# Ch 20 Electric Fields and Forces

-Explains the basic electric phenomena
-There are 2 kinds of charge, called “positive”and “negative”charge
-There is an attractive force between charges of opposite kind, a repulsive force between charges of the same kind
describe the “charge model” of electricity (what explains, what forms part of it, explain)
-The attractive and repulsive forces between two charged particles can be calculated from this law
-Coulomb’s law tells us how the force between charges depends on their charge and the distance between them
Colomb’s law (Using it, what you can calculate, what does it tell us?)
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repel each other
same charges_
attract each other
opposite charges_
-Every charge alters the space around it, creating and electric field at every point
-This electric field then exerts a force on other charges
Electric field (how interacts with environment)
-electrons and protons are the basic charges of ordinary matter
explain the atomic nature of matter
The process of charging an object by rubbing can be understood as a transfer of electrons from one material to another
explain what happens at the molecular level when you charge an object by rubbing it?
-Charges in electric fields experience forces and torques due to the field
-The force on a positive charge is in the same direction as the field; the force on a negative charge is opposite the field
explain how forces and torques in electric field work when charges are within it? (describe each case of having a positive and a negative charge)
two equal but opposite charges form an electric dipole
electric dipole
an electric dipole in an electric field experiences a torque
what happens when we have an electric dipole within an electric field?
-Nothing happens to either rod
-Explanation: There are no special electrical properties to these undistributed rods. We say that they are neutral
Describe what would happen if you have a plastic rod that has been undisturbed for a long period of time hanged by a thread and, then, you pick up another undisturbed plastic rod and bring it close to the hanging rod. give explanation of this
-The hanging rod moves away from the handled rod when you bring the two close together
-The same would happen if you bring together two glass rods that were rubbed with silk
-summary: the two rods repel each other
-Explanation: Rubbing a rod changes its properties so that forces now act between two such rods.
-We call this process of rubbing as charging
-the rubbed rod is charged
Describe what would happen if you hang a plastic rod that vigorously was rubbed with wool and, then, you take another plastic rod that was vigorously rubbed by wool and put them close together? what would happen if you repeat this experiment with glass rods? explain why this happens. How you call this process? how you would call the affected object?
that the object has acquired a charge
what is meant when we say that an object is charged?
-long-range repulsive force (force requiring no contact)
what kind of force is found in the previous experiment of rods charged by rubbing them (or charged) in the same way and bringing them close together?
the force between charged objects
electric force
-The two rods attract each other
-Explanation:
-The two materials obtained two different kinds of charge: a positive charge acquired by the glass rod and the negative charge acquired by the plastic rod
what happens if you bring a glass rod that has been rubbed with silk close to a hanging plastic rod that has been rubbed with wool? Explain the experiment.
-like charges (positive/positive and negative/negative) exert repulsive forces on each other
-opposite charges (positive/negative) exert attractive forces on each other
summarize the main idea behind the previous experiments
-If two rod are held farther form each other, the force between them decreases
-The strength of the force is greater for rods that have been rubbed more vigorously
Explanation:
-the electric force decreases with the distance between the charged objects
-the greater the charge on the 2 objects, the greater the force between them
How is the intensity of repulsion/attraction in the experiment:
-change distance
-change intensity of rubbing
EXPLAIN
long range force as gravitational force
what kind of force is electric force?
positive
negative
what are the kinds of charges?
-gives an schematic picture of the distribution of charge on an object
-negative charge is represented as a minus sign
-positive charge is represented as a plus sign
-equal amounts of positive and negative charges can be represented by drawing the same number of + and – signs
-more charge is represented by more + or – signs
-In a charge diagram, we would only draw the excess charge
charge diagram (what shows, describe it)
-the rod is attracted to the wool
-Explanation: Plastic rod obtains negative charge. Because the wool attracts the rod, the wool must have a positive charge
Describe what would happen if you start with a neutral hanging plastic rod and a piece of wool. Then, you rub the plastic rod with the wool, then hold the wool close to the rod? explain why
-a neutral object is not one that has no charge at all, but it contains equal amounts of positive and negative charge
-the equal amounts of opposite charge “cancel,” leaving no overall or net charge
define what a “neutral object” is?
-when the amount of positive charge on it exceeds the amount of negative charge
-an object is negatively charged when the amount of negative charge on it is greater than the amount of positive charge
when an object becomes positively charged? how it applies when to a negative charged object?
the rubbing process works by transferring charge form one object to other
explain how rubbing process works in charging?
-when two objects are rubbed together, negative charge is transferred from the wool to the rod
-This clearly leaves the rod with an excess of negative charge
-But the wool, having lost some of its negative charge to the rod now has an excess of positive charge, leaving it positively charged
describe how is the flow of charges when two objects are rubbed together? (use the example of wool and plastic rod)
the negative charge moves
which of the two charges moves when rubbing two objects together?
