Presentation on theme: "Chapter 12: Electric Field Lines and Electric Charges"— Presentation transcript:
1Chapter 12: Electric Field Lines and Electric Charges J. PulickeelNovember 2008SPH 3U1
2Contents of this Presentation In this presentation we will learnWhat are Electric Force Fields?What are the properties of Electric Force Fields?How do we calculate the magnitude of the Force between two charged particles?How do we measure the charge of a particle?
3Electric Force FieldsA field force is sometimes referred to as an action at a distanceThe space surrounding a charged object is affected by the presence of the force or charge.Gravitational FieldsMagnetic FieldsElectrical Fields
4Electric Field LinesElectric Field Lines are sometimes called lines of force. The lines are drawn to show the direction of the force, due to the electric field, as it wouldact on a test charge.Generally we state that the field starts at the positive charge and goes to the negative charge
5Electric Field Lines All charged objects create electric field lines. Similar charges attract each otherLike charges will repel each other.
6Drawing Electric Field Lines Field Lines always go from positive to negativeField lines NEVER cross. They represent the net force on a test charge. If they were to cross it would mean that the charge had two different net forces with different directions. This is not possible. The charge will experience a single net force in the direction of the field.The number of field lines leaving the (+) is proportional to the magnitude of the charge.The density of field lines in a given area can represents the relative strength of the field
7Field StrengthA measure of the strength of a field is how dense the field lines in a certain area are.As you get closer to the source of the field the force gets stronger, and the field line density increases.
8Drawing Electric Field Lines The number of field lines leaving the (+) is proportional to the magnitude of the charge. Thus there are more field lines around the 4q charge that the q charge.The density of field lines does not always indicate relative strength. In this case, color indicates the relative strength of the field.
9Calculating the Charge… How do we calculate the charge (Q) for each particle?The charge of 1 electron is 1.60 x CTherefore the charge of a particle isWhere N is the number of electrons
10Coulombs (C) The charge of a particle is measured in Coulombs. A Coulomb is approximately the amount of electric charge that passes through a 100 light bulb in 1 secondThe earth stores approximately C of chargeA person walking across a woollen rug can store 10-8 C
12Recall: Gravitational Force Fields The Universal Law of Gravitation states that all objects with mass exert a force of attraction on others.The force is dependant on theMass of the two objects (F m)The distance between them (F 1/d)
13Electric and Gravitational Forces Electric ForcesA Force of attraction between two objects.A force between two charged particles. It can be attractive or repulsive.
14Electric Force FieldsCharles de Coulomb (19th Century) discovered a similar relationship between charged particlesCoulomb’s Law states that all objects with charge exert a force of attraction on others.The force is dependant on theCharge of the two objects (F q)The distance between them (F 1/d)
15True/ False QuestionsIf electrons are added to an object, it will be charged negatively.A negatively charged ebonite rod is brought near a neutral, metallic-coated pith ball. Some of the electrons are repelled by the ebonite rod and move to the far side of the pith ball. This process is called charging by inductionTrueFalseAnswerTrueFalseAnswer
16True/ False QuestionsCoulomb’s law states that the electric force is directly proportion to the product of the charges on each sphere and inversely proportional to the distance between the charges.When the distance between two charged spheres is decreased by a factor of 2, then the electric force decreases by a factor of 2.One of the ways in which Newton’s law of universal gravitation differs from Coulomb’s law is that gravitational force can only attract, whereas the electric force can only repel.TrueFalseAnswerTrueFalseAnswerTrueFalseAnswer
17Electrons flow from the earth through the wire to the electroscope. Multiple Choice A negatively charged rod is held near, but does not touch the knob of an electroscope. The leaves of the electroscope move apart from one another. A wire is connected to the knob and to a water tap with the negatively charged rod staying in the same position. Which of the following would occur?Electrons flow from the earth through the wire to the electroscope.No electron flow takes place.The leaves of the electroscope remain still.The leaves of the electroscope move closer together.Electrons flow from the electroscope through the wire to the earth.My AnswerOfficial Answer
18is opposite to the charge on the charging object Multiple Choice When a charge separation has been induced on an object, the charge on the objectis opposite to the charge on the charging objectis proportional to the size of the object being chargedis permanentis the same as the charge on the charging objectis inversely proportional to the size of the object being chargedMy AnswerOfficial Answer
19a negatively charged rod two objects of the same charge Multiple Choice To charge an electroscope positively by induction you needa negatively charged rodtwo objects of the same chargea negatively charged rod and a groundtwo objects with opposite chargesa positively charged rod and a groundMy AnswerOfficial Answer
20Multiple Choice The law of electric charges states that opposite charges attract each other, similar charges attract neutral objects, and charged objects repel one anotherrepel each other, similar charges attract neutral objects, and charged objects attract one anotherattract neutral objects, similar charges repel each other, and charged objects attract one anotherattract each other, similar charges repel one another, and charged objects attract some neutral objectsattract neutral objects, similar charges attract each other, and charged objects repel one anotherMy AnswerOfficial Answer
21e = 1.6 x10-19C N= ? Q = Ne Q = 7.5 x10-11 C N= 4.7 x 108 electrons Sample Question 1 How many electrons have been removed from a positively charged pith ball electroscope if it ahs a charge of 7.5 x10-11C?Q = 7.5 x10-11 Ce = 1.6 x10-19CN= ?Q = NeN= 4.7 x 108 electrons
22Sample Question 2 Two charged spheres are 2. 00 m apart Sample Question 2 Two charged spheres are 2.00 m apart. One sphere has a charge of 5.7 x10-5C and the other sphere has a charge of 1.6 x10-1C . What is the electric force between the two spheres?The electric force between the two spheres is 2.05 x 104 N [APART].