

• A repulsive electrostatic force of magnitude F exists between two metal spheres having identical charge q.  The distance between their two centers is r.  Which combination of changes would produce no change in the electrostatic force between the two spheres?

1.doubling q on one sphere while doubling r 2.doubling q on both spheres while doubling r 3.doubling q on one sphere while halving r 4.doubling q on both spheres while halving r

Electrical Force and Newton's Laws

• Electric force, like any force, is analyzed by Newton's laws of motion.

• The analysis usually begins with the construction of a free-body diagram. The magnitudes of the forces are then added as vectors in order to determine the resultant sum, also known as the net force. The net force can then be used to determine the acceleration of the object.

• In some instances, the free-body diagram is used to determine the spatial separation or charge of two objects that are at static equilibrium. In this case, the free-body diagram is combined with an understanding of vector principles in order to determine some unknown quantity.

example• A 0.90x10-4 kg balloon with a charge of -7.5 x 10-10 C is

located a distance of 0.12 m above a plastic golf tube which has a charge of -8.3 x 10-10 C. Determine the acceleration of the balloon at this instant?

free body diagram

find individual forces and Fnet & a

Fgrav = m•g = )0.90x10-4 kg(•)9.81 m/s2(

Fgrav = 8.82 x 10-4 N, down

Felect = k • q1 • q2 /r2

Felect= )8.99x109 N•m2/C2(•)-75 x 10-9 C(•)-83 x 10-9 C( / )0.12m(2

Felect = 3.89 x 10-7 N, up

Fnet = Fgrav )down( + Felect )up(

Fnet = - 8.82 x 10-4 N + 3.89 x 10-7 N

Fnet = - 8.82 x 10-4 N, down

a = Fnet / m

a = )8.82 x 10-4 N/ )0.90x10-4 kg ( a = 9.8 m/s2, down

example• Balloon A and Balloon B are charged in a like manner by

rubbing with animal fur. Each acquires 4.0 x 10-6 C. If the mass of each balloon is 1 gram, then how far below Balloon B must Balloon A be held in order to levitate Balloon B at rest? Assume the balloons act as point charges.

Felec

Fg

Fg=m•g= 0.0098 N.

Fe=m•g= 0.0098 N.

Fe = k•q1•q2 / r2

r = √ k)q1• q2( / Fe

r = 3.83 meters

Lesson 4: Electric Fields1. Electric force and gravitational force are

field force

2. Electric field and Gravitational Field Concept

3. Electric Field Intensity )magnitude and direction(

4. Electric Field Lines

5. Electric Fields and Conductors

6. Lightning

Electric force and gravitational force are field forces

Gravitational force - the mass of the Earth exerted an influence, affecting other masses which were in the surrounding neighborhood.

electrical force – The charges exerts an influence over a distance affecting other charges which were in the surrounding neighborhood

Electric Field and Gravitational Field Concept

• Similarly, a charged object creates an electric field. Other charges in that field would feel its effect in the space. Whether a charged object enters that space or not, the electric field exists.

• How can an apple reach across space and falls toward Earth?

• The massive Earth creates a Gravitational field. Other masses in that field would feel its effect in the space. Whether a massive object enters that space or not, the gravitational field exists.

Electric and gravitational Field

Gravitational field strength

• Force per mass ratio

• Direction is toward center of Earth

Electric field strength• Force per charge ratio

• The direction of the electric field vector is the direction that a positive test charge is pushed or pulled when in the presence of the electric field.

g = Fg / mE = Fe / q

2

2 )(

d

kQ

qd

kQq

E

q

FE e

• Electric field strength )E( is the force per charge ratio. The unit for electric field is N/C

• q is the charge in the field )test charge( – in Coulombs

• Fe is the force on the test charge q – in Newton

• k: constant, k = 8.99 x 109 Nm9/C2

• Q: source charge – in Coulombs

• d: distance from the source charge - in m

Electric Field Strength (magnitude)

• Note that there are two charges here - the source charge and the test charge. Electric field is the force per quantity of charge on the test charge.

• The electric field strength is not dependent upon the quantity of charge on the test charge.

• The electric field strength is dependent upon the quantity of charge on the source charge Q and the distance of separation d from the source charge. Just like the gravitational field does not depends how much you weigh!

e

2d

kQE

An Inverse Square Law• Electric field strength is location dependent, and its

magnitude decreases as the distance from a location to

the source increases. And by whatever factor the

distance is changed, the electric field strength will

change inversely by the square of that factor.

E

d

E = d2

k∙Q

example• What is the magnitude of the electric force

acting on an electron located in an electric field with an intensity of 5.0 x 103 N/C?