-In any charging process, no electrons are created or destroyed. Electrons are simply transferred from one material to another. (by electrons we mean charges)
-If a certain amount of positive charge appears somewhere, an equal amount of negative charge must appear somewhere so that the net charge doesn’t change
law of conservation of charge
1. Frictional forces, such as rubbing, add something called charge to an object or remove it from the object. The process itself is called charging. More vigorous rubbing produces a larger quantity of charge
2. There are 2 kinds of charge: positive and negative
3. 2 objects with like charge (positive/positive or negative/negative) exert repulsive forces on each other. Two objects with opposite charge (positive/negative) exert attractive forces on each other. We call these electric forces
4. The force between 2 charged objects is a long-range force. The magnitude of the force increases as the quantity of charge increases and decreases as the distance between the charges increases
5. Neutral objects have an equal mixture of positive and negative charge
6. The rubbing process charges the objects by transferring charge (usually negative) from one to the other. The objects acquire equal but opposite charges
7. Charge is conserved; It cannot be created or destroyed
8. There are two types of materials. Conductors are materials through or along which charge easily moves. Insulators are materials on or in which charges remain fixed in place
9. Charge can be transferred from one object to another by contact
list the postulates of the “charge model” (9)
The metal sphere then repels a charged, hanging plastic rod. The metal sphere appears to have acquired a charge of the same sign as the plastic rod
if you charge a plastic rod by rubbing it with wool, then you touch a neutral metal sphere with the rubbed area of the rod. Describe what happens if you put close together the metal sphere with a hanging charged plastic rod
-Afterward, the metal sphere that was touched repels a charged, hanging plastic rod. The other metal sphere does not
-The metal sphere that was touched acquires charge
-The metal sphere that was not touched by the charged plastic rod, but was in contact with the other metal sphere by a neutral plastic rod in the middle, remains neutral
-no charge moved along the plastic rod connecting the 2 metal spheres
-If you have a metal rod connecting the two metal spheres and you touch one of the metal spheres with a charged plastic rod, afterward, both metal spheres repel a charged, hanging plastic rod
-charge moved along the metal rod connecting the spheres, transferring some charge from the first sphere to the second
If you place two metal spheres close together with a plastic rod connecting them. Then, charge a second plastic rod, by rubbing, and touch it to one of the metal spheres. what would happen if you put the metal spheres close to a hanging charged plastic rod? What would happen if you repeat this experiment, but in this case replace the neutral plastic rod would be replaced by a metal rod connecting the two spheres?
-it happens when charge is transferred from one object to another only when the objects touch
-Contact is required
-Removing charge from an object, which you can do by touching it, is called discharging
discharging (when it happens, what is required, definition)
those materials through or along which charge easily moves
ex: metal
conductors (definition and example)
those materials on or in which charges remain immobile
ex: glass and plastic
insulators (definition and 2 example)
-If you rub 2 objects together, there will be a transfer of electrons (negative charge) and the objects will end up with different charges
-If you touch 2 objects together, there will be a transfer of charges in which the 2 objects end up with the same charge (both are positive or both are negative). In the case of the experiment in which both metal spheres end up charged, touching the metal sphere with a charged plastic rod caused a second metal sphere, connected by a metal rod to the first, to become charged with the same type of charge as the rod
what is the difference between by charging 2 object by rubbing 2 objects together and by touching 2 objects together?
-charged
-the ability of charge to move
both insulators and conductors can be_. They differ in_
contact
charge is transferred upon_
-on the place that was charged. In the insulator charges are not free to move around the rod
-In its surface. In the conductor the charges are free to move around. Because like charges repel, these negative charges quickly move as far apart as they possibly can. In the case of the experiment with the spheres and the metal rod between them, some of the charges move through the connecting metal rod to the 2nd sphere. The repulsive forces drive the negative charges as far apart as they can possibly get, causing them to end up on the surfaces of the conductors
where the charge is localized in a charged insulator? where is localized in a charged conductor? explain
-Once the charge is placed on the conductor it rapidly distributes itself over the conductor’s surface. This movement of charge is extremely fast
-Other than this brief interval during which the charges are adjusting, the charges on an isolated conductor are in static equilibrium with the charges at rest. This condition is called electrostatic equilibrium
electrostatic equilibrium (explain what happens before this. define it)
-Because the charged rod doesn’t touch the sphere, no charge is added to or removed from the sphere. Instead, the rod attracts some of the sphere’s negative charge to the side of the sphere near the rod. This leaves a deficit of negative charge on the opposite side of the sphere, so that side is now positively charged. The negative charge on the sphere is close to the rod so it is strongly attracted to the rod. The positively charge on the sphere is far from the rod, so it is weakly repelled by the rod. This results in the movement of the sphere in direction to the road because the net force is toward the rod.
-This slight separation of the positive and negative charge in a neutral object when a charged object is brought near is called charge polarization
Explain how a neutral conductor object (in this case a metallic neutral sphere) is attracted toward a charged one (a rod with excess positive charge)? -Give the term for this and define it
the slight separation of the positive and negative charge in a neutral object when a charged object is brought near
charge polarization
polarization force
what kind of force is presented in the charge polarization process?
it arises because the charges in the metal are slightly separated, not because the rod and metal are oppositely charged
why the polarization force arises?
the polarization force between a charged object and a neutral one is always attractive
Describe how the polarization force would be between a charged object and a neutral one?
-In an insulator charge cannot move through it. Nevertheless, the attractive force between a charged object and an insulator is also a polarization force.
