Ch18_HallowellC

toc = = = = = = =9/8/11=

1. What is the structure and properties of an atom? An atom is composed of a variety of subatomic particles. The three main subatomic particles are protons, electrons, and neutrons. The protons and neutrons are located in the nucleus of the atom. Outside the nucleus are concentric spherical spaces called electron shells. An atom that is charged is called an ion. A positive ion is called a cation. A negative ion is called an anion. 2. What is the symbol and unit of electric charge? Coulomb (c) 3. Distinguish between positive and negative charges in as many ways as possible. When an atom has more protons than electrons, it is said to be positively charged. If an atom has more electrons than protons, it is said to be negatively charged. (Q) or (q)- chargee= -1.6 x 10^-19 Cp= 1.6 x 10^-19 C positive charges- ions (mass = 1.67 x 10^-27) negative charges- electrons (mass= 9.11 x 10^-31 kg) Protons do not move as much as electrons.

4. Describe the properties of electric forces. It is a non-contact force. Any charged object can exert this force on other objects, charged or uncharged. Opposites attract and likes repel. 5. Distinguish between insulators and conductors. Insulators are materials that impeded the free flow of electrons from atom to atom and molecule to molecule. Conductors are materials that permit the free flow of electrons from atom to atom and molecule to molecule. 6. What is polarization? It is the process of separating opposite charges within an object.

7. How does a neutral object acquire charge? It can acquire charge through three processes. The processes are charging by friction, charging by induction, and charging by conduction.

8. Distinguish between the 3 charging processes. Induction charging is a method used to charge an object without actually touching the object to any other charged object Friction charging results in a transfer of electrons between two objects that are rubbed together. Conduction charging involves the contact of charged object to a neutral object.

9. What is the law of electric charge? The charge is always conserved. #1-4    Summarizing Lesson 1: Basic Terminology and Concepts (Method 2a) 1. What (specifically) did you read that you already understood well from our class discussion? Describe at least 2 items fully. I read about the structure of the atom and had previously understood it well from our class discussion. I understand that in every atom there is a nucleus that is made up of positively charged protons and neutral neutrons. Surrounding the nucleus are negatively charged electrons. If the atom has less electrons than protons, then the atom is positively charged. If the atom has more electrons than protons, then the atom is negatively charged. I also read about the idea that opposites attract and likes repel. In class, I understood this idea very well and after reading the lesson, I further understood the topic. When an object that is negatively charged is placed near an object that is also negatively charged, the objects will repel each other, whereas when that original object is placed near an object that is positively charged, the two objects will attract to one another. I also understand that any charged object will attract to a neutral object.

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. In class, I was a little shaky on the idea of Coulombs and how to perform operations using electrons and Coulombs. I did not understand how the Coulombs fit with the idea of electrons and charges. However, after reading the article, I now understand Coulombs are just a unit to measure the charge of an object. They are in the same category as meters and grams. I also now understand why they use microCoulombs.

3. What (specifically) did you read that you still don’t understand? Please word these in the form of a question. Do two objects need to touch to polarize?

4. What (specifically) did you read that was not gone over during class today? We did not go over conductors and insulators during class today, however, after reading the lesson, I understand the topics well.

=9/9/11= Summarizing Lesson 2: Methods of Charging (Method 2b) (I missed class so I did 2b.) 1. What (specifically) did you read that you understand well? Describe at least 2 items fully. I understood the process of charging by friction very well. Electrons are transferred when two objects are rubbed together. The object with the higher electron affinity will receive electrons while the other object will lose electrons. As a result, the object with the excess amount of electrons will be charged negatively, while the object with a shortage of electrons will be charged positively. I also understood the law of conservation of charge. It makes a lot of sense that no matter how many electrons were transferred from one object to another, the net charge of the two objects will be the same at the end as the net charge was at the beginning.