E = 5.0 x 103 N/Cq = 1.6 x 10-19 C

F =? N

E = F/q F = Eq

F = )5.0 N x 103 N/C( x )1.6 x 10-19 C(

= 8.0 x 10-16 N

example• What is the magnitude of an electrostatic

force experienced by one elementary charge at a point in an electric field where the electric field intensity is 3.0 × 103 N/C?

E = 3.0 x 103 N/Cq = 1.6 x 10-19 C

F =? N

E = F/q F = Eq

F = )3.0 N x 103 N/C( x )1.6 x 10-19 C(

= 4.8 x 10-16 N

example

• The diagram above represents a uniformly charged rod.  Which graph below best represents the relationship between the magnitude of the electric field intensity )E( and the distance from the rod as measured along line AB?

A B C D

example

• Suppose that two equally charged spheres attract each other with a force of -0.5 N )"-" means attractive( when placed a distance of 30. cm from each other. Determine the charge of the spheres. PSYW

The Direction of the Electric Field Vector

• Electric field strength is a _______quantity.

• the direction of the electric field vector is defined as the direction that a ______________________________ is pushed or pulled when in the presence of the electric field.

• the electric field vector would always be directed _______ from positively-charged objects.

• electric field vectors are always directed ____________ negatively-charged objects

vector

positive test charge

awaytowards

Electric Field Maps

+ -

Electric Field Maps

+ +

Electric Field Maps

+ +--

Rules for Drawing Electric Field Patterns

1. The lines must begin on positive charges and terminate on negative charges

2. Surround more charged objects by more lines.

The electric field is greatest at locations closest to the surface of the charge and least at locations further from the surface of the charge.

3. draw the lines of force ___________________ to the

surfaces of objects at the locations where the lines

connect to object's surfaces.

perpendicular

4. Electric field lines should _____________________. never cross

• Examples of electric field lines

example• The diagram shows the electric field in the vicinity of

two charged conducting spheres, A and B. What is the static electric charge on each of the conducting spheres?

1. A is negative and B is positive. 2. A is positive and B is negative. 3. Both A and B are positive. 4. Both A and B are negative.

example• Two small metallic spheres, A and B, are separated by a

distance of 4.0 × 10-1 meter, as shown. The charge on each sphere is +1.0 × 10-6 coulomb. Point P is located near the spheres. Which arrow best represents the direction of the resultant electric field at point P due to the charges on spheres A and B?

1 2 3 4

Fields between two oppositely charged parallel plates

• If the distance separating

two oppositely charged

parallel plates is small

compared to their area, the

electric field between the

plates is ____________.

• The field lines are from

positive plate to the

negative plate.

uniform

• Since F = E∙q, the __________________________ on a

charged particle anywhere inside the plates.

• A charged particle will accelerate toward the plate with the

opposite charge.

force is the same

─

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

┼

example• As an electron moves

between two charged parallel plates from point B to point A, as shown in the diagram, the force of the electric field on the electron

1. decreases

2. increases

3. remains the same

example• In the diagram, proton p, neutron n, and electron e are

located as shown between two oppositely charged plates. The magnitude of acceleration will be greatest for the

1. neutron, because it has the greatest mass 2. neutron, because it is neutral 3. electron, because it has the smallest mass 4. proton, because it is farthest from the negative plate

Millikan’s oil-drop experiment

• In 1909, Robert Millikan performed the oil-drop experiment to measure the elementary electric charge. The experiment entailed balancing the downward gravitational force with the upward electric forces on tiny charged droplets of oil suspended between two metal plates.

Fg = Fe

m∙g = E∙q

q = mg / EFg

Fe

• Milliken measured the forces on charged oil drops in a

uniform electric field.

• He found no drop with a charge less than 1.60 x 10-19

coulomb. The charges on other drops were integral

multiples of this value.

• This finding demonstrated that there is a ______________

unit of charge. This elementary charge of 1.60 x 10-19

coulomb is called the charge on a single electron.

fundamental

example• What did Milliken conclude after performing his

oil-drop experiment?

1. The charge on an electron is 1.0 C. 2. The mass of an electron is 1.7 × 10-27 kg. 3. The charge on any oil drop is an integral

multiple of the charge on an electron. 4. The charge on an oil drop may have any value

larger than 1.6 × 10-19 C.

example• The diagram, which illustrates the Milliken oil drop

experiment, shows a 3.2 × 10-14-kilogram oil drop with a charge of -1.6 × 10-18 coulomb.  The oil drop was in equilibrium when the upward electrical force on the drop was equal in magnitude to the gravitational force on the drop.  What was the magnitude of the electric field intensity when this oil drop was in equilibrium?