-The reason is that the charge in each atom that makes up an insulator can be slightly polarized
-Although the charge separation in one atom is exceedingly small, the net effect over all the countless atoms in an insulator is shift a perceptible amount of charge form one side of the insulator to the other. This is just what’s needed to allow a polarization force to arise
explain how an neutral insulator can be attracted by a charged object? (what name is given to this and why it happens)
-made up of a very small and denser positively charged nucleus, containing positively charged protons and as well as neutral particles called neutrons, surrounded by much-less-massive orbiting negatively charged electrons that form an electron cloud surrounding the nucleus
-The atom is held together by the attractive electric forces between positive nucleus and the negative electrons
describe the current model of the atom (what are its parts and how it is held together)
charge, like mass, is an inherent property of electrons and protons
what is the inherent property of electrons and protons?
electrons
protons
*There are no other sources of charge*
what are the basic charges of ordinary matter?
electrons and protons have charges of opposite sign by exactly equal magnitude
-Thus, because charge is due to electrons and protons, and object is charged if it has an unequal number of electrons and protons
how the charge of electrons compared with the charge of protons? How this explains that an object is charged?
-more electrons than protons
-more protons than electrons
An object with negative charge has_(in terms of electrons and protons). An object with positive charge has_
-object that has equal number of protons and electrons.
-such object has no net charge
describe an object that is “electrically neutral”
-In practice, objects acquire a positive charge not by gaining protons but by losing electrons
-Protons are extremely tightly bound within the nucleus and cannot be added to or removed from atoms. Electrons, on the other hand, are bound much more loosely than the protons and can be removed with little effort
How an object becomes positively charged? why?
the process of removing an electron from the electron cloud of an atom
ionization
an atom that is missing an electron due to ionization
positive ion
atoms that accommodate an extra electron
negative ion
-Friction (rubbing)
-Contact with a charged object
-Polarization
the charging mechanisms involve_(3)
-The forces of friction often cause molecular bond at the surface to break as 2 materials slide past each other
-Molecules are electrically neutral, but molecular ions can be created when one of the bonds in a large molecule is broken
-If the positive molecular ions remain on one material and the negative ions on the other, one of the objects being rubbed ends up with a net positively charge and the other with a net negative charge
-summary: charging by friction may result from molecular ions produced as bonds are broken
explain how rubbing can charge an object by looking it at the molecular view
-“q” or sometimes is “Q”
-coulomb (C)
what is the symbol of charge? what is it SI unit?
-“e”
-the magnitude of the charge of an electron or a proton
fundamental charge (symbol and definition)
e=1.60X10^-19 C/proton
e=-1.60X10^-19 C/electron
what is the fundamental charge of an electron? proton?
proton mass=1.67X10^-27kg
electron mass=9.11X10^-31kg
what is the mass of an electron? proton?
-charge is conserved
-associated charge is conserved as well
That charge is associated with electrons and protons explains why_. Because electrons and protons are neither created nor destroyed in ordinary processes, their_
The electrons in the insulator are all tightly bound to the positive nuclei and not free to move around
explain how an insulator looks at the molecular level
charging and insulator by friction leaves patches of molecular ions on the surface, but these patches are immobile
what happens when you charge an insulator at the molecular level?
In conductors (like metals), the outer atomic electrons (called valence electrons) are only weakly bound to the nuclei
Describe how a conductor looks at the molecular level
As the atoms come together to form a solid, the outer electrons become detached from their parent nuclei and are free to wander about through the entire solid
-The solid as a whole remains electrically neutral, because we have not added or removed any electrons, but the electrons are now rather like a negatively charged gas or liquid (called a sea of electrons) permeating an array of positively charged ion cores
-However, although the electrons are highly mobile within the metal, they are still weakly bound to the ion cores and will not leave the metal
what happens when a conductor’s atoms come together to form a solid? describe it at the molecular level
-if you bring a positive charge near a neutral atom, the charge polarizes the atom by attracting the electron cloud while repelling the nucleus
-Because there is an enormous number of atoms in an insulator, added together, their net polarization–and the resulting polarization force–can be quite significant
-electric dipole
Explain how an insulator becomes polarized when brought near a charged object from the molecular view. give the term for this
2 equal but opposite charges with a separation between them (in one molecule)
electric dipole
-where the polarization is caused by the external charge, the atom has become an induced electric dipole
induced electric dipole
-Because the negative end of the dipole is slightly closer to the positive charge, the attractive force on the negative end slightly exceeds the repulsive force on the positive end, and there is a net force toward the external charge
why a neutral object would move toward a charged object? (attracted) explain at the molecular level
-permanent electric dipoles
-some molecules have an asymmetry in their charge distribution that makes them permanent electric dipoles
what is another type of dipole? define it
-water molecule
-bonding between the hydrogen and oxygen atoms result in an unequal sharing of charge that leaves the hydrogen atoms with a small positive charge and the oxygen atoms with a small negative charge
what is an example of a permanent electric dipole? why?
-weak bond
-When 2 water molecules are close, the attractive electric force between the positive hydrogen atom of one molecule and the negative oxygen atom of the second molecule can form a weak bond called hydrogen bond
-weak bonds result in certain “stickiness”between water molecules that is responsible for water’s special properties like expansion when freezing, wide range of temperatures over which it is liquid, and its high heat of vaporization
hydrogen bond (type of bond, define it, and how it defines some of water properties)
-DNA molecule has the structure of a double helix. Information in DNA is coded in the nucelotides, the 4 molecules guanine, thymine, cytosine, and adenine. The nucleotides on one strand of DNA helix form hydrogen bonds with nucleotides on the opposite strand
-The nucleotides bond only in certain pairs: cyotsine and guanine, adenine and thymine. This preferential bonding is crucial for DNA replication. When 2 strands of DNA are taken apart, each separate strand of DNA forms a template on which another complementary strand can form, creating 2 identical copies of the original molecule
-The preferential bonding in DNA is explained by hydrogen bonding. In each of the nucleotides, the hydrogen atoms have a small positive charge, oxygen and nitrogen a small negative charge. The positive hydrogen atoms on one nucleotide attract the negative oxygen or nitrogen atoms on another. The geometry of the nucleotides allows cytosine to form a hydrogen bond with only guanine, adenine only with thymine.
how hydrogen bonds are extremely important for biological systems?
