Chapter 22- Charge Flashcards
what is charging ?
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.
at first they discovered that there is only 2 types of charges. what are they ?
There are two and only two kinds of charge. For now we will call these “plastic charge” and “glass charge.” Other objects can sometimes be charged by rubbing, but the charge they receive is either “plastic charge” or “glass charge.”
what happens when two likes charge are close together
and two opposite charge ?
Two like charges (plastic/plastic or glass/glass) exert repulsive forces on each other. Two opposite charges (plastic/glass) attract each other.
what is the type of force between 2 charges ?
the force between two charges is a long-range force. The size of the force increases as the quantity of charge increases and decreases as the distance between the charges increases.
neutral objects has what ?
Neutral objects have an equal mixture of both “plastic charge” and “glass charge.” The rubbing process somehow manages to separate the two.
how do you tell if an object is charge ?
However, an important characteristic of any charged object appears to be that a charged object picks up small pieces of paper. This behavior provides a straightforward test to answer the question, Is this object charged? An object that passes the test by picking up paper is charged; an object that fails the test is neutral.
what is charge model , part 2 .
1) 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.
2) Charge can be transferred from one object to another by contact.
what is discharging ?
Removing charge from an object, which you can do by touching it, is called discharging.
example of insulators ?
glass and plastic
example of conductor ?
metal
In Experiment 12, touching one metal sphere with a charged plastic rod caused a second metal sphere to become charged with the same type of charge as the rod. Use the postulates of the charge model to explain this.
Charge is transferred upon contact.
Metal is a conductor, and charge moves through a conductor
Like charges repel.
The plastic rod was charged by rubbing with wool. The charge doesn’t move around on the rod, because it is an insulator, but some of the “plastic charge” is transferred to the metal upon contact. Once in the metal, which is a conductor, the charges are free to move around. Furthermore, because like charges repel, these plastic charges quickly move as far apart as they possibly can. Some move through the connecting metal rod to the second sphere. Consequently, the second sphere acquires “plastic charge.”
To determine if an object has “glass charge,” you need to
See if the object attracts a charged plastic rod.
See if the object repels a charged glass rod.
Do both a and b.
Do either a or b.
do both a and b ?
electrons and protons has what kind of charge ?
Thus by convention electrons have a negative charge and protons a positive charge.
Franklin established a convention about charges .
what is it ?
It’s entirely up to us! Franklin established the convention that a glass rod that has been rubbed with silk is positively charged. That’s it. Any other object that repels a charged glass rod is also positively charged. Any charged object that attracts a charged glass rod is negatively charged. Thus a plastic rod rubbed with wool is negative.
electron and protons have opposite signs of charge but has equal…
has exactly the same magnitude.
charge is represented by the symbol..
Q
what is the formula for charged proton ?
q =( Np -Ne) e
where Np is number of protons
Ne is the number of electron
an object with equal number of proton and electron has what kind of charge ?
An object with an equal number of protons and electrons has no net charge (i.e., q=0) and is said to be electrically neutral.
a object is postively charged when ..
an object is negatively charged when …
A charged object has an unequal number of protons and electrons. An object is positively charged if Np>Ne. It is negatively charged if Np
objects acquire postive charge by ,,,
why ?
what is it called ?
objects acquire a positive charge not by gaining protons, as you might expect, 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 rather loosely and can be removed without great difficulty. The process of removing an electron from the electron cloud of an atom is called ionization. An atom that is missing an electron is called a positive ion. Its net charge is q=+e.
how does friction between 2 objects causes it to become + and -.
The forces of friction cause molecular bonds at the surface to break as the two materials slide past each other. Molecules are electrically neutral, but Figure 22.3 shows that molecular ions can be created when one of the bonds in a large molecule is broken. The positive molecular ions remain on one material and the negative ions on the other, so one of the objects being rubbed ends up with a net positive charge and the other with a net negative charge. This is the way in which a plastic rod is charged by rubbing with wool or a comb is charged by passing through your hair.
law of conservation of charge
what is it ?
Charge is neither created nor destroyed. Charge can be transferred from one object to another as electrons and ions move about, but the total amount of charge remains constant. For example, charging a plastic rod by rubbing it with wool transfers electrons from the wool to the plastic as the molecular bonds break. The wool is left with a positive charge equal in magnitude but opposite in sign to the negative charge of the rod: q wool=−q plastic. The net charge remains zero.
explain how the electrons and protons are in an insulator ?