2. What (specifically) did you read that made you feel little confused/unclear/shaky, but further reading helped to clarify? Describe the misconception(s) you were having as well as your new understanding. Originally, I was a little unclear on what the difference was between charging by induction and charging by conduction. However, after reading, I now understand that charging by induction does not involve contact in order to transfer electrons, whereas charging by conduction does involve contact in order to transfer electrons.

3. What (specifically) did you read that you don’t understand? Please word these in the form of questions. Why is there a need for a pathway? Why am I not able to just touch the object in order to ground it.

4. What (specifically) did you read that you thought was pretty interesting, that you didn't know before, or can easily apply to your every day life? I can easily apply the method of charging by friction into my everyday life. This method is present every time I put my shirt on in the morning. When the shirt is pulled over my head, it rubs against my hair and charging by friction occurs. This occurrence will usually result in messy hair.

=9/12/11= Lab: Sticky Tape

Observations:

In order to obtain these results, I used tape, foil, and paper. I also used a ring stand to hang my materials off of.

For the second part of the lab, I used a PVC rod, a lucite rod, animal fur, plastic, and tape. These were the results:

Discussion Questions: 1. Explain how materials become charged through their interaction with one another. The materials become charged through the method of charging by friction. When the pieces of tape are pulled from one another at a rapid pace, friction occurs and as a result, they become charged. The same idea happens when the PVC rod and animal fur, as well as the lucite rod and plastic are rubbed together rapidly. Charging by friction occurs, therefore resulting in a negatively charged PVC rod and a positively charged lucite rod.

2. 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. The crackling sound happens as a result of the protons in the atoms of one material interacting with the electrons on the other material. This method of charging by friction results in two materials that end up being charged.

3. Photocopying machines use the principles of electric charges. Do research to find out how photocopying machines work. Be sure to list your sources. There is a copier drum inside the machine. The drum is given a positive charge and the image from the original copy illuminates the drum. An "invisible" image is formed. Then, using static electricity, ink (toner) is attracted to the positively charged drum and an image is visible. After the new copy is lifted from the drum and cleaned using rollers and blades, it is exposed to a neon light that erases any remaining static charge left on the copy. http://www.sil.org/lingualinks/literacy/ReferenceMaterials/glossaryofliteracyterms/WhatIsAPhotocopyMachine.htm



=9/13/11= Electrostatic Forces

+q +q

Fe= (ke) |q1| |q2| / d^2

The "q" has to be in Coulombs. The d in meters and the F in N. Similar to force of gravity equation, but this is electrostatic force. Electrostatic force can attract and repel.


 * 1) 5-7 of Guiding Questions

Summarizing Lesson 3: Electric Force (Method 2a) 1. What (specifically) did you read that you already understood well from our class discussion? Describe at least 2 items fully. I understand that the electric force is a vector quantity that has both magnitude and direction. The magnitude is determined by how charged the two interacting objects are. The magnitude can also be determined by the distance between the two interacting objects. The direction can be determined by whether the two objects are repelling or attracting. I feel that I also understand the equation for electric force as well. I understand that the quantity of the charges and the force are directly proportional, while the distance between the charges and the force are inversely proportional. I also understand that the charges are in Coulombs, the distance is in meters, and the force is in newtons. The value of "k" is equal to 9.0 x 10^9 N(m)^2/C^2.

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. Originally, I was a little unclear on how this force could help me to determine an acceleration. However, after reading the lesson, I now understand that it is very similar to the original F=ma equation that we used last year.

3. What (specifically) did you read that you still don’t understand? Please word these in the form of a question. I truly felt that I understood the reading well.

4. What (specifically) did you read that was not gone over during class today? We did not go over how to solve the problems that include 3 or 4 total charges, but I can see that we are going to have to use vector addition from last year.

=9/14/11= More advanced problems using vector addition
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 * 17

HW: Guide Questions #10-14

10. What is an electric field? An electric field is the region around a charge where another charge will feel an electric force.