-The force law that describes the behavior of: electric force increases for objects with more charge and decreases as charged objects are moved farther apart
-mathematical formulation
Coulomb’s law (what describes, what kind of formulation is?)
charges only, not the sign
lql (absolute value of charge (q))
*It is always a positive number, whether the charge is positive or negative*
In the formulation of Coulomb’s law, we will use the magnitude of_
-If two charged particles having charges q1 and q2 are a distance r apart, the particles exert forces on each other of magnitude:
F1on2=F2on1=(K lq1l lq2l)/(r^2)
where the charges are in coulombs (C), and K=8.99X10^9 N*m^2/C^2 is called the electrostatic constant
Define Coulomb’s law by its magnitude
-These forces are an action/reaction pair, equal in magnitude and opposite in direction
-Coulomb’s law
How are electrostatic forces between two objects compared? this is part of which law?
-The electric forces are directed along the line joining the two particles
-The forces are repulsive for 2 like charges and attractive for 2 opposite charges
Define Coulomb’s law in terms of its direction
-between charged particles
Coulomb’s law describes the force between what two identities?
point charges
Coulomb’s law applies only to_
-A point charge is an idealized material object with charge and mass but with no size or extension
-For practical purposes, 2 charged objects can be modeled as point charges if they are much smaller than the separation between them
point charges (definition and when an object can be considered a point charge)
inverse square relation
Coulomb’s law is an_relation
either positive or negative, so the forces can be attractive or repulsive
The charge q in Coulomb’s law can be_, so the forces can be_
-always positive
-The direction must be determined from the second part of the law
The magnitude of the force in Coulomb’s law is_. How this applies to direction?
-force
-forces are vectors
Coulomb’s law is a _law, and _are vectors
-superimposed
-If multiple charges are present,the net electric force on a particular charge (like j) due to all other charges is therefore the sum of all the individual forces due to each charge
Fnet=F1onj + F2onj + F3onj +….
Where each of the forces is given by the equation of Coulomb’s law
Electric forces, like other forces, can be_. explain
B
The two forces are an action/reaction pair, opposite in direction but equal in magnitude
Charges 1 and 2 exert repulsive forces on each other q1=4q2. Which statement is true? Explain
A. F1on2>F2on1
B. F1on2=F2on1
C. F1on2
-electrostatics
-positive charge exerts on nearby negative charge
Coulomb’s law is the basic law of_. We can use Coulomb’s law to calculate the force a_
-Coulomb’s law tells us how to calculate the magnitude and direction of the force, but it doesn’t tell us how the force is transmitted through empty space from one charge to the other
-To answer this question, we will introduce the field model (introduced by Michael Faraday)
How does the negative charge “know”that the positive charge is there? Include the name of the law that can explain this
-the grass seeds line up to form a regular pattern
-the pattern suggests that some kind of electric influence from the charges fills the space around the charges
-This alteration of the space around the charges could be the mechanism by which the long-range Coulomb’s law force is exerted
-This is the essence of the field model
When charged spheres, one positive and one negative, touch the surface of the oil with tiny grass seeds floating on it, what happens? what this suggest? This is the essence of _
-In the field model, it is the alteration of space around charge A that is the agent that exerts a force on charge B
-This alteration of space is what we call a field
-The charge makes an alteration everywhere in space. Other charges then respond to the alteration at their position
describe the field model (use example of having an attractive force between positive charge A and a negative charge B)
-Consider having an interaction between a positive charge A and a negative charge B
-Force model: A exerts a force directly on B
-Field model: A alters the space around it. Particle B then responds to the altered space. The altered space is the agent that exerts the force on B
Describe how the force model is different from the field model (use particle A and B as example)
-part of field model
-alteration of space around a charged particle
field (part of which model, definition)
-electric field
-gravitational field
-the magnetic field
The space around a charge is altered to create the _. The alteration of the space around a mass is called the_. The alteration of space around a magnet is called_
the electric field model
what is another name for the field model?
-for visualizing and calculating forces for complex arrangements of charges
-When we begin to study fields that change with time, we will find phenomena that can be understood only in terms of fields
the field model is used for_(2)
the field model
the investigation of electric fields is done by using_
-It describes how charges interact:
1. A group of charges, which we will call the “source charges”, alter the space around them by creating an electric field E (with arrow above E).
2. If another charge is then placed in this electric field, it experiences a force F(with arrow above F) exerted by the field
describe the postulates of field model (2)
Suppose charge q experiences an electric force Fonq due to other charges:
-The strength and direction of this force vary as q is moved from point to point in space.
-This suggests that “something” is present at each point in space to cause the force that charge q experiences.
Suppose charge q experiences an electric force Fonq due to other charges. how is the strength and direction of the force? what this suggest?
E at (x,y,z)=(Fonq at (x,y,z))/q
the electric field at the point (x, y, z) is defined as_(formula)
the force-to-charge ratio
the definition of electric field is_
Newstons per coulomb N/C
what are the units of electric field?
the magnitude of E of the electric field
electric field strength
probe to determine if an electric field is present at a certain point
you can think of using charge q as a _to determine_
If charge q experiences an electric force at a point in space, then there is an electric field at that point causing the force
When an electric field is present?
the force on the charge tells us that there is an electric field at these points
how can you know if there is an electric field?
