electric forces and fields chapter 17. section 17-1 objectives understand the basic properties of...
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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