Advanced Physics Chapter 16 Electric Charge and Electric Field.

Advanced PhysicsAdvanced PhysicsChapter 16Chapter 16Electric Charge and Electric Electric Charge and Electric FieldField

Electric Charge and Electric Charge and Electric FieldElectric Field

• 16-1 Static Electricity16-1 Static Electricity• 16-2 Electric Charge in the Atom16-2 Electric Charge in the Atom• 16-3 Insulators and Conductors16-3 Insulators and Conductors• 16-4 Induced Charge: the Electroscope16-4 Induced Charge: the Electroscope• 16-5 Coulomb’s Law16-5 Coulomb’s Law• 16-6 Solving Problems Involving Coulomb’s Law and 16-6 Solving Problems Involving Coulomb’s Law and

VectorsVectors• 16-7 The Electric Field16-7 The Electric Field• 16-8 Field lines16-8 Field lines• 16-9 Electric Fields and Conductors16-9 Electric Fields and Conductors• 16-10 Electric Forces in Molecular Biology (DNA)16-10 Electric Forces in Molecular Biology (DNA)

Static ElectricityStatic ElectricityStatic ElectricityStatic Electricity = ? = ?Electric chargeElectric charge = ? = ?• TypesTypes• InteractionsInteractions• Giving an object a net electric Giving an object a net electric

chargecharge

Static ElectricityStatic ElectricityLaw of conservation of electric Law of conservation of electric

chargecharge——the net amount of electric the net amount of electric charge produced in any process is charge produced in any process is zerozero

• This means = ?This means = ?

Electric Charge in the Electric Charge in the AtomAtom• So what are the electric charges in So what are the electric charges in

an atom?an atom?• What happens if we “charge” an What happens if we “charge” an

atom?atom?• What is a polar molecule?What is a polar molecule?

Insulators and ConductorsInsulators and Conductors• What are What are insulatorsinsulators and and conductorsconductors??• What are examples of insulators and What are examples of insulators and

conductors?conductors?• Can you charge insulators and/or Can you charge insulators and/or

conductors?conductors?– Explain how.Explain how.

• What is a What is a semiconductorsemiconductor? ?

Induced Charge: the Induced Charge: the ElectroscopeElectroscope• What is an What is an electroscopeelectroscope??• Can you use an electroscope to Can you use an electroscope to

find the type of charge on an find the type of charge on an object?object?

Induced Charge: the Induced Charge: the ElectroscopeElectroscope• How do you How do you chargecharge an an

electroscope electroscope byby conductionconduction??• What charge does it get?What charge does it get?

Induced Charge: the Induced Charge: the ElectroscopeElectroscope• How do you How do you chargecharge an an

electroscope electroscope byby inductioninduction??

Induced Charge: the Induced Charge: the ElectroscopeElectroscope• How do you How do you chargecharge an an

electroscope electroscope byby inductioninduction??• What is What is groundinggrounding??• What charge does it get?What charge does it get?

Coulomb’s LawCoulomb’s LawF = (kQF = (kQ11QQ22)/r)/r22

• Where:Where:– F = force between two charged objects (1 & F = force between two charged objects (1 &

2)2)– k = k = Coulomb's constant (9 x 10Coulomb's constant (9 x 1099 N*m N*m22/C/C22))– Q = net charges on objects (Q = net charges on objects (CoulombCoulomb))

• 1 electron has the charge of 1.6 x 101 electron has the charge of 1.6 x 10-19-19 C C– r = distance between objectsr = distance between objects

Coulomb’s LawCoulomb’s LawF = (kQF = (kQ11QQ22)/r)/r22

• k can be represented byk can be represented byk = 1/(4k = 1/(400))

• Where Where 0 0 = permittivity of free space = permittivity of free space – 8.85 x 108.85 x 10-12-12 C C22/N*m/N*m22

• You will see You will see 00 again later again later

Coulomb’s LawCoulomb’s Law

F = (kQF = (kQ11QQ22)/r)/r22

• What is the relationship between F, What is the relationship between F, Q’s and r?Q’s and r?

Coulomb’s LawCoulomb’s LawF = (kQF = (kQ11QQ22)/r)/r22

• If the force is between two objects with If the force is between two objects with the same charge the force is repulsive the same charge the force is repulsive and positiveand positive

• If the force is between two objects with If the force is between two objects with the opposite charges the force is the opposite charges the force is attractive and negativeattractive and negative

Solving Problems Involving Solving Problems Involving Coulomb’s Law and VectorsCoulomb’s Law and Vectors• The net force acting on an object is the The net force acting on an object is the

sum of all the forces acting on itsum of all the forces acting on it• For charged objects, these forces can be For charged objects, these forces can be

calculated using Coulomb’s Law and calculated using Coulomb’s Law and remembering that force vectors have remembering that force vectors have directiondirection