-we define the electric field at that point to be the vector given by field model equation
-The dots where vectors start are the points at which the field is known
we define the electric field (graphically)
the field is the agent that exerts an electric force on charge q
the basic idea of the field model is that_
1. the electric field, a vector, exists at a every point in space. Electric field diagrams will show a sample of the vectors, but there is an electric field vector at every point whether one is shown or not
2. If the probe charge q is positive, the electric field vector points in the same direction as the force on the charge; if negative, the electric field vector points opposite the force
3. because q appears in Equation of electric field model, it may seem that the electric field depends on the magnitude of the charge used to probe the field. It doesn’t. We know from Coulomb’s law that the force Fonq is proportional to q. Thus the electric field defined in Equation of electric field is independent of the charge q that probes the field. The electric field depends only on the source charges that create the field.
3 important things about the field:
1. place q’ at the point to probe the field (assume both charges are positive)
2. measure the force on q’
3. The electric field is E=Fonq’lq’. It is a vector in the direction of Fonq’ (if the probe is positive)
-If q is negative, the magnitude of the force on the probe charge is the same as the equation E=Klql/r^2 , but toward q
describe the steps to determine what is the electric field of point q at specific point
E=Klql/r^2 where:
-It is away from q if q>0
-It is toward q if q<0
the general expression for the field is_ (include direction of electric field)
Coulomb’s law
the expression of electric field is similar to_
-Coulomb’s law has the product of charges in the numerator because it describes the force between 2 charges
-The electric field has a single charge in the numerator because it is the field of a single charge
differentiate Coulomb’s law and the electric field equations
– In the electric field diagram of a positive point charge the field vectors point away from a positive charge
-In the electric field diagram of a negative point charge the field vectors point toward a negative charge
describe how the electric field diagram of a positive point charge and a negative point charge
-positive: a positive person likes to give to other people, so the field vectors of the electric field are directed away from the positive charge
-negative: a negative person is selfish and does not want to help other people, so the field vectors of the electric field are directed toward the negative charge
Trick to remember the direction of an electric field in a positive and negative point charge
-By drawing electric field vectors at ta number of points around a positive point charge or a negative point charge
electric field diagram (how to construct it)
-the arrow indicates the direction and the strength of the electric field at the point to which it is attached–at the point where the tail of the vector is placed
-The length of any vector is significant only relative to the lengths of other vectors
describe what the field vector shows at a electric field diagram
Although we have to draw a vector across the page, from one point to another, an electric field vector does not “stretch” from one point to another. Each vector represents the electric field at one point in space
While drawing electric fields, take into account that although we have to draw a vector _, an electric field vector does not “_” from one point to another. Each vector represents the _
-sample of electric field vectors.
-all other points
-good indication of what the field would be like at a neighboring point
the electron field diagram is just a _. The field exists at _. A well-drawn diagram gives a _
-charge
-the square of the distance
the field is proportional to_and inversely proportional to_
-found by looking at the force on a probe charge, no mater what the number of source charges is
-vector sum of the forces due to all of the individual charges
-*the electric field due to multiple charges is the vector sum of the electric field due to each of the charges*
suppose we want to find the electric field due to more than one source charge, the electric field at a point in space can be found by_. Because the net force on the probe charge is_, the _
between 10^3 and 10^6 N/C
what is the range of electric field (power of ten)
how charges alter the space around them
a electric field diagram shows how charges_
-2 conduction plates, which are face-to-face with a narrow gap between them
-One electrode has total +Q and the other has total charge -Q
-This arrangement of 2 electrodes, closely spaced and charged equally but oppositely is called parallel-plate capacitor
electrodes (describe them, how their arrangement is called)
-At any point, the electric field is the vector sum of the fields from all the positive charges and all the negative charges on the plates
-However, the field of a point charge decreases inversely with the square of its distance, so in practice only the nearby charges contribute to the field (the charges on the inner surfaces of the plates)
-The horizontal components of the individual fields cancel, while the vertical components add to give an electric field vector pointing from the positive plate toward the negative plate
-By mapping the electric field at many points, we find that the field inside a parallel-plate capacitor is the same–in both strength and direction–at every point. This is called a “uniform electric field”
what is the electric field between the two plates in a parallel-plate capacitor? Just focus on the field of the central region, far from the edges. include the term for the phenomena happening between the plates
-it is represented with parallel electric field vectors of equal length
how is the uniform electric field represented?
Ecapacitor=Q/ε₀A
ε₀=epsilon zero or epsilon naught
A=area
Q=charge
what is the equation for the electric field inside a parallel-plate capacitor?
-permittivity constant
-Its value is related tot he electrostatic constant as
ε₀=1/(4piK)=8.85X10⁻¹² C²²/N*m²
what is another name for the epsilon zero/epsilon naught?
-charge-to-area ration Q/A, which is often called the charge density
-If the charges are packed more closely, the field will be larger
the fields depends on the_. (Include the term for this and when the field will be larger)
-separation
-small
-size
-the spacing between the plates does not affect the electric field, and this spacing does not appear in the equation of the electric field for the interior of a parallel-plate capacitor
Our analysis requires that the _of the plates be_(size) compared with their _. If this is true, _
The shape of the electrodes is not relevant as long the electrodes are very close together
does the shape of the electrodes affect the electric field between them?