E∙q = m∙g

E)-1.6x10-18C( = 3.2x10-14kg)9.81 N/kg(

E = 1.96 x 105 N/C = -2.0 x 105 N/C

Fnet = Fe - Fg = 0

Fe = Fg

example• An object with a net charge of 4.80 × 10-6 coulomb

experiences an electrostatic force having a magnitude of 6.00 × 10-2 newtons when placed near a negatively charged metal sphere. What is the magnitude and direction of electric field strength at this location? [show all work including substitution with units]

Given: q = 4.8 x 10-6 C

F = 6.00 x 10-2 N

Unknown: E = ? N/C

Solve: E = F / q = 6.00 x 10-2 N / 4.0 x 10-6 C

E = 1.25 x 104 N/C directed toward the sphere.

Electric field and conductors

• A _______________ is material which allows electrons

to move relatively freely from atom to atom.

• Electrostatic equilibrium is the condition established

by charged conductors in which the excess charge has

optimally distanced itself so as to reduce the total

amount of repulsive forces. Once a charged conductor

has reached the state of electrostatic equilibrium, there

is no further motion of charge about the surface.

conductor

+

+

+

+ -

-

-

-

Four properties of conductor in electric equilibrium

1. The electric field anywhere beneath the surface of a

charged conductor is ___________.

• This principle of shielding is commonly utilized today

as we protect delicate electrical equipment by

enclosing them in metal cases.

zero.

2. Any excess charge on an isolated conductor resides

entirely on the conductor’s ____________________.outer surface

3. The electric field upon the surface of the conductor is

directed entirely _________________ to the surface. perpendicular

4. The electric fields are strongest at locations along the

surface where the object is

______________________________. most curved

example• A metallic sphere is positively charged.

The field at the center of the sphere due to this positive charge is

1. positive

2. negative

3. zero

4. dependent on the magnitude of the charge

Lightning • Perhaps the most

known and powerful displays of electrostatics in nature is a lightning storm.

• What is the cause and mechanism associated with lightning strikes?

• How do lightning rods serve to protect buildings from the devastating affects of a lightning strike?

Static Charge Buildup in the Clouds

• The precursor of any lightning strike is the polarization of positive and negative charges within a storm cloud. The tops of the storm clouds are known to acquire an excess of positive charge and the bottom of the storm clouds acquire an excess of negative charge.

• When a thunderhead passes over the ground, electrons on Earth's outer surface are repelled by the negatively charged cloud's bottom surface. This creates an opposite charge on the Earth's surface. Buildings, trees and even people can experience a buildup of static charge as electrons are repelled by the cloud's bottom.

• The electric field between the cloud and the Earth is similar to the electric field between two oppositely charged plates.

• When the difference in negative and positive charges between ground and cloud gets large enough, a lightning bolt begins. The excess electrons on the bottom of the cloud start a journey through the conducting air to the ground at speeds up to 60 miles per second.

• As electrons travel close to the Earth, it encounters the positive charges traveling upward, when the two types of charges meet, lightning begins.

• The enormous and rapid flow of charge along this pathway between the cloud and Earth heats the surrounding air, causing it to expand violently. The expansion of the air creates a shockwave which we observe as thunder

Lightning Rods and Other Protective Measures

• Tall buildings, farm houses and other structures susceptible to lightning strikes are often equipped with lightning rods.

• the lightning rod serves to safely divert the lightning to the ground in event that the cloud discharge its lightning via a bolt.

Check Your Understanding1. TRUE or FALSE:

The presence of lightning rods on top of buildings prevents a cloud with a static charge buildup from releasing its charge to the building.

 

 

2. TRUE or FALSE:

If you place a lightning rod on top of your home but failed to ground it, then it is unlikely that your home would be struck by lightning.

Lesson 5: Electric Potential Difference

1. Electric Field and the Movement of Charge

2. Electric Potential Energy

3. Electric Potential Difference

Electric Field and the Movement of Charge

• A charged object creates an electric field. Electric field is a vector quantity. As another charged object enters into the field, its movement is affected by the field.

Electric Potential Energy• Electric potential energy are similar to gravitational potential energy -

both involve field forces.

-

-

Work done by electric field

Work done by external force

Low PE

High PE

Low PE

High PE

PEg = mg∆h

Gravitational potential energy is a result of interaction between masses. It depends on the mass and the field strength and the relative position.

Similarly, electric potential energy is a result of interaction between charges. It depends on the charge and field strength and relative position.

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

To increase PE

+ +

To decrease PE

+-

Change Electrical Potential Energy

+ + +-

example

B

A

+e

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

As a positive charge moves for B to A, it potential energy is _____.

a. increased b. decreased c. stays the same

• Electric energy can be produce from many sources and also can be converted into other types of energy. Electric potential energy is a form of mechanical energy:

TME = KE + PEg + PEs + PEe

• In a uniform field, when a charge is released, work done by the field on the charge equals to its lose PE e and it gain in KE because there is no friction.

