Chapter+18

toc = Electric Charge, Forces, and Fields =


 * Guiding Questions**
 * 1) What is the structure and properties of an atom?
 * 2) The nucleus, or center of the atom, is packed with a specific number of protons and neutrons. The electrons orbit the nucleus at a fixed radius. Most of the atom is space surrounding the nucleus.
 * 3) What is the symbol and unit of electric charge?
 * 4) Electric charge uses a unit known as the Coulomb (C).
 * 5) Distinguish between positive and negative charges in as many ways as possible.
 * 6) When an atom has a positive charge, there are more protons in the atom than there are electrons. When an atom has a negative charge, it is vice versa. An atom becomes positively charged when an electron leaves the atom, and it becomes positively charged when a new electron enters the shell.
 * 7) Positive Charges - ions, ten thousand times more massive, immobile
 * 8) Negative Charges - electrons, 9.11 x 10^-31 kg, mobile
 * 9) Describe the properties of electric forces.
 * 10) Electric forces are non-contact forces. A charged object can exert a force on charged and uncharged objects. The biggest rule about electric forces is that opposites attract and likes repel.
 * 11) Distinguish between insulators and conductors.
 * 12) Insulator – a material that prevents electrons from flowing freely from atom to atom. Does not transfer a charge through its surface
 * 13) Conductor – a material that allows electrons to flow freely from atom to atom. Transfers a charge to objects that are not touching each other, but are both touching the conductor
 * 14) What is polarization?
 * 15) Polarization is separating opposite charges within the same object. This causes one side of the object to be left with an excess of positive charge and one side with an excess of negative charge. Ever hear of a magnet?
 * 16) How does a neutral object acquire charge?
 * 17) An overall loss or gain of electrons
 * 18) Distinguish between the 3 charging processes.
 * 19) Friction – occurs when two objects are rubbed together. One object might have a greater attraction for electrons, thus causing the electrons from the other object to attach to the first object. This object would not have a negative charge, while the object that just lost the electrons will have a positive charge.
 * 20) Induction – occurs when two conductors are placed next to one another. Example: A positively charged object is brought next to sphere A causing the negatives to flow into A and the positives to flow into B. When the object is drawn away, the charges distribute themselves evenly in each sphere and now the two objects are charged.
 * 21) Conduction – occurs when a charged object and neutral object are touching. A charged object has many charges that are the same, thus they are looking to repel one another. When this object touches a neutral object, some of this charge is rubbed off onto the neutral surface, now giving it a charge.
 * 22) What is the law of electric charge?
 * 23) Charge is neither created nor destroyed during the charging process.
 * 24) What is an electric field?
 * 25) A region around a charged object where a force would be exerted on other charged objects
 * 26) What are the characteristics and properties of an electric field? See applet: [|http://www.gel.ulaval.ca/~mbusque/elec/main_e.html]
 * 27) The intensity of electric fields depend on the charged object. This measured by a force per charge ratio.
 * 28) What are the “players” involved in an electric field?
 * 29) The charged object and all other objects within the field
 * 30) What are electric field lines?
 * 31) Visually represents the vector nature of an electric field
 * 32) What are 4 characteristics of electric field lines?
 * 33) Electric field lines should never cross.
 * 34) Always directed perpendicular to the surface of an object.
 * 35) Density = strength
 * 36) Always flows into negative objects and out of positive
 * 37) Go to []. Scroll to the bottom of the page and do the “Check Your Understanding” questions.
 * 38) 1. C,D,E
 * 39) Electric field lines should never cross!
 * 40) A – negative, B – positive
 * 41) DAECB
 * A,C,F,G,H,I are positive
 * 1) B < A, C<D, G<E<F, J<H<I

=**Practice Questions**= 1. What is the charge on a rod that has 15 excess electrons? 2. What is the charge on a pith ball that has 3.15 x 1016 electrons? 3. How many electrons are missing from a balloon that has a charge of 4.19 x 10-5 C? 4. If a metal object receives a positive charge, does its mass increase, decrease, or stay the same? What happens to its mass if the object receives a negative charge? 5. A 4.5 x 10-9 C charge is located 3.2 m from a –2.8 x 10-9 C charge. Find the electrostatic force exerted by one charge on the other. 6. What is the magnitude and direction of the force between two identical 10 microcoulomb charges 0.015-m apart? 7. Consider the electric force between a pair of charged particles a certain distance apart. By Coulomb's law: Concept Map
 * 1) If the charge on one of the particles is doubled, the force is //_.//
 * If, instead, the charge on both particles is doubled, the force is //_.//
 * 1) If instead the distance between the particles is halved, the force is //_.//
 * 2) If the distance is halved, //and// the charge of both particles is doubled, the force is ___ as great.