-the charges on the plates are equal and opposite +Q and -Q, so the net charge is zero.
*where Q is the magnitude of the charge on each plate
How the charges of each plate are compared?
uniform
the electric field between the plates is_
-magnitude
-direction
when giving answers of electric field, what you should include always?
electric field lines
what is another way to visualize electric fields besides the electric field diagram?
-these are imaginary lines drawn through a region of space so that:
1. the tangent to a field line at any point is in the direction of the electric field E at that point, and
2. The field lines are closer together where the electric field strength is greater; far apart where the field is weak
electric field lines
-The electric field vector is tangent to the electric field line
-The electric field is stronger where the electric field vectors are longer and where the electric field lines are closer together
what is the relationship between the field vectors and field lines? (2)
-The field is directed away from the positive charge, so the field lines are directed radially outward
-The field lines are closest together near the charge, where the field strength is the greatest
describe how a positive point charge would be represented in the field lines diagram (2)
-The field vectors are directed from the positive to the negative plate so the field lines are as well
-The field is constant, so the field lines are evenly spaced
describe how the field lines would be represented in an ideal capacitor (2)
how the field looks like
field lines helps us to look _
1. Field lines cannot cross. The tangent to the field line is the electric field vector, which indicates the direction of the force on a positive charge. The force must be in a unique, well-defined direction, so two field lines cannot cross (draw the lines tangent to the field vector at each point)
2. The electric field is created by charges. Field lines start on a positive charge and end on a negative charge
what are the 2 rules to keep in mind while generating a field line picture of an arrangement of charges?
-close together
-far apart
where the field is strong, the field lines need to be drawn_. Where the field is weak, the field lines need to be drawn_
-nerve and muscle cells
-a cell membrane is an insulator that encloses a conduction fluid and is surrounded by conduction fluid.
-While resting, the membrane is polarized with positive charges on the outside of the cell, negative charges on the inside
-When a nerve or a muscle cell is stimulated, the polarity of the membrane switches; we say that the cell depolarizes
-Later, when the charge balance is restored, we say the that the cell repolarizes
what 2 biological cells have prominent electrical nature? How its walls are conformed that base this and explain the processes?
-depolarization
-heart
all nerve and muscle cells generate an electrical signal when_occurs, but the largest electrical signal in the body comes from the_
-The rhythmic beating of the heart is produced by a highly coordinated wave of depolarization that sweeps across the tissue of the heart
-the surface of the heart is positive on one side of the boundary between tissue that is depolarized and tissue that is not yet depolarized, negative on the other
-the heart is a large electric dipole
-The orientation and strength of the dipole changes during each beat of the heart as the depolarization wave sweeps across it
explain the beating of a heart using the electricity (include where the charges are localized and give the term for this) (include localization and strength of charges during each beating)
dipole electric field that extends throughout the torso
the electric dipole of the heart generates a_
-The electric field is zero at all point inside a conductor in electrostatic equilibrium
-Reason: electric field exert forces on charges, so an internal electric field would exert forces on the charges in the conductor. Because charges in a conductor are free to move, these forces would cause the charges to move. But that would violate the assumption that all the charges are at rest
Consider a conductor in electrostatic equilibrium (this means that none of the charges are moving), how would be the electric field inside the conductor? why?
-The excess charge on the conductor must lie at its surface.
-Reason: Any charge in the interior of the conductor would create an electric field there, in violation of our conclusion that the field inside is zero. Physically, excess charge ends up on the surface because repulsive forces between like charges cause them to move as far apart as possible without leaving the conductor
Because the electric field inside a conductor in electrostatic equilibrium is zero, where the excess charge is? why?
-the electric field right at the surface of a charged conductor is perpendicular to the surface
(keep in mind that the electric field inside would still be 0)
describe the electric field of a charged conductor.
-Suppose the electric field had a component tangent to the surface.This component of electric field would exert a force on charges at the surface and cause them to move along the surface, thus violating the assumption that all charges are at rest
-The only exterior electric field consistent with electrostatic equilibrium is on that is perpendicular to the surface
why the electric field has to be perpendicular to the surface?
-the electric field inside a conductor is zero (at all points) in electrostatic equilibrium
-all excess charge is on the surface
-the electric field right at the surface of a charged conductor is perpendicular to the surface
Summarize the electric field of conductors at electrostatic equillibrium (3)
*The excess charge on the conductor is at the surface and the electric field within the conductor is zero, so there’s nothing that could create an electric field within the enclosure*
-a region of space enclosed by conducting walls is screened from electric field (E=0)
-If we have a void completely enclosed by the conductor, the electric field inside the enclosed void is zero
A region of space enclosed by conduction walls will have what kind of electric field?
zero
the electric field within a conducting enclosure is_
conducting box, which can be used to exclude electric field from a region of space
screening
such a bag, when sealed, forms a conducting shell around its interior. All excess charge is on the surface of the bag and the electric field inside is zero. A chip or component inside the bag is protected from damaging charges and fields
computer chips and other electronic components are very sensitive to electronic charges and fields. Even small static charge or field may damage them. Such components are shipped and stored in conducting bags. How do these bag protect the components stored inside?