2

2

1mvKEPEdEqdFW e

d

-

-

-

-

+

+

+

+

Energy is conserved

The Gravitational Potential

GPE = mg∆h

g∆h, is a quantity that could be used to rate various locations about the surface of the planet in terms of how much potential energy each kilogram of mass would possess when placed there.

Gravitational potential is defined as the PE/mass. It is mass independent. Gravitational potential describes the affects of a gravitational field upon objects that are placed at various locations within it.

g∆h, is known as gravitational potential. m

GPEhg

• Electric potential )V( is defined as potential energy per charge.

q

PEV e

The electric potential is the same for all charges at a given location. A test charge with twice the quantity of charge would possess twice the potential energy at that location.

• Electric potential is a property of the location within an electric field. Electric potential )V( does not depend on q.

Equipotential lines• Equipotential lines connect positions of equipotential

energy. As a charge moves on an equipotential line, there is _________________in potential energy. As the charge crosses equipotential lines, the potential energy changes.

+e

no change

+e

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

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

• Electric potential difference between point A and point B is the change is potential between point A and B

Electric Potential Difference

q

W

q

PE

q

PEVVV AB

AB

The standard metric unit on electric potential difference is the volt or voltage. 1 Volt = 1 Joule / Coulomb.

If 1 joule of work is needed to move 1 C of charge from point A to point B, the potential difference between point A & B is 1 Volt.

If 3 joule of work is needed to move 1 C of charge from point A to point B, the potential difference between point A & B is 3 Volts

A

B

+e

q

WV

Electric Potential Energy - 2 units

If 1 C of charge is moved across 1 V of potential difference, 1 Joule of work/energy is needed.

If 1 e of charge is moved across 1 V of potential difference, 1 eV (electron-volt) of work/energy is needed.

Both Joule and eV are units of electric energy.

Since 1 e = 1.6 x 10-19 C1 eV = 1.60 x 10-19 J

VqW q

WV

• 6.0 joules of work are done in pushing an object with +3.0 coulombs of charge toward a charged plate.– What type of charge does the plate have on it?– How much potential energy was stored in the electric fields?– How much electrical potential was generated?

Positive

Example #1

6.0 J

V = W/q

V = 6.0 J / 3.0 C

V = 2.0 V

• An object with a 2.0 coulomb charge is accelerated through a potential difference of 10 volts.– How much kinetic energy does the object gain?

Example #2

V = W/q

W = Vq

W = (10 V)(2.0 C) = 20 J

Electron-volts• Alternate unit for work/energy:

• Raises 1e to an electrical potential of 1 V• 1 eV = 1.6 x 10-19 J

What is the energy needed to raise two electrons to a potential of 1.0 volt?

V = W /q

1.0 V = W / 2e

W = 2.0eV

What is the energy needed to raise four electrons to a potential of 2.5 volts?

V = W /q

2.5 V = W / 4e

W = 10 eV

• An electron travels a distance of 2.0 x 10-3 meter as its electrical potential is raised by 300 volts.– How much work is done on the electron?

Example #3

V = W/q

300 V = W / 1e

W = 300 eV

V = W/q

300 V = W / 1.6 x 10-19 C

W = 4.8 x 10-17 J

Electric Potential in CircuitsA battery powered electric circuit has locations of high and low potential.

Within the cells of the battery, the electric field is directed from the positive terminal towards the negative terminal. As a positive test charge move through the cells from the negative terminal to the positive terminal, it would require work, thus the potential energy of the charge would increase. It is for this reason that the positive terminal is described as the high potential terminal.

• As a positive charge move through the wires from the positive terminal to the negative terminal, it would move in the direction of the electric field and would not require work. The charge would lose potential energy. The negative terminal is described as the low potential terminal.

example• How many eV is required to move 3.2 x 10-19 C

of charge through a potential difference of 5.0 volts?

V = W / q

5.0 V = W / (3.2 x 10-19 C) = W / (2 elem. Charges)

W = 10 eV

Example

• Moving +2.0 coulombs of charge from infinity to point P in an electric field requires 8.0 joules of work.  What is the electric field potential at point P?

The electric potential at any point in an electric field is the work required to bring a unit positive charge from infinity to that point.

V = W / q = 8.0 J / (2.0 C) = 4.0 V

Example

• The graph shows the relationship between the work done on a charged body in an electric field and the net charge on the body.   What does the slope of this graph represent?

Slope = rise / run

Slope = W / q = V

The slope represent the potential difference.