=**Summary - Lesson 1**= 1. What (specifically) did you read that you already understood well from our class discussion? Describe at least 2 items fully. I definitely understood the section on "Charge as a Quantity" due to our discussion today in class. It made sense to me when reading that Coulumbs measures the amount of charge an object possesses. I already knew what the Physics Classroom was describing when it wrote about the amount of charge on single electrons and protons and how to calculate certain charges. I already knew what the Physics Classroom was talking about when it described how objects of different charges attract and repel. I know that when a neutral object is rubbed against another neutral object, electrons are rubbed onto one of them (because they are more likely to attract the electrons) and now both objects have a certain charge. This fact changes their behavior when they come close to other charged objects.

2. What (specifically) did you read that you were a little confused/unclear/shaky about from class, but the reading helped to clarify? Describe the misconception you were having as well as your new understanding. The reading helped clarify how objects are polarized. It explained well that when a charged (let's use negative for this example) object is placed next to a neutral object, the negative charges go to the end of the object closest to the other one and the positive charges go to the other side. Now, there is one end with positive charges and another end with negative charges.

3. What (specifically) did you read that you still don’t understand? Please word these in the form of a question. How can an insulator be polarized?

4. What (specifically) did you read that was not gone over during class today? This took me in depth on conductors and insulators. I learned what they are (conductors allow the flow of electrons, while insulators do not) and how they function. I learned about how when two objects are touching an insulator, electrons are able to move through the material and reach the other object.

=**Summary - Lesson 2**= 1. What (specifically) did you read that you already understood well from our class discussion? Describe at least 2 items fully. Charging by friction made a lot of sense to me. Two objects are rubbed together and one has a greater attraction for electrons than the other. When they are rubbed, the one that has a greater attraction receives electrons from the other object. This object now has a negative charge with excess electrons. The other object lost a lot of electrons and now has a positive charge. I understood well how to charge by conduction using a negatively charged object. A negatively charged object makes contact with a neutral object. Since the electrons inside the negatively charged object repel, many of them flow into the neutrally charged object upon contact. This makes the once neutral object have a negative charge.

2. What (specifically) did you read that you were a little confused/unclear/shaky about from class, but the reading helped to clarify? Describe the misconception you were having as well as your new understanding. The grounding section was explained well. I understand now that when a person touches a charged object, the object becomes neutral. Let's say that the object is negatively charged. These negative charges are repelling one another. When a hand makes contact with the object, these negative charges get to disperse into the human, causing the charged object to lose its negative charge and become neutral.

3.What (specifically) did you read that you still don’t understand? Please word these in the form of a question. Why is a pathway needed?

4. What (specifically) did you read that was not gone over during class today? We really didn't go that in depth about charging by induction like the reading did, so it clarified many of my questions about it.

=**Summary - Lesson 3**= 1. What (specifically) did you read that you already understood well from our class discussion? Describe at least 2 items fully. I fully understood when it talked about how charge interactions are actually forces. The balloon demonstration was a perfect explanation even though I already got the point. Two objects that have the same charge each have a force acting on them pushing them away from the other object. It's just like any other force and can be analyzed using a free body diagram and Newton's laws. The equation and its explanation was pretty easy for me to grasp given the knowledge I already had. I knew that this type of force could be measured quantitatively, like all other forces. The examples were pretty easy given the explanation of the equation above.

2. What (specifically) did you read that you were a little confused/unclear/shaky about from class, but the reading helped to clarify? Describe the misconception you were having as well as your new understanding. I was a bit uncertain on how the electric and gravity forces related but the reading helped clarify this. It now makes sense that the two are so related because it is the force on objects without any contact. The differences also are apparent as G is much larger than k, while gravitational forces are attractive and electrical forces are both attractive and repulsive.

3.What (specifically) did you read that you still don’t understand? Please word these in the form of a question. How do you work out problems with electric force and static equilibrium?

4. What (specifically) did you read that was not gone over during class today? The inverse square relationship between separation distance and electrostatic force was gone over much more in depth in the reading. I fully understand how the two relate and how the numbers work when one of the two variables are changed.