• It is usually better to ignore signs and It is usually better to ignore signs and just deal with direction of force when just deal with direction of force when looking at net force acting on an objectlooking at net force acting on an object

Solving Problems Involving Solving Problems Involving Coulomb’s Law and VectorsCoulomb’s Law and VectorsExampleExample• Three charged particles QThree charged particles Q11, Q, Q22, and Q, and Q33

are arranged in a straight line. They are arranged in a straight line. They have charges of –8.0have charges of –8.0C, 3.0C, 3.0C and – C and – 4.04.0C respectively. QC respectively. Q1 1 is 0.3 m from Qis 0.3 m from Q22 and 0.5 m from Qand 0.5 m from Q33. Find the net force on . Find the net force on QQ33

AnswerAnswer• 1.5 N toward Q1.5 N toward Q11

Solving Problems Involving Solving Problems Involving Coulomb’s Law and VectorsCoulomb’s Law and VectorsExampleExample• Three charged particles QThree charged particles Q11, Q, Q22, and Q, and Q33 are are

arranged where Qarranged where Q11is 52 cm east of Qis 52 cm east of Q22 and and QQ33 is 30 cm north of Q is 30 cm north of Q22 They have charges They have charges of –86.0of –86.0C, 50.0C, 50.0C and 65.0C and 65.0C C respectively. Find the net force on Qrespectively. Find the net force on Q33

AnswerAnswer• 282.36 N at 64.6282.36 N at 64.6°° north of east or north of east or

25.4° east of north25.4° east of north

The Electric FieldThe Electric FieldElectric fieldElectric field—property of all —property of all

charged objectscharged objects• the vector sum of the force exerted the vector sum of the force exerted

on a test charge when it is by on a test charge when it is by charged object divided by the charged object divided by the charge on the test chargecharge on the test charge

Test chargeTest charge must be ______ and must be ______ and _______. Why?_______. Why?

The Electric FieldThe Electric FieldElectric fieldElectric field

E = F/qE = F/q''• Why is the electric field not just Why is the electric field not just

determined by the force on the test determined by the force on the test charge?charge?

• What is the unit for electric field strength?What is the unit for electric field strength?

The Electric FieldThe Electric FieldElectric fieldElectric fieldE = F/qE = F/q''So…So…F = EqF = Eq''So…So…F = kQq‘/rF = kQq‘/r22 = Eq' = Eq'So…So…E = kQ/rE = kQ/r22 for an electric field for an electric field

generated by charge Qgenerated by charge Q

The Electric FieldThe Electric FieldElectric fieldElectric fieldWhen the electric field is due to more than When the electric field is due to more than

one charge, the total field at any point is one charge, the total field at any point is the vector sum of the individual fields the vector sum of the individual fields ((superposition principle)superposition principle)

E = EE = E11 + E + E22 + ….. + …..

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– Positive chargePositive charge• How do the lines tell an electric field’s How do the lines tell an electric field’s

magnitude and direction?magnitude and direction?

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– Negative chargeNegative charge

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– Two negative chargesTwo negative charges

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– Two positive chargesTwo positive charges

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– One negative and one positive chargeOne negative and one positive charge

Field LinesField LinesElectric fieldElectric field• How do you diagram electric field?How do you diagram electric field?

– Between two parallel plates of opposite Between two parallel plates of opposite chargecharge

Electric Fields and Electric Fields and ConductorsConductors• The electric field inside a good The electric field inside a good

conductor is zero in the static situation conductor is zero in the static situation (when charges are at rest)(when charges are at rest)

• Any net charge on a good conductor Any net charge on a good conductor distributes itself on the surfacedistributes itself on the surface

• The direction of the electric field is The direction of the electric field is perpendicular to the surface of the perpendicular to the surface of the conductorconductor

Electric Fields and Electric Fields and ConductorsConductors• For a hollow sphere with a charge For a hollow sphere with a charge

in it; there is an electric field due to in it; there is an electric field due to net charges on inside and outside net charges on inside and outside surface so there is an electric field surface so there is an electric field inside and outside but not within inside and outside but not within the metalthe metal

Electric Fields and Electric Fields and ConductorsConductors• For a hollow box inside an electric For a hollow box inside an electric

field; there is no electric field field; there is no electric field because of the accumulation of because of the accumulation of charges along its surface (safety in charges along its surface (safety in a car)a car)

Electric Forces in Molecular Electric Forces in Molecular Biology (DNA)Biology (DNA)

• What are the four bases What are the four bases in DNA?in DNA?– Thymine, cytosine, Thymine, cytosine,

adenine, and guanineadenine, and guanine• How do they match up?How do they match up?

– Thymine and adenineThymine and adenine– Cytosine and guanineCytosine and guanine

• What holds the two DNA What holds the two DNA strands together?strands together?– Hydrogen bonds Hydrogen bonds