Electric Forces and Fields Chapter 17

Section 17-1 Objectives Understand the basic properties of electric charge Understand the basic properties of electric charge Differentiate between conductors and insulators Differentiate between conductors and insulators Distinguish between charging by contact, charging by induction and charging by polarization Distinguish between charging by contact, charging by induction and charging by polarization

Electric Charge What’s happening to this little girl?

Electric Charge There are two kinds of electric charge There are two kinds of electric charge –Positive and negative –Like charges repel –Unlike charges attract

Electric Charge Electric charge is conserved!! Electric charge is conserved!! –What else is conserved??  Mass, Energy, Momentum  Mass, Energy, Momentum –Electric charge cannot be created or destroyed but it can be transferred –Example: Transferring electrons from someone’s hair to a balloon

Electric Charge Electric Charge is quantized Electric Charge is quantized –One unit of charge : e= x C  C stands for Coulomb, the unit of electric charge –A proton has a charge of x C –An electron has a charge of x C

Conductors & Insulators Conductors: Materials in which electric charges move freely Conductors: Materials in which electric charges move freely –Examples: most metals Insulators: Materials in which electric charges do not move freely Insulators: Materials in which electric charges do not move freely –Examples: Plastic, glass, silk, rubber

Charging by contact The two objects are rubbed together and electrons are transferred from one to the other The two objects are rubbed together and electrons are transferred from one to the other –electrons from the fur are transferred to the rod

Charging by Induction To charge by induction, a charged object is brought close to (not touching!) a conductor and then a conducting wire connects the conductor to the ground and the electrons travel to the ground To charge by induction, a charged object is brought close to (not touching!) a conductor and then a conducting wire connects the conductor to the ground and the electrons travel to the ground

Charging by Polarization Charging by polarization creates a surface charge Charging by polarization creates a surface charge –A charged object is brought close to an insulator and the electrons and protons realign themselves to create one side that is more positive and one that is more negative

Section 17.2 Objectives Calculate electric force using coulomb’s law Calculate electric force using coulomb’s law Compare electric force with gravitational force Compare electric force with gravitational force Apply the superposition principle to find the resultant force on a charge and to find the position at which the net force on a charge is zero Apply the superposition principle to find the resultant force on a charge and to find the position at which the net force on a charge is zero

Coulomb’s Law Coulomb’s Law describes the mathematical relationship between electric force, distance and electric charge for two objects Coulomb’s Law describes the mathematical relationship between electric force, distance and electric charge for two objects Electric force= Coulomb’s Constant x (charge 1)(charge 2) distance 2 k C = 8.99 x 10 9 Nm 2 C 2

Coulomb’s Law The force between two charges is proportional to the magnitude of the charges The force between two charges is proportional to the magnitude of the charges The force between two charges is inversely proportional to the distance 2 between them The force between two charges is inversely proportional to the distance 2 between them

Coulomb’s Law Remember that force is a vector! Remember that force is a vector! –For problems involving two charges, the direction is either “attractive” or “repulsive” –i.e. the direction of the force between a positive charge and negative charge is attractive and the direction of the force between two negative charges is repulsive

Example Problem p. 636 #2 Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of +12 x C and the other is given a charge of -18 x C Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of +12 x C and the other is given a charge of -18 x C –A. Find the electric force exerted on one sphere by the other –B. The spheres are connected by a conducting wire. After equilibrium has occurred, find the electric force between the two spheres.

p.636 #2 (part a) Use Coulomb’s Law Use Coulomb’s Law

p.636 #2 (part b) What does it mean “after equilibrium has occurred”? What does it mean “after equilibrium has occurred”? –The charge on each sphere is the same

Things to Remember Electric charge is conserved Electric charge is conserved Conductors and insulators can be charged by contact Conductors and insulators can be charged by contact Conductors can be charged by induction Conductors can be charged by induction Insulators can have a surface charge due to polarization Insulators can have a surface charge due to polarization Electric force is a vector! Electric force is a vector!

Sources Balloon Hair Pic: 011/01/static-electricity.html Balloon Hair Pic: 011/01/static-electricity.html 011/01/static-electricity.html 011/01/static-electricity.html searchers_studying_static_ge.html searchers_studying_static_ge.html