Chapter 16: Electric Charge and Electric Fields

electric flux

-a scalar quantity and refers to the electric field passing thru a given area and can be interpreted in terms of field lines (N number of field lines) (E=N/A) -the e flux thru an area is proportional to the total number of field lines crossing the area. therefore Electric field thru an area is proportional to the number of field lines (I=EA=N)

what is net charge on a conductor?

-is the sum of all of the positive and negative charges in the conductor -if a neutral conductor has extra electrons added to it then the net charge is negative -if a neutral conductor has electrons removed from it then the net charge is poisitve -if a neutral conductor has the same amt of positive and negative charges then the net charge is zero

what happens when there is a ballon on the wall how does it stick

-wall is initially neutral...wall becomes negative -the balloon induces a charge in the wall -there is an electric force there -when balloon falls...the air neutralizes the charge so objects will lose charge and force of gravity will take over and balloon will fall -induction attracts object -conduction does not occur bc its not a metal...conduction only occurs in conductors...charge is not flowing here

atoms form molecules when bind together

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electrons are less massive than p, n

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electrons orbit around nucleus and p,n are inside nucleus

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friction produces separation of charge chrage can be transferred between two materials

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mass is concentrated in the nucleus

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conductors in electrostatic equilibrium

1. the electric field inside a conductor is zero. if there was an E different from 0 the field would exert a force on every charge in the conductor, giving the free charges a net motion. by definition electrostatic charges are at rest and have no net motion, thus field must be 0 2. the E is always perpendicular to the surface outside the conductor 3. any excess charge on a conductor distributes itself on the surface 4. the surface charge density is highest a sharp points.

electric dipole

2 equal charges, opposite in sign -direction of electric field at any point is tangent to the field line passing thru that point -the e field produced by two alike charges is zero at any point on the line joining them. a charge in that point will not experience any force

Electric Field and Electric Force directions when have q negative and q positive

F=qE. the force on a point charge in an electric field. if q is positive E and F have the same direction if q is negative E and F have the opp direction

for a uniform electric field, E passing thru an area A what is the electric flux defined as and what does it mean

I=EAcos where the angle is the angle between the E direction and a line drawn perpendicular to the area the maximum angle is 0 when the angle of E filed and line perpependicular to the area electric flux will be minumum when the angle is 90 therefore no flux -when E field encounter a perpendicular plane the angle will be zero

two neutral conductors are connected by a wire and a charged rod is brought near, but doesnt touch then the wire is taken away and then the charged rod is removed. what are the charges on the conductors

While the conductors are connected, positive charge will flow from the blue to the green ball due to polarization. Once disconnected, the charges will remain on the separate conductors even when the rod is removed.

what happens when you move farther away from +Q...

then we feel less of an electric force and less electric field but if we move closer to it we increase electric field and force

what are the free charges in the conductor

they are electrons that can move about freely within the materials bc they are loosely bound to their atoms. free electrons are also called conduction electrons. a conductor may have a zero net charge but have substantial free charges

insulators

water dont conduct electricity almost no charge flows e are tightly bound

note

cannot charge an insulator by a conductor

one way we can charge an object: conduction: what happens:

charge flows from one to another when touch object. you transfer positive charge to other object and now both have positive charge so they repel -conduction involves touch of object -for conduction: two objects end up with the same charge -start w/ neutral metal rod and charge it by contact with a charge metal object -transfers charge

electroscope:

device that can be used to detect charges can be charged either by induction or conduction

gravitational field:

earth produces a gravitational force in the space around it and field exerts a force Fg on test mass m0. g field acts with attractive force only -g field exerts a force Fg on the test mas mo. it doesnt depend on the mass that produced it (earth) -its defined as the force per unit mass

fluid is transferred from one objet to another -we start out as neutral chrage then when we rub and combine what happends

either one loses and one gains electrons therefore the total charge is always conserved

why can electric field lines never cross

electric field lines are drawn to indicate direction of a postive test charge. the electric force has a unique direction at each point. if two field lines crossed it would indicate the electric force is goin in two directions at once which is not possible -field lines never cross bc cant have two directions at the same point

ion

first start out neutral then an atom loses or gains an electron then it will have a net positive or negative charge bond is ionic

static electricity

friction produces electrical charge if want body to move= need a force (electric force)

superposition principle

if several charges are present the net force that is acting on any charges present will be the vector sum of the forces on that charge due to each of the others

when an object is neutral:

it contains an equal amt of p and e charges

conductors:

metals, gold silver -conduct electricity -charge flows freely; electrons move -e are bound loosely to nucleus

metal ball hands from ceiling by an insulated thread. the ball is attracted to a positive charged rod held near ball. charge of ball must be:

negative or neutral neutral bc of induction bc if metal ball neutral then we can have a polarization of charge and the left side will be negative and right side positive

how does object become charged

object becomes charged bc of rubbing and possesses then a net electric charge

action at a distance

or noncontact electrical force

there are two kinds of charges:

positve charge and negative charge unlike charges attract like charges repel

the number of field lines (starting on a positive charge or ending on a negative charge is proportional to what

proportional to the charge and therefore the net number of field lines thru a closed surface must be proportional to the charge enclosed =Bc the number of field lines is proportional to the electric flux, also the flux thru the closed surface must be proportional to the enclosed charge

F=qE

q will experience force which is proportional to the electric field and magnitude of q

2 charged balls are repelling each other what can you say bout charge

since they are repelling one another they are either both positive or both negative

what do we call -Q:

sink of electric field

what do we call +Q:

source of electric field

what determines the structure, properties and fxn of matter

the atomic structure and bonds

proton and an electron are held apart a distance of 1m and then released. which particle has the largest acceleration at any moment

the electron bc two particle feel the same force. since f=ma, the particle with the smaller mass will have the largest acceleration

what if distance is doubled

the force decreases 1/4 to its original value

what if charge on both the objects is doubled

the force increases to 4x the original values

if charge is doubled on either one of the objects... what happens to the force

the force is doubled

what prevents the leaves from moving farther apart

the horizonal comp of this tension force balances the electric force of repulsion the vertical comp of the tension force balances the weight of the leaves or the force of gravity

line density:

the number of lines that cross a specific area is proportional to the intensity

according to coloumbs law..what does the force depend on

the product on the magnitude of both charges -therefore force will be equal

how to represent an atom what are the two things:

atomic number (up top): number of protons atomic mass: (bottom): number of protons and neutrons the number of protons equals the number of electrons

two balls with positive charges are fixed at a separation distance of 3r. is it possible to place another charged ball on the line betweent he two charges such that the net force on the ball is zero?

a positive charge would repel both which means forces would then cancel a negative charge would attract both which means the forces would then cancel yes-independent of the sign or value of the ball placed in middle

when Q interacts with electric field and experiences a force what does it also produce

an acceleration

can charge object by induction: how

when positive charge go to neutral but dont tocuh, free electrons dont leave the rod but move within the rod, the charges are then separated and no net charge is created (its still zero) -separation of charges ocurs (polarization) has been induced. also occurs in nonconductors -dont touch object positive charge attract to negative charge -since we do not transfer charge...when remove the charged object, the object that was neutral will go back to neutral

whenever a certain amount of charge is produced on one object, an equal amount of the opposite type of charge is produced on another object: therefore what is the net charge

zero