-not be uniformly distributed
-where the density of charge is highest (the charges are closer together), the electric field will be the strongest
Although any excess charge on a conductor will be found on the surface, it may _. How this would affect the strength of the electric field on the surface?
the sharper the point
the _(shape), the stronger the field
force on a charge
the electric field was defined in terms of the _
-if a charge q is placed at the point in space where the electric field is E then according to the equation the charge experiences an electric force:
Fonq=qE
E=electric field
q=charge
[Force on a charge due to an electric field]
what will experience a charge q placed in space where the electric field is? include equation
-If q is positive, the force on charge q is in the direction of E (electric field)
-The force on a negative charge is opposite the direction of E (electric field)
describe how to determine the direction of the electric force that the charge q experiences
The electric force on its negative charge is equal in magnitude but opposite in direction tot he force on its positive charge. Thus an electric dipole in an uniform electric field experiences no net force. However, there is a net torque on the dipole that causes it to rotate
what happens if you put an electric dipole in an uniform electric field?
-electric dipole moment
-is a vector that points from the negative to the positive charge
Dipole moment (other name and definition)
1. Because the forces on the positive and negative charges are equal in magnitude but oppositely directed, there is no net force on the dipole
2. However, there is a net torque on the dipole that causes it to rotate
3. When the dipole lines up with the field, the net torque is zero. The dipole is in static equilibrium. We can say the the “dipole moment” tries to align itself with the field
Describe in steps what would happen if you place a dipole in an uniform electric field
the equilibrium position of a dipole in an electric field is with the electric dipole moment aligned with the field
the equilibrium position of a dipole in an electric field is_
1. the electric field polarizes the seed, including opposite charges on their ends. The seeds are induced electric dipoles, with dipole moments along the axis of each seed
2. Torques on the dipole moments cause them to line up with the electric field, revealing its structure
explain why the grass seeds lined up in the oil with the electric field from two charged electrodes
the electric field (E)
charges interact with each other via_
-the force on a probe charge q
electric field is identified and measured in terms of_
all points in space
electric field exists at_
the field only at one point, the point at the tail of the vector
electric field vector shows_
parallel
electric field lines are always_to the field vectors (geometry relation)
from positive to negative charges
electric field lines go from_to_
-net charge
-field
-the two charges are separated and of equal magnitude
Dipole has no_, but has a_because_
opposite charges attract one another; like charges repel
central rule of electricity
-If the charges are alike in sign, the force is repelling; if the charges are not alike, the force is attractive
-Electric force between 2 particles
Coulomb’s law (simplify) (2)
underlies the bonding forces between molecules
What Coulomb’s law underlies?
-Ancient mystery: “Amber effect”
-J. J. Thompson: identified negatively charged electrons
Electron theory of charge (name of antique mystery, who? what he did?)
atom
basic unit of matter
Charges in matter
what is an inseparable property of certain particles?
electric force
charge interaction is due to_force
non-zero net charge from loss/gain of electrons
ions
Stationary charge confined to an object
electrostatic charge
reorientation of charges induced without changing net charge
polarization definition
-Electrons are free to move throughout material
-Examples: metals, graphite (carbon)
3 main characteristics of electrical conductors (definition, what happens to added charge, examples (2))
-Electron motions restricted
-Added charge tends to remain on object
-Examples: Glass, wood, diamond (carbon)
3 main characteristics of electrical insulators (definition, what happens to added charge, examples (3))
-Conduct/insulate depending on circumstances
-Applications: Computer chips, solar cells, etc
2 characteristics of semiconductors (definition, applications (2))
-Charges on an insulator do not move; they would stay at the same place where it was touched
when an insulator is touched by a charged object, what happens to the charges?
-Charges on a conductor adjust until there is no net force on any charge. We call this electrostatic equilibrium.
-They will spread over the surface
when an conductor is touched by a charged object, what happens to the charges?
Coulomb
metric unit of charge
Equivalent to charge of 6.24×10^18 electrons!
1 coulomb is equivalent to how many electrons?
q=ne
q=quantity charge
n=number of electrons
e=electron charge
formula to get quantity of charge?
1.60 x10⁻¹⁹ Coulombs/electron
what is the value of the charge of electrons? what about protons?
-when a conductor is charged, electrostatic equilibrium happens when there is an equal distribution of the new charges on the surface
explain electrostatic equilibrium with simple worlds
-the new charges acquired by the conductor cannot move anymore after that
what happens after electrostatic equilibrium?
-Depends on the situation
-if they are identical, after they contact, they will have the same amount of charge on each object. Thus, the total extra charge of two objects will be divided between them
when 2 objects touch each other (one being charged or both), do they end up with the same amount of charge?
-On both cases, charges will end up on the surface of the objects
-The difference is that in conductors charges will distribute evenly on the surface and on insulators the charges will remain on the spot that were charges on the suface
where do the additional charges end up in conductors? insulators?
A
A negatively charged rod is brought near a neutral metal sphere. Which of the following is true?
A.There is an attractive force between the rod and sphere.
B.There is a repulsive force between the rod and sphere.
C.There is no electric force between the rod and sphere
-generate static charge by transferring electrons between objects when you have friction between objects
-Electrons are loose bond and the protons are strongly attached in the nucelus. Easier to remove electron than proton
when do you generate static charge? why?
same number of electrons and protons
neutral atom has_
atoms are formed because there is an electrical charge interaction between negative (electron) and positive (proton)
why atoms are put together? (why the particles simply spread out?)