=**Summary - Lesson 4**= Action-at-a-distance forces are sometimes referred to as field forces. A charged object creates an electric field - an alteration of the space in the region that surrounds it. Other charges in that field would feel the unusual alteration of the space. Whether a charged object enters that space or not, the electric field exists. Space is altered by the presence of a charged object. Other objects in that space experience the strange and mysterious qualities of the space. An electric charge creates an electric field - it has altered the nature of the space surrounding the charge. And if another charge gets near enough, that charge will sense that there is an affect when present in that surrounding space. And electric field is sensed by the detector charge. The strength of the electric field is dependent upon the amount of charge that creates the field and the distance from the charge. Electric Field Intensity Electric field strength is [|a vector quantity] ; it has both magnitude and direction. The magnitude of the electric field strength is defined in terms of how it is measured. Let's suppose that an electric charge can be denoted by the symbol ** Q **. This electric charge creates an electric field; since ** Q ** is the source of the electric field, we will refer to it as the **source charge**. The strength of the source charge's electric field could be measured by any other charge placed somewhere in its surroundings. The charge that is used to measure the electric field strength is referred to as a ** test charge ** since it is used to //test// the field strength. The test charge has a quantity of charge denoted by the symbol ** q **. When placed within the electric field, the test charge will experience an electric force - either attractive or repulsive. As is usually the case, this force will be denoted by the symbol ** F **. The magnitude of the electric field is simply defined as the force per charge on the test charge. E=F/q In this case, the standard metric units are Newton/Coulomb or N/C.
 * Action at a Distance **
 * The Electric Field Concept **
 * The Force per Charge Ratio **

If the expression for electric force as given by Coulomb's law is substituted for force in the above E =F/q equation, a new equation can be derived as shown below.

E = (k*Q)/d^2

The strength of an electric field as created by source charge ** Q ** is inversely related to square of the distance from the source. This is known as an ** inverse square law **. If you want to know the strength of an electric field, you simply use a charge detector - a test charge that will respond in an attractive or repulsive manner to the source charge. And of course the strength of the field is proportional to the affect upon the detector. The strength of a source charge's electric field is dependent upon how charged up the source charge is. The electric field equation shows that as you get closer and closer to the source of the field, the affect becomes greater and greater and the electric field strength increases.

The magnitude of the electric field vector is calculated as the force per charge on any given test charge located within the electric field. The force on the test charge could be directed either towards the source charge or directly away from it. The worldwide convention that is used by scientists is to define the direction of the electric field vector as the direction that a **positive test charge** is pushed or pulled when in the presence of the electric field. Electric Field Lines Electric field lines always extend from a positively charged object to a negatively charged object, from a positively charged object to infinity, or from infinity to a negatively charged object.
 * The Direction of the Electric Field Vector **
 * Electric field lines **, point in the direction that a positive test charge would accelerate if placed upon the line. As such, the lines are directed away from positively charged source charges and toward negatively charged source charges.
 * Rules for Drawing Electric Field Patterns **
 * Electric field lines never cross each other.
 * Electric field lines are most dense around objects with the greatest amount of charge.
 * At locations where electric field lines meet the surface of an object, the lines are perpendicular to the surface.

