Electric Forces and Electric Forces and FieldsFields

Chapter 17Chapter 17

Section 17-1 ObjectivesSection 17-1 Objectives

Understand the basic properties of Understand the basic properties of electric chargeelectric charge

Differentiate between conductors Differentiate between conductors and insulatorsand insulators

Distinguish between charging by Distinguish between charging by contact, charging by induction and contact, charging by induction and charging by polarizationcharging by polarization

Electric ChargeElectric Charge

What’s happening to this little girl?

Electric ChargeElectric Charge

There are two kinds of electric There are two kinds of electric chargecharge– Positive and negativePositive and negative– Like charges repelLike charges repel– Unlike charges attractUnlike charges attract

Electric ChargeElectric Charge

Electric charge is conserved!!Electric charge is conserved!!– What else is conserved??What else is conserved??

Mass, Energy, Momentum Mass, Energy, Momentum

– Electric charge cannot be created or Electric charge cannot be created or destroyed but it can be transferreddestroyed but it can be transferred

– Example: Transferring electrons from Example: Transferring electrons from someone’s hair to a balloonsomeone’s hair to a balloon

Electric ChargeElectric Charge

Electric Charge is quantizedElectric Charge is quantized– One unit of charge : e= 1.60219 x 10One unit of charge : e= 1.60219 x 10-19-19 C C

C stands for Coulomb, the unit of electric C stands for Coulomb, the unit of electric chargecharge

– A proton has a charge of +1.60 x 10A proton has a charge of +1.60 x 10-19-19 C C

– An electron has a charge of -1.60 x 10An electron has a charge of -1.60 x 10-19-19 C C

Conductors & InsulatorsConductors & Insulators

Conductors: Materials in which Conductors: Materials in which electric charges move freelyelectric charges move freely– Examples: most metalsExamples: most metals

Insulators: Materials in which electric Insulators: Materials in which electric charges do not move freelycharges do not move freely– Examples: Plastic, glass, silk, rubberExamples: Plastic, glass, silk, rubber

Charging by contactCharging by contact

The two objects The two objects are rubbed are rubbed together and together and electrons are electrons are transferred from transferred from one to the otherone to the other– electrons from the electrons from the

fur are transferred fur are transferred to the rodto the rod

Charging by InductionCharging by Induction To charge by induction, a charged object is To charge by induction, a charged object is

brought close to (not touching!) a brought close to (not touching!) a conductor and then a conducting wire conductor and then a conducting wire connects the conductor to the ground and connects the conductor to the ground and the electrons travel to the groundthe electrons travel to the ground

Charging by PolarizationCharging by Polarization

Charging by polarization creates a Charging by polarization creates a surface chargesurface charge– A charged object is brought close to an A charged object is brought close to an

insulator and the electrons and protons insulator and the electrons and protons realign themselves to create one side realign themselves to create one side that is more positive and one that is that is more positive and one that is more negativemore negative

Section 17.2 ObjectivesSection 17.2 Objectives

Calculate electric force using coulomb’s lawCalculate electric force using coulomb’s law

Compare electric force with gravitational Compare electric force with gravitational forceforce

Apply the superposition principle to find the Apply the superposition principle to find the resultant force on a charge and to find the resultant force on a charge and to find the position at which the net force on a charge position at which the net force on a charge is zerois zero

Coulomb’s LawCoulomb’s Law

Coulomb’s Law Coulomb’s Law describes the describes the mathematical mathematical relationship relationship between electric between electric force, distance and force, distance and electric charge for electric charge for two objectstwo objects

1 22electric C

q qF k

r

Electric force= Coulomb’s Constant x (charge 1)(charge 2)distance2

kC= 8.99 x 109 Nm2

C2

Coulomb’s LawCoulomb’s Law

The force between two charges is The force between two charges is proportional to the magnitude of the proportional to the magnitude of the chargescharges

The force between two charges is The force between two charges is inversely proportional to the inversely proportional to the distancedistance22 between them between them

Coulomb’s LawCoulomb’s Law

Remember that force is a vector!Remember that force is a vector!– For problems involving two charges, the For problems involving two charges, the

direction is either “attractive” or direction is either “attractive” or “repulsive”“repulsive”

– i.e. the direction of the force between a i.e. the direction of the force between a positive charge and negative charge is positive charge and negative charge is attractive and the direction of the force attractive and the direction of the force between two negative charges is between two negative charges is repulsiverepulsive

Example Problem p. 636 #2Example Problem p. 636 #2

Two identical conducting spheres are Two identical conducting spheres are placed with their centers 0.30 m apart. placed with their centers 0.30 m apart. One is given a charge of +12 x 10One is given a charge of +12 x 10-9-9 C and C and the other is given a charge of -18 x 10the other is given a charge of -18 x 10-9-9 C C– A. Find the electric force exerted on one A. Find the electric force exerted on one

sphere by the othersphere by the other– B. The spheres are connected by a B. The spheres are connected by a

conducting wire. After equilibrium has conducting wire. After equilibrium has occurred, find the electric force between the occurred, find the electric force between the two spheres.two spheres.

p.636 #2 (part a)p.636 #2 (part a)

Use Coulomb’s LawUse Coulomb’s Law

1 22electric C

q qF k

r

9 929 5

22

12 10 18 108.99 10 2.2 10 Attractive

0.30

x C x CNmF x x N

C m

p.636 #2 (part b)p.636 #2 (part b)

What does it mean “after equilibrium What does it mean “after equilibrium has occurred”?has occurred”?– The charge on each sphere is the sameThe charge on each sphere is the same

9 929 7

22

3 10 3 108.99 10 8.99 10 Repulsive

0.30

x C x CNmF x x N

C m

Things to RememberThings to Remember

Electric charge is conservedElectric charge is conserved Conductors and insulators can be Conductors and insulators can be

charged by contactcharged by contact Conductors can be charged by Conductors can be charged by

inductioninduction Insulators can have a surface charge Insulators can have a surface charge

due to polarizationdue to polarization Electric force is a vector! Electric force is a vector!

SourcesSources

Balloon Hair Pic: Balloon Hair Pic: http://heyheyheyparty365.blogspot.com/2011/01/static-electricity.html

http://startswithabang.com/?p=1643 http://blogs.nature.com/news/http://blogs.nature.com/news/

2011/06/2011/06/researchers_studying_static_ge.htmlresearchers_studying_static_ge.html