11. What are the characteristics and properties of an electric field? See applet: [] - Anywhere beneath the surface of a conductor, the electric field is zero. - Electric fields are perpendicular to the surface. - Electric fields are strongest at points where the surface is most curved (pointed).

12. What are the “players” involved in an electric field? There is a "source of field", which gives off the field lines. Its charge is represented by Q. There are also experiencing charges. There can be multiple experiencing charges and they can vary in size. They feel the effects of the electric field. Their charge is represented by q.

13. What are electric field lines? They are used to visually represent the forces of electric fields.

14. What are 4 characteristics of electric field lines? - They go on to infinity, unless there is another charge of the opposite charge present. - They are perpendicular to the surface of the charges. - They cannot intersect. - The more field lines mean a stronger electric field.

15. Check your Understanding Questions 1. In C, the lines are directed towards a positively charged object. In D, the lines are not symmetrically positioned despite the fact that the object is a symmetrical sphere. In E, the lines are directed away from a negative charge. 2. Electric field lines should never intersect each other. Erin crossed his lines. 3. D- Electric field lines are directed towards object A so object A must be negative. They are directed away from object B so object B must be positive. 4 **.** DAECB- Electric field strength is greatest where the lines are closest together and weakest where lines are furthest apart. 5. Objects A, C, F, G, H, and I are positively charged. Objects B, D and E are negatively charged. The principle is: electric field lines always approach negatively charged objects and are directed away from positively charged objects. 6. B < A; C < D; G < E < F; J < H < I

=9/15/11= Balloon Class Activity 1st Half

2nd Half

Electric field- region around a charge where another charge will feel an electric force.

Q (source of field) q (experiencing charge)

The bigger the experiencing charge, the less affect the source of field has on it. Strength of the source field is determined by magnitude and distance.

E= Fe/q = kQ/d^2

Characteristics of electric field: Can't intersect out of positive, into negative more lines=stronger E perpendicular to surface

Guidesheet Problems #10,13
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Summarizing Lesson 4: Electric Fields using Method 1 Action at a Distance C__ [|harge interactions] __ - the affect of a charged object upon other objects of the same type of charge, of an opposite type of charge and of no charge whatsoever. The concept of **action-at-a-distance** using a different concept known as the **electric field**. It was mentioned that there are two categories of forces - contact forces and non-contact forces. Electrical force and gravitational force were both listed as non-contact forces. The __ [|gravitational force] __, are action-at-a-distance forces that act between two objects even when they are held some distance apart. If you watch a roller coaster car move along its course, then you are witnessing an action-at-a-distance. The Earth and the coaster car attract even though there is no physical contact between the two objects.

A charged plastic golf tube might be held above bits of paper on a lab bench. The plastic tube attracts the paper bits even though physical contact is not made with the paper bits. The charged plastic tube might also be held near a charged rubber balloon; and even without physical contact, the tube and the balloon act over a distance. The charged plastic tube exerts its influence over a distance, affecting other charged objects that were in the surrounding //neighborhood//.

**The Electric Field Concept** Contact forces are quite usual and customary to us. Explaining a contact force that we all feel and experience on a daily basis is not difficult. Non-contact forces require a more difficult explanation. The best explanation to this question involves the introduction of the concept of **electric field**. Action-at-a-distance forces are sometimes referred to as field forces. The concept of a **field force** is utilized by scientists to explain this rather unusual force phenomenon that occurs in the absence of physical contact. While all masses attract when held some distance apart, charges can either repel or attract when held some distance apart. 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.