One characteristic of a conductor at electrostatic equilibrium is that the electric field anywhere beneath the surface of a charged conductor is zero. A second characteristic of conductors at electrostatic equilibrium is that the electric field upon the surface of the conductor is directed entirely perpendicular to the surface. A third characteristic of conducting objects at electrostatic equilibrium is that the electric fields are strongest at locations along the surface where the object is most curved. Lightning The precursor of any lightning strike is the [|polarization] of positive and negative charges within a storm cloud. The tops of the storm clouds are known to acquire an excess of positive charge and the bottoms of the storm clouds acquire an excess of negative charge. The cloud's [|electric field] stretches through the space surrounding it and induces movement of electrons upon Earth. Electrons on Earth's outer surface are repelled by the negatively charged cloud's bottom surface. This creates an opposite charge on the Earth's surface. As the static charge buildup in a storm cloud increases, the electric field surrounding the cloud becomes stronger. The strong electric fields surrounding a cloud are capable of ionizing the surrounding air and making it more conductive. The ionization involves the shredding of electrons from the outer shells of gas molecules. The gas molecules that compose air are thus turned into a soup of positive ions and free electrons. The insulating air is transformed into a conductive ** plasma **. The ability of a storm cloud's electric fields to transform air into a conductor makes charge transfer (in the form of a lightning bolt) from the cloud to the ground (or even to other clouds) possible. A lightning bolt begins with the development of a ** step leader **. Excess electrons on the bottom of the cloud begin a journey through the conducting air to the ground at speeds up to 60 miles per second. These electrons follow zigzag paths towards the ground, branching at various locations. The quantity of positive charge residing on the Earth's surface becomes even greater. This upward rising positive charge - known as a ** streamer ** - approaches the step leader in the air above the surface of the Earth. Once contact is made between the streamer and the leader, a complete conducting pathway is mapped out and the lightning begins. The contact point between ground charge and cloud charge rapidly ascends upward at speeds as high as 50 000 miles per second. As many as a billion trillion electrons can transverse this path in less than a millisecond.
 * Electric Field Lines for Configurations of Two or More Charges **
 * Electric Fields and Conductors **
 * Electrostatic equilibrium ** is the condition established by charged conductors in which the excess charge has optimally distanced itself so as to reduce the total amount of repulsive forces. Once a charged conductor has reached the state of electrostatic equilibrium, there is no further motion of charge about the surface.

=**Notes**=

Electrostatic Forces

Fe =(k*[q1]*[q2])/d^2 k = 9x10^9 Nm^2/C^2

Balloon Activity 9/16

=Sticky Tape Lab=
 * __Objectives:__**
 * 1) How do different materials react in the presence of a charged object?
 * 2) How do you distinguish between positively and negatively charged objects?
 * 3) How do you determine the exact net charge on an object?

Hypothesis: Materials: Tape, Paper, Foil, Fur, PVC Rod, Glass Rod, Plastic strip Observations:




 * Discussion Questions: **
 * 1) Explain how materials become charged through their interaction with one another.
 * 2) In this case, when materials are rubbed against each other, electrons are shed. The object with a greater affinity for electrons will take the electrons that are shed by the other object. Now this object is negative and the other is positive.


 * 1) Why, when you stroke a cat's fur, or comb your hair on a cold, dry day can you hear a crackling sound? Doing these things in a darkened room, you can actually see sparks. Explain.
 * 2) The crackling sound and sparks are side effects of the transfer of electrons.


 * 1) Photocopying machines use the principles of electric charges. Do research to find out how photocopying machines work. Be sure to list your sources.
 * 2) A document is placed upside down onto the glass. A bright light scans across the document. More light reflects off the white areas than off the black areas. An electric shadow of the page forms on the photoconductor. The photoconductor is a rotating conveyer belt coated with a chemical called selenium. As the belt rotates, it carries this electrical shadow. An ink drum touching it coats it with tiny particles of ink. The toner has been given an electrical charge, so it sticks to the electrical shadow and makes an inked image of the original page on the belt. A sheet of paper from a hopper on the other side of the copier feeds up toward the first belt on another conveyor belt. As it moves along, the paper is given a strong electrical charge. When the paper moves near the upper belt, its strong charge attracts the charged toner particles away from the belt. The image is rapidly transferred from the belt onto the paper. The inked paper passes through two hot rollers. The heat and pressure from the rollers fuse the toner particles permanently onto the paper. The final copy emerges from the side of the copier.
 * 3) http://www.explainthatstuff.com/photocopier.html

PVC(++), Wool (+++++), Styrene (++++), Teflon (+), Polyester (+++)
 * 1) Materials have a characteristic which evaluates their attraction for electrons. The Triboelectric Series orders materials by their affinity for gathering electrons through contact from other materials. The materials toward the top of the list are likely to give up electrons in these interactions whereas those at the bottom are more likely to gain electrons. Five materials are ranked as follows, with more positives meaning least desiring electrons.


 * 1) Rank the materials on the scale below:




 * 1) Determine the net charge on each item when the following pairs of materials are rubbed together. (In other words, which ends up giving up electrons and which ends up accepting them?)
 * 2) PVC and Wool
 * 3) PVC


 * 1) PVC and Teflon
 * 2) Teflon


 * 1) PVC and Polyester
 * 2) PVC


 * 1) Teflon and Polyester
 * 2) Teflon


 * 1) Styrene and Wool
 * 2) Syrene