The electrons in the insulator are all tightly bound to the positive nuclei and not free to move around. Charging an insulator by friction leaves patches of molecular ions on the surface, but these patches are immobile.
check pic in book
explain how the electrons and protons are in an conductor ?
In metals, the outer atomic electrons (called the valence electrons in chemistry) are only weakly bound to the nuclei. As the atoms come together to form a solid, these 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—what physicists like to call a sea of electrons—permeating an array of positively charged ion cores.
what is the consequence of the metal structure ?
The primary consequence of this structure is that electrons in a metal are highly mobile. They can quickly and easily move through the metal in response to electric forces.
what is a current ?
The motion of charges through a material is what we will later call a current,
what are charge carriers ?
and the charges that physically move are called the charge carriers. The charge carriers in metals are electrons.
what are other example of conductors ?
and why we dont really talk about them ?
Metals aren’t the only conductors. Ionic solutions, such as salt water, are also good conductors. But the charge carriers in an ionic solution are the ions, not electrons. We’ll focus on metallic conductors because of their importance in applications of electricity.
insulators, how are they charged ?
what happens ?
Insulators are often charged by rubbing. The charge diagrams of Figure 22.5 show that the charges on the rod are on the surface and that charge is conserved.
There are only negative charge on the part that was rub.
It doesnt move around the rod.
The charge can be transferred to another object upon contact, but it doesn’t move around on the rod.
insulators, how are they charged ?
what happens ?
Insulators are often charged by rubbing. The charge diagrams of Figure 22.5 show that the charges on the rod are on the surface and that charge is conserved.
There are only negative charge on the part that was rub.
It doesnt move around the rod.
The charge can be transferred to another object upon contact, but it doesn’t move around on the rod.
check image
metal, how are they charged ?
Metals usually cannot be charged by rubbing, but Experiment 9 showed that a metal sphere can be charged by contact with a charged plastic rod. Figure 22.6 gives a pictorial explanation. An essential idea is that the electrons in a conductor are free to move. Once charge is transferred to the metal, repulsive forces between the negative charges cause the electrons to move apart from each other.
check image
what happens when new charges are added to the metal. ?
Note that the newly added electrons do not themselves need to move to the far corners of the metal. Because of the repulsive forces, the newcomers simply “shove” the entire electron sea a little to the side. The electron sea takes an extremely short time to adjust itself to the presence of the added charge, typically less than 10−9 s. For all practical purposes, a conductor responds instantaneously to the addition or removal of charge.
the charges in a isolated conductor static or dynamic ?
why ?
and what is this condition called ?
Other than this very brief interval during which the electron sea is adjusting, the charges in an isolated conductor are in static equilibrium. That is, the charges are at rest (i.e., static) and there is no net force on any charge. This condition is called electrostatic equilibrium. If there were a net force on one of the charges, it would quickly move to an equilibrium point at which the force was zero.
In an isolated conductor, any excess charge is located on the surface of the conductor.
what does this mean ?
To see this, suppose there were an excess electron in the interior of an isolated conductor. The extra electron would upset the electrical neutrality of the interior and exert forces on nearby electrons, causing them to move. But their motion would violate the assumption of static equilibrium, so we’re forced to conclude that there cannot be any excess electrons in the interior. Any excess electrons push each other apart until they’re all on the surface.
Many electricity demonstrations are carried out with the help of an electroscope like the one shown in Figure 22.7. Touching the sphere at the top of an electroscope with a charged plastic rod causes the leaves to fly apart and remain hanging at an angle. Use charge diagrams to explain why.
check answer in the book
the human body, is it a conductor or an insulator ?
explain ?
The human body consists largely of salt water. Pure water is not a terribly good conductor, but salt water, with its Na+ and cl− ions, is. Consequently, and occasionally tragically, humans are reasonably good conductors. This fact allows us to understand how it is that touching a charged object discharges it,
the net effect of touching a charged metal is that it and the conducting human together become a much larger conductor than the metal alone. Any excess charge that was initially confined to the metal can now spread over the larger metal + human conductor. This may not entirely discharge the metal, but in typical circumstances, where the human is much larger than the metal, the residual charge remaining on the metal is much reduced from the original charge. The metal, for most practical purposes, is discharged. In essence, two conductors in contact “share” the charge that was originally on just one of them.
is the earth a conductor
why ? or why not ?