A Van de Graaff generator is a large conducting sphere that acquires a charge as electrons are scuffed off of a rotating belt as it moves past sharp elongated prongs inside the sphere. The buildup of static charge on the Van de Graaff generator is much greater than that on a balloon rubbed with animal fur or an aluminum plate charged by induction. On a dry day, the buildup of charge becomes so great that it can exert influences on charged balloons held some distance away. The Van de Graaff generator, like any charged object, alters the space surrounding it. Electric forces are exerted upon those charged objects when they enter that space. **A Stinky Analogy** Anyone who has ever walked into a room of an infant with a soiled diaper has experienced a //stinky field//. There is something about the space surrounding an infant's soiled diaper that exerts a strange influence upon other people who enter that space. When that //little stinker// needs a diaper change, you can't help but to notice it. When you walk into a room with such a diaper present, your detectors (i.e., the nose) begin to detect the presence of a stinky field. As you move closer and closer to the infant, the stinky field becomes more and more intense. The diaper has altered the nature of the surrounding space and when your nose gets near, you know it.

An electric charge creates an electric field - it has altered the nature of the space surrounding the charge. And electric field is sensed by the detector charge in the same way that a nose senses the stinky field. The strength of the stinky field is dependent upon the distance from the stinky diaper and the amount of //stinky// in the diaper. And in an analogous manner, 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 **The Force per Charge Ratio** 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** 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 standard metric units are Newton/Coulomb or N/C. Electric field is the force per quantity of charge //on the test charge//. The electric field strength is not dependent upon the quantity of charge on the test charge. Increasing the quantity of charge on the test charge - say, by a factor of 2 - would increase the denominator of the equation by a factor of 2. But according to __ [|Coulomb's law] __, more charge also means more electric force ( **F** ). So as the denominator in the equation increases by a factor of two (or three or four), the numerator increases by the same factor. These two changes offset each other such that one can safely say that the electric field strength is not dependent upon the quantity of charge on the test charge. So regardless of what test charge is used, the electric field strength at any given location around the source charge **Q** will be measured to be the same.

**Another Electric Field Strength Formula** Coulomb's law states that the electric force between two charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between their centers. The formula for electric force can be written as 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. One feature of this electric field strength formula is that it illustrates an inverse square relationship between electric field strength and distance. 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**. Electric field strength is location dependent, and its magnitude decreases as the distance from a location to the source increases.
 * An Inverse Square Law**

**The Stinky Field Analogy Revisited** 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. A more sensitive detector (a better nose or a more charged test charge) will sense the affect more intensely. Yet the field strength is defined as the affect (or force) per sensitivity of the detector; so the field strength of a stinky diaper or of an electric charge is not dependent upon the sensitivity of the detector.

**The Direction of the Electric Field Vector** Electric field strength is a vector quantity. 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 precise direction of the force is dependent upon whether the test charge and the source charge have the same type of charge (in which repulsion occurs) or the opposite type of charge (in which attraction occurs). 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. By using the convention of a positive test charge, everyone can agree upon the direction of **E**. A positive source charge would create an electric field that would exert a repulsive affect upon a positive test charge. Thus, the electric field vector would always be directed away from positively charged objects. On the other hand, a positive test charge would be attracted to a negative source charge. Therefore, electric field vectors are always directed towards negatively charged objects.

Electric Field Lines For any given location, the arrows point in the direction of the electric field and their length is proportional to the strength of the electric field at that location. Note that the lengths of the arrows are longer when closer to the source charge and shorter when further from the source charge.

A more useful means of visually representing the vector nature of an electric field is through the use of electric field lines of force. These pattern of lines, sometimes referred to as **electric field lines**, point in the direction that a positive test charge would accelerate if placed upon the line. The lines are directed away from positively charged source charges and toward negatively charged source charges. Each line must include an arrowhead that points in the appropriate direction.

**Rules for Drawing Electric Field Patterns** One common convention is to surround more charged objects by more lines. Objects with greater charge create stronger electric fields. The density of lines at a specific location in space reveals information about the strength of the field at that location. These cross-sections represent regions of space closer to and further from the source charge. The field lines are closer together in the regions of space closest to the charge; and they are spread further apart in the regions of space furthest from the charge. One would reason that the electric field is greatest at locations closest to the surface of the charge and least at locations further from the surface of the charge.