All is based on electron configuration to see which ones become what kind of ion
How can you know when an atom of specific element will get or give away electron(s)?
electron transfer
electrostatic charge is generated by_(look at it in the molecular perspective)
-It is not. It is current
-there is no microscopic move in electrostatic charge
charging a laptop can be considered electrostatic charge? why?
-In conductors, inside the material electrons are free to move throughout it. electron are free to move from one atom to another atom
-atoms hold electrons, do not let them flow or jump to another atom
a conductor is a conductor because? what about insulators? (look at it in microscopic perspective)
low temperature= insulator
hot temperature=conductor
silicon
when semiconductors are conductors? insulators? give an example of a semiconductor
A
A hollow sphere made out of electrically insulating material is electrically neutral (no excess charge). A small amount of negative charge is suddenly placed at one point P on the outside of this
sphere. If we check on this excess negative charge a few seconds later we will find one of the following possibilities:
(a) All of the excess charge remains right around P.
(b) The excess charge has distributed itself evenly over the outside surface of the sphere.
(c) The excess charge is evenly distributed over the inside and outside surface.
(d) Most of the charge is still at point P, but some will have spread over the sphere.
(e) There will be no excess charge left.
B
A hollow metal sphere is electrically neutral (no excess charge). A small amount of negative charge is suddenly placed at one point P on this metal sphere. If we check on this excess negative charge a few seconds later we will find one of the following possibilities:

(a) All of the excess charge remains right around P.
(b) The excess charge has distributed itself evenly over the outside surface of the sphere.
(c) The excess charge is evenly distributed over the inside and outside surface.
(d) Most of the charge is still at point P, but some will have spread over the sphere.
(e) There will be no excess charge left.

-Touch one with the other (one is charged by rubbing)
Describe a procedure by which you could give two identical metal spheres exactly equal charges.
-have the neutral metal spheres to touch each other
-bring close a charged rod by the lateral side of the alienation of the balls
-separate them
Describe a procedure by which you could give two identical metal spheres charges of opposite sign but exactly equal magnitude.
False
B because it is quantized in the formula of q=ne, n has to be integer because you cannot have half electrons, only full electrons
True or False: The statement “A plastic ball can carry a static charge with the amount of 1004.5 × e” is (where e is the unit of charge,1.60×10-19 C) Explain why
-3 electrons
-There cannot be half charges because you cannot split an electron. This is the reason charge is quantized
triple charge would be how many electrons? why? This is the reason charge is_
fundamental charge
what is the smallest charge that can be found in nature?
-vector
-magnitude and direction
force is a _, for this reason when giving an answer you need to provide_
presence of charge
Fields exists due to_
energy
field carry on_ (what contains?)
The net electric field can only be zero if the electric fields due to the two charges point in opposite directions and have equal magnitudes. Therefore, first determine the region(s) where the two constituent electric fields point in opposite directions. Then, in each region determine whether a point exists where the fields have equal magnitude. If there is such a point, then select that region.
when do you have an net electric field equal to zero when you have two charges of the same magnitude and a charged point in the middle?
on the surface
where is located the charge in an object?
-vector
-scalar
-vector
Force is a_, charge is a_quantity, and electric field_
-we have a source charge that generates the electric field, we want to define electric field at specific point by putting a small positive charge as a test
-For test will be always posiitve charge. we use the test charge as positive to define electrical field. thus it will be always like that (positive) and the electric fields of the source charge will always have the following directions of electric field:
How do we define an electric field?
-by source charge, not by tiny test charge
the electric field is defined solely by_,not by_
POSITVE= the electric field point away
NEGATIVE= the electric field points toward
describe the direction of the electric fields of positive and negative sources
capacitor
a uniform electric field between two plates is called_
-two same identical plates that are parallel to each other and they have the same charge magnitude but opposite signs
-Electric field magnitude is the same throughout =uniform field
define capacitor (definition and define how it would be its electric field)
-exactly the same magnitude and direction
-direction=from positive to negative
uniform field
uniform
The field inside a charged parallel-plate capacitor is_
It will move at a constant acceleration.
A positive charge is placed at rest at the center of a region of space in which there is a uniform, three-dimensional electric field. When the positive charge is released from rest in the uniform electric field, what will its subsequent motion be?
What do the electric field lines do for you?
they are radiant around a charge
How are the electric field lines around a charge?
uniform
the electric field between plates is_
tangent
the electric field vector is_to the electric field line
-electric field vectors are longer
-electric lines are closer
the electric field is stronger where electric field vectors_and where electric lines_
they are equally spaced
how the lines are spaced between each other in a capacitor?
-increase the magnitude of the charge on both plates
-decrease the area of the plates (while keeping the magnitude of the charges the same)
Two parallel plates have charges of equal magnitude but opposite sign. What change could be made to increase the field strength between the plates? (2)
You will destroy the uniform electrical field in the field because the electric field and charges will organize to have the inside of the object to be keep as zero
what will happen if put a conductor inside a uniform electric field (between two plates)?
zero field
The figure below shows a hollow conducting metal sphere which was given initially an evenly distributed positive (+) charge on its surface. Then a positive charge +Q was brought up near the sphere as shown. What is the direction of the electric field at the center of the sphere after the positive charge +Q is brought up near the sphere?
the electrical force will point in opposite direction to the electrical field because the electron is negative and this affects the F=Eq
what is the direction of the electrical force of an electron with respect to the electrical field?
T=qEL
what is the formula for torque of dipole in electric field?
the dissipated power
what heats the wire?

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