Chapter 12: Electric Field Lines and Electric Charges

1. Chapter 12: Electric Field Lines and Electric Charges J. Pulickeel November 2008 SPH 3U1

2. Contents of this Presentation In this presentation we will learn • What 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?

3. Electric Force Fields • A field force is sometimes referred to as an action at a distance • The space surrounding a charged object is affected by the presence of the force or charge. Gravitational Fields Magnetic Fields Electrical Fields

4. Electric Field Lines • Electric 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 would • act on a test charge. • Generally we state that the field starts at the positive charge and goes to the negative charge

5. Electric Field Lines • All charged objects create electric field lines. Similar charges attract each other Like charges will repel each other.

6. Drawing Electric Field Lines • Field Lines always go from positive to negative • Field 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

7. Field Strength • A 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.

8. Drawing 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.

9. Calculating the Charge… • How do we calculate the charge (Q) for each particle? • The charge of 1 electron is 1.60 x 10-19 C • Therefore the charge of a particle is • Where N is the number of electrons

10. Coulombs (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 second • The earth stores approximately 400 000 C of charge • A person walking across a woollen rug can store 10-8 C

11. Drawing Electric (and Magnetic) Field Lines

12. Recall: 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 the • Mass of the two objects (F  m) • The distance between them (F 1/d)

13. Electric and Gravitational Forces Gravitational Forces Electric Forces • A Force of attraction between two objects. • A force between two charged particles. It can be attractive or repulsive.

14. Electric Force Fields • Charles de Coulomb (19th Century) discovered a similar relationship between charged particles • Coulomb’s Law states that all objects with charge exert a force of attraction on others. • The force is dependant on the • Charge of the two objects (F  q) • The distance between them (F 1/d)

15. True/ False Questions • If 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 induction True True False False Answer Answer

16. True/ False Questions • Coulomb’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. True True True False False False Answer Answer Answer

17. Multiple ChoiceA 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 Answer Official Answer

18. Multiple Choice When a charge separation has been induced on an object, the charge on the object • is opposite to the charge on the charging object • is proportional to the size of the object being charged • is permanent • is the same as the charge on the charging object • is inversely proportional to the size of the object being charged My Answer Official Answer

19. Multiple Choice To charge an electroscope positively by induction you need • a negatively charged rod • two objects of the same charge • a negatively charged rod and a ground • two objects with opposite charges • a positively charged rod and a ground My Answer Official Answer

20. Multiple Choice The law of electric charges states that opposite charges • attract each other, similar charges attract neutral objects, and charged objects repel one another • repel each other, similar charges attract neutral objects, and charged objects attract one another • attract neutral objects, similar charges repel each other, and charged objects attract one another • attract each other, similar charges repel one another, and charged objects attract some neutral objects • attract neutral objects, similar charges attract each other, and charged objects repel one another My Answer Official Answer

21. Sample Question 1How 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 C • e = 1.6 x10-19C • N= ? Q = Ne N= 4.7 x 108 electrons

22. Sample Question 2Two 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].