A second rule involves drawing the lines of force perpendicular to the surfaces of objects at the locations where the lines connect to object's surfaces. The electric force, and thus the electric field, is always directed perpendicular to the surface of an object. Once a line of force leaves the surface of an object, it will often alter its direction.

A final rule for drawing electric field lines involves the intersection of lines. Electric field lines should never cross.

**Electric Field Lines for Configurations of Two or More Charges** Suppose that there are two positive charges - charge A (QA) and charge B (QB) - in a given region of space. At any given location surrounding the charges, the strength of the electric field can be calculated using the expression kQ/d2. Since there are two charges, the kQ/d2 calculation would have to be performed twice at each location - once with kQA/dA2 and once with kQB/dB2. The strength of the field is represented by the length of the arrow and the direction of the field is represented by the direction of the arrow.

Since electric field is a vector, the usual operations that apply to vectors can be applied to electric field. That is, they can be added in head-to-tail fashion to determine the resultant or net electric field vector at each location.

Note that for each location, the electric field vectors point tangent to the direction of the electric field lines at any given point.



In each of the above diagrams, the individual source charges in the configuration possess the same amount of charge. Subsequently, the pattern is symmetrical in nature and the number of lines emanating from a source charge or extending towards a source charge is the same. If the quantity of charge on a source charge is not identical, the pattern will take on an asymmetric nature, as one of the source charges will have a greater ability to alter the electrical nature of the surrounding space.

After plotting the electric field line patterns for a variety of charge configurations, the general patterns for other configurations can be predicted.
 * 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.
 * 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.

**Electric Field Lines as an Invisible Reality** The concept of the electric field was first introduced by 19th century physicist Michael Faraday. It was Faraday's perception that the pattern of lines characterizing the electric field represents an invisible reality. Faraday used the concept of a field to propose that a charged object affects the space that surrounds it. As another object enters that space, it becomes effected by the field established in that space. A charge is seen to interact with an electric field as opposed to with another charge. Each charge or configuration of charges creates an intricate web of influence in the space surrounding it. While the lines are invisible, the affect is ever so real.

Electric Fields and Conductors A__ [| conductor] __ is material that allows electrons to move relatively freely from atom to atom. It was emphasized that when a conductor acquires an excess charge, the excess charge moves about and distributes itself about the conductor in such a manner as to reduce the total amount of repulsive forces within the conductor. **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.

**Electric Fields Inside of Charged Conductors** One characteristic of a conductor at electrostatic equilibrium is that the electric field anywhere beneath the surface of a charged conductor is zero. If an electric field did exist beneath the surface of a conductor (and inside of it), then the electric field would exert a force on all electrons that were present there. The electric field lines either begin or end upon a charge and in the case of a conductor, the charge exists solely upon its outer surface.  This principle of **shielding** is commonly utilized today as we protect delicate electrical equipment by enclosing them in metal cases. Even delicate computer chips and other components are shipped inside of conducting plastic packaging that shields the chips from potentially damaging affects of electric fields. 

**Electric Fields are Perpendicular to Charged Surfaces** 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. There cannot be a component of electric field (or electric force) that is parallel to the surface.

The motion of electrons, like any physical object, is //governed// by __ [|Newton's laws] __. One outcome of Newton's laws was that __ [|unbalanced forces cause objects to accelerate in the direction of the unbalanced force and a balance of forces causes objects to remain at equilibrium] __. This truth providesthe foundation for the rationale behind why electric fields must be directed perpendicular to the surface of conducting objects. If there were a component of electric field directed parallel to the surface, then the excess charge on the surface would be forced into accelerated motion by this component.

**Electric Fields and Surface Curvature** 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. The curvature of a surface can range from absolute flatness on one extreme to being curved to a //blunt// point on the other extreme. We will consider an irregularly shaped object that is negatively charged. Such an object has an excess of electrons. These electrons would distribute themselves in such a manner as to reduce the affect of their repulsive forces. Since electrostatic forces vary inversely with the square of the distance, these electrons would tend to position themselves so as to increase their distance from one another. On an irregularly shaped object, excess electrons would tend to accumulate in greater density along locations of greatest curvature.