The earth itself is a giant conductor because of its water, moist soil, and a variety of ions. Any object that is physically connected to the earth through a conductor is said to be grounded. The effect of being grounded is that the object shares any excess charge it has with the entire earth! But the earth is so enormous that any conductor attached to the earth will be completely discharged.
what is the purpose of grounding ?
The purpose of grounding objects, such as circuits and appliances, is to prevent the buildup of any charge on the objects.
Charge polarization
what is it ?
Charge polarization is a slight separation of the positive and negative charges in a neutral object.
How do charged objects exert an attractive force on a neutral object?
positively charged rod held close to—but not touching—a neutral electroscope.
The charged rod doesn’t touch the electroscope, so no charge is added or removed. Instead, the metal’s sea of electrons is attracted to the positive rod and shifts slightly to create an excess of negative charge on the side near the rod. The far side of the electroscope now has a deficit of electrons—an excess positive charge. We say that the electroscope has been polarized.
Because there’s no net charge, the electron sea quickly readjusts when the rod is removed.
Why don’t all the electrons rush to the side near the positive charge?
when we put a positive charge rod near a neutral object.
Once the electron sea shifts slightly, the stationary positive ions begin to exert a force, a restoring force, pulling the electrons back to the right. The equilibrium position for the sea of electrons shifts just enough that the forces due to the external charge and the positive ions are in balance. In practice, the displacement of the electron sea is usually less than 10−15 m!
what if we place negative rod near a neutral object ?
A negatively charged rod would push the electron sea slightly away, polarizing the metal to have a positive upper surface charge and a negative lower surface charge.
what is an electric dipole ?
Two opposite charges with a slight separation between them
what happens when you bring a positive charge near a atom ?
consider what happens when we bring a positive charge near an atom. As Figure 22.12 shows, the charge polarizes the atom. The electron cloud doesn’t move far, because the force from the positive nucleus pulls it back, but the center of positive charge and the center of negative charge are now slightly separated.
what happens to an insulator when an external charge is brought close ?
An insulator has no sea of electrons to shift if an external charge is brought close. Instead, as Figure 22.13 shows, all the individual atoms inside the insulator become polarized. The polarization force acting on each atom produces a net polarization force toward the external charge. This solves the puzzle. A charged rod picks up pieces of paper by
Polarizing the atoms in the paper,
Then exerting an attractive polarization force on each atom.
what is coloumbs law ?
If two charged particles having charges q1 and q2 are a distance r apart, the particles exert forces on each other of magnitude
(22.2)
F1 on 2=F2 on 1= ( K x q1 x q2) / r2
where K is called the electrostatic constant. These forces are an action/reaction pair, equal in magnitude and opposite in direction.
The forces are directed along the line joining the two particles. The forces are repulsive for two like charges and attractive for two opposite charges.
what is coloumbs law ?
If two charged particles having charges q1 and q2 are a distance r apart, the particles exert forces on each other of magnitude
(22.2)
F1 on 2=F2 on 1= ( K x q1 x q2) / r2
where K is called the electrostatic constant. These forces are an action/reaction pair, equal in magnitude and opposite in direction.
The forces are directed along the line joining the two particles. The forces are repulsive for two like charges and attractive for two opposite charges.
Experiment 4 in Section 22.1 found that the electric force decreases with distance. The force law that describes this behavior is known as Coulomb’s law.
point charges
the force between charged particles,
as described by coloumb law
A charged particle, which is an extension of the particle model we used in Part I, has what
has a mass and a charge but has no size.
unit of charge e,
e=1.60×10^−19
what are the two important observation about coloumbs law ?
Coulomb’s law applies only to point charges. A point charge is an idealized material object with charge and mass but with no size or extension. For practical purposes, two charged objects can be modeled as point charges if they are much smaller than the separation between them.
Electric forces, like other forces, can be superimposed. If multiple charges 1, 2, 3, … are present, the net electric force on charge j due to all other charges is
(22.4)
F⃗ net= F1 + F2 +f3 ⋯