The fact that surfaces that are sharply curved to a blunt edge create strong electric fields is the underlying principle for the use of lightning rods.

Lightning
 * What is the cause and mechanism associated with lightning strikes?
 * How do lightning rods serve to protect buildings from the devastating affects of a lightning strike?

**Static Charge Buildup in the Clouds** 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. One mechanism involves a separation of charge by a process that bears resemblance to __ [|frictional charging] __. This upwardly rising moisture collides with water droplets within the clouds. In the collisions, electrons are ripped off the rising droplets, causing a separation of negative electrons from a positively charged water droplet or a cluster of droplets. The second mechanism that contributes to the polarization of a storm cloud involves a freezing process. These cooler temperatures cause the cluster of water droplets to undergo freezing. The frozen particles tend to cluster more tightly together and form the central regions of the cluster of droplets. The frozen portion of the cluster of rising moisture becomes negatively charged and the outer droplets acquire a positive charge. Air currents within the clouds can rip the outer portions off the clusters and carry them upward toward the top of the clouds. The frozen portion of the droplets with their negative charge tends to gravitate towards the bottom of the storm clouds. Thus, the clouds become further polarized. 

The polarization of the clouds has an equally important affect on the surface of the Earth. 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. Buildings, trees and even people can experience a buildup of static charge as electrons are repelled by the cloud's bottom.

**The Mechanics of a Lightning Strike** As the static charge buildup in a storm cloud increases, the electric field surrounding the cloud becomes stronger. 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. It is believed that the presence of impurities or dust particles in various parts of the air might create regions between clouds and earth that are more conductive than other regions. The step leader is not the actual lightning strike; it merely provides the roadway between cloud and Earth along which the lightning bolt will eventually travel.

The quantity of positive charge residing on the Earth's surface becomes even greater. This charge begins to migrate upward through buildings, trees and people into the air. 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. As many as a billion trillion electrons can transverse this path in less than a millisecond. This initial strike is followed by several secondary strikes or charge surges in rapid succession. The enormous and rapid flow of charge along this pathway between the cloud and Earth heats the surrounding air, causing it to expand violently. The expansion of the air creates a shockwave that we observe as thunder.

**Lightning Rods and Other Protective Measures** The attachment of a grounded lightning rod to a building is a protective measure that is taken to protect the building in the event of a lightning strike. First, the rod serves to prevent a charged cloud from releasing a bolt of lightning. And second, the lightning rod serves to safely divert the lightning to the ground in event that the cloud does discharge its lightning via a bolt.

The first of Franklin's two proposed theories is often referred to as the **lightning dissipation theory**. According to the theory, the use of a lightning rod on a building protects the building by preventing the lightning strike. The idea is based upon the principle that the __ [|electric field strength is great around a pointed object] __. The intense electric fields surrounding a pointed object serve to ionize the surrounding air, thus enhancing its conductive ability.

The second of Franklin's proposed theories on the operation of the lightning rod is the basis of the **lightning diversion theory**. The lightning diversion theory states that a lighting rod protects a building by providing a conductive pathway of the charge to the Earth. It is indeed true that the tip of a lightning rod is capable of ionizing the surrounding air and making it more conductive. However, this affect only extends for a few meters above the tip of the lightning rod. A few meters of enhanced conductivity above the tip of the rod is not capable of discharging a large cloud that stretches over several kilometers of distance.

=9/16/11= Practice Problem #19c, from physics homework practice 133

1st Half 2nd Half

=9/19/11=

There is an excess of charge on the pointier edges of a surface rather than the flatter edges. That is why we create a shock when we touch something with our finger if we are charged.

Guide Question #17 